Automotive Braking Systems

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Automotive Braking Systems

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MASTER AUTOMOTIVE TECHNICIAN SERIES

Automotive Braking Systems Nicholas Goodnight Automotive Instructor, Ivy Tech Fort Wayne, Indiana

Kirk VanGelder ASE Certified Master Automotive Technician & L1 & G1 Technology Educators of Oregon – President Certified Automotive Service Instructor Vancouver, Washington

World Headquarters Jones & Bartlett Learning 5 Wall Street Burlington, MA 01803 978-443-5000 [email protected] www.jblearning.com Jones & Bartlett Learning books and products are available through most bookstores and online booksellers. To contact Jones & Bartlett Learning directly, call 800-832-0034, fax 978-443-8000, or visit our website, www.jblearning.com. Substantial discounts on bulk quantities of Jones & Bartlett Learning publications are available to corporations, professional associations, and other qualified organizations. For details and specific discount information, contact the special sales department at Jones & Bartlett Learning via the above contact information or send an email to [email protected] Copyright © 2019 by Jones & Bartlett Learning, LLC, an Ascend Learning Company All rights reserved. No part of the material protected by this copyright may be reproduced or utilized in any form, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission from the copyright owner. The content, statements, views, and opinions herein are the sole expression of the respective authors and not that of Jones & Bartlett Learning, LLC. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not constitute or imply its endorsement or recommendation by Jones & Bartlett Learning, LLC and such reference shall not be used for advertising or product endorsement purposes. All trademarks displayed are the trademarks of the parties noted herein. Automotive Braking Systems is an independent publication and has not been authorized, sponsored, or otherwise approved by the owners of the trademarks or service marks referenced in this product. There may be images in this book that feature models; these models do not necessarily endorse, represent, or participate in the activities represented in the images. Any screenshots in this product are for educational and instructive purposes only. Any individuals and scenarios featured in the case studies throughout this product may be real or fictitious, but are used for instructional purposes only. Production Credits General Manager: Kimberly Brophy Content Services Manager: Kevin Murphy Product Manager: Jesse Mitchell Senior Vendor Manager: Sara Kelly Marketing Manager: Amanda Banner VP, Manufacturing and Inventory Control: Therese Connell Composition and Project Management: Integra Software Services Pvt. Ltd.

Cover Design: Scott Moden Rights & Media Specialist: Robert Boder Media Development Editor: Shannon Sheehan Cover Image (Title Page): Getty Images/4X-image Printing and Binding: LSC Communications Cover Printing: LSC Communications

Library of Congress Cataloging-in-Publication Data unavailable at time of printing.

6048

Printed in the United States of America 22 21 20 19 18  10 9 8 7 6 5 4 3 2 1

BRIEF CONTENTS CHAPTER 1    Strategy-Based Diagnostics���������������������������������������������������������������������������������� 1 CHAPTER 2    Principles of Braking�������������������������������������������������������������������������������������������� 23 CHAPTER 3    Hydraulic Brake Systems������������������������������������������������������������������������������������ 43 CHAPTER 4    Disc Brake Systems���������������������������������������������������������������������������������������������� 91 CHAPTER 5    Drum Brake Systems���������������������������������������������������������������������������������������� 137 CHAPTER 6    Refinishing Brake Rotors and Drums �������������������������������������������������������������� 175 CHAPTER 7    Parking Brakes �������������������������������������������������������������������������������������������������� 197 CHAPTER 8    Power-Assist Systems���������������������������������������������������������������������������������������� 207 CHAPTER 9    Wheel Bearing Service�������������������������������������������������������������������������������������� 227 CHAPTER 10    Hybrid Vehicle Braking Systems ���������������������������������������������������������������������� 255 CHAPTER 11    Advanced Braking Systems: Electronic Brake Controls���������������������������������� 265 CHAPTER 12    Electronic Stability Control Systems �������������������������������������������������������������� 295 Appendix A    2011 NATEF Automobile Accreditation Task List Correlation Guide ���������� 305 Glossary������������������������������������������������������������������������������������������������������������������������������������������ 307 Index������������������������������������������������������������������������������������������������������������������������������������������������ 315

CONTENTS CHAPTER 1  Strategy-Based Diagnostics�������������1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Vehicle Service History. . . . . . . . . . . . . . . . . . . . . . . . . 2 Strategy-Based Diagnostic Process. . . . . . . . . . . . . . . . 5 Documenting the Repair. . . . . . . . . . . . . . . . . . . . . . . 16 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Key Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 ASE Technician A/Technician B Style Questions. . . . . 22 CHAPTER 2  Principles of Braking . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Braking Systems Fundamentals. . . . . . . . . . . . . . . . . . Physics Behind Braking System Application . . . . . . . . How Energy Is Transformed. . . . . . . . . . . . . . . . . . . . How Brake Fade Affects Vehicle Stopping. . . . . . . . . . Rotational Force. . . . . . . . . . . . . . . . . . . . . . . . . . . . . Different Applications of Braking Systems. . . . . . . . . Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . Key Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . ASE Technician A/Technician B Style Questions. . . . .

23 24 24 26 29 31 33 34 38 39 40 40

CHAPTER 3  Hydraulic Brake Systems. . . . . . . . Hydraulic Braking System. . . . . . . . . . . . . . . . . . . . . . Hydraulic Components in Braking Systems. . . . . . . . Master Cylinder Service. . . . . . . . . . . . . . . . . . . . . . . Brake Pedals Help the Driver. . . . . . . . . . . . . . . . . . . Brake-Line and Hose Usage. . . . . . . . . . . . . . . . . . . . Hydraulic Braking System Control. . . . . . . . . . . . . . . Brake Lines, Hardware, and Hoses. . . . . . . . . . . . . . . Brake-Warning-Light System . . . . . . . . . . . . . . . . . . . Stop-Light Operation. . . . . . . . . . . . . . . . . . . . . . . . . Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . Key Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . . ASE Technician A/Technician B Style Questions. . . . .

43 44 46 52 55 56 60 68 79 84 86 87 88 88

CHAPTER 4  Disc Brake Systems . . . . . . . . . . . . 91 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Disc Brake System Component Application. . . . . . . . 92

Disc Brake Caliper Operation . . . . . . . . . . . . . . . . . . 95 Disc Brake Pads and Friction Material . . . . . . . . . . . . 98 Wear Indicators on Brake Pads . . . . . . . . . . . . . . . . 102 Various Disc Brake Rotors. . . . . . . . . . . . . . . . . . . . 105 Disc Brake Service . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Removing and Inspecting Calipers . . . . . . . . . . . . . . 111 Inspecting and Measuring Disc Brake Rotors. . . . . . 119 Wheel Stud Evaluation and Installation . . . . . . . . . . 126 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . 134 ASE Technician A/Technician B Style Questions. . . . 135 CHAPTER 5  Drum Brake Systems. . . . . . . . . . 137 Types of Drum Brake Systems . . . . . . . . . . . . . . . . . 138 Drums and Related Components. . . . . . . . . . . . . . . 142 Wheel Cylinders. . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Brake Shoes and Lining Construction. . . . . . . . . . . . 146 Springs and Hardware Inside a Drum Brake System.148 Self-Adjusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Drum Parking Brake Systems. . . . . . . . . . . . . . . . . . 152 Drum Brakes Operation. . . . . . . . . . . . . . . . . . . . . . 152 Brake Drum Repair. . . . . . . . . . . . . . . . . . . . . . . . . . 154 Removal and Inspection of Brake Shoes and Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . 173 ASE Technician A/Technician B Style Questions. . . . 174 CHAPTER 6  Refinishing Brake Rotors and Drums���������������������������������������������������������������175 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 Rotors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176 Brake Rotor Deformation . . . . . . . . . . . . . . . . . . . . 180 Drums. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . 194 ASE Technician A/Technician B Style Questions. . . . 194

CONTENTS v

CHAPTER 7  Parking Brakes�����������������������������197 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 Parking Brakes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 Diagnosis and Service. . . . . . . . . . . . . . . . . . . . . . . . 201 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Key Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . 203 ASE Technician A/Technician B Style Questions. . . . 204 CHAPTER 8  Power-Assist Systems. . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vacuum Booster Operation. . . . . . . . . . . . . . . . . . . Hydraulic Power-Assist Operation. . . . . . . . . . . . . . Diagnosis and Service. . . . . . . . . . . . . . . . . . . . . . . . Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . Key Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . ASE Technician A/Technician B Style Questions. . . .

207 208 208 212 217 224 224 224 225

CHAPTER 9  Wheel Bearing Service. . . . . . . . . 227 How Wheel Bearings Affect Braking Components. . 228 Wheel Bearing Types. . . . . . . . . . . . . . . . . . . . . . . . . 229 Wheel Bearing Arrangements for Rear Drive Axles.233 Wheel Bearing Failures. . . . . . . . . . . . . . . . . . . . . . . 237 Serviceable Wheel Bearings. . . . . . . . . . . . . . . . . . . 238 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 Key Terms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . 253 ASE Technician A/Technician B Style Questions. . . . 254 CHAPTER 10  Hybrid Vehicle Braking Systems�������������������������������������������������������������255 The Science Behind Regenerative Braking. . . . . . . . 256 Hybrid Systems That Use an ICE . . . . . . . . . . . . . . . 256 Diagnosis Strategy on a Regenerative Braking System. . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 261

Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . 262 ASE Technician A/Technician B Style Questions. . . . 262 CHAPTER 11  Advanced Braking Systems: Electronic Brake Controls. . . . . . . . . . . . . . . . 265 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 Electronic Brake Control Systems . . . . . . . . . . . . . . 266 Antilock Brake System Components. . . . . . . . . . . . 268 Antilock Brake System Operation . . . . . . . . . . . . . . 269 ABS Hydraulic Components. . . . . . . . . . . . . . . . . . . 270 Wheel Speed Sensors Operate . . . . . . . . . . . . . . . . 273 Electrical Components in an ABS. . . . . . . . . . . . . . . 277 Diagnostic Process for Antilock Brake and Electronic Brake Control Systems. . . . . . . . . . . . . . . . . . . . . 279 Wheel Speed Sensors and Tone Wheel Diagnosis . . 286 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . 292 ASE Technician A/Technician B Style Questions. . . . 292 CHAPTER 12  Electronic Stability Control Systems. . . . . . . . . . . . . . . . . . . . . . . . 295 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 Electronic Stability Control Operation. . . . . . . . . . .296 Traction Control System Operation . . . . . . . . . . . . 298 Electronic Stability System with Other Driver Safety Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Ready for Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Key Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Review Questions. . . . . . . . . . . . . . . . . . . . . . . . . . . 302 ASE Technician A/Technician B Style Questions. . . . 302 Appendix A  2011 NATEF Automobile Accreditation Task List Correlation Guide��������������������������������������305 Glossary ����������������������������������������������������������������������307 Index ����������������������������������������������������������������������������315

NOTE TO STUDENTS This book was created to help you on your path to a career in the transportation industry. Employability basics covered early in the text will help you get and keep a job in the field. Essential technical skills are built in cover to cover and are the core building blocks of an advanced technician’s skill set. This book also introduces “strategy-based diagnostics,” a method used to solve technical problems correctly on the first attempt. The text covers every task the industry standard recommends for technicians, and will help you on your path to a successful career. As you navigate this textbook, ask yourself, “What does a technician need to know and be able to do at work?” This book is set up to answer that question. Each chapter starts by listing the technicians’ tasks that are covered within the chapter. These are your objectives. Each chapter ends by reviewing those things a technician needs to know. The content of each chapter is written to explain each objective. As you study, continue to ask yourself that question. Gauge your progress by imagining yourself as the technician. Do you have the knowledge, and can you perform the tasks required at the beginning of each chapter? Combining your knowledge with hands-on experience is essential to becoming a Master Technician. During your training, remember that the best thing you can do as a technician is learn to learn. This will serve you well because vehicles keep advancing, and good technicians never stop learning. Stay curious. Ask questions. Practice your skills, and always remember that one of the best resources you have for learning is right there in your classroom… your instructor.

Best wishes and enjoy! The CDX Automotive Team

ACKNOWLEDGMENTS Editorial Board Nicholas Goodnight, CMAT Ivy Tech Community College Fort Wayne, Indiana Tim Dunn Sydney, New South Wales, Australia

Contributors Carl Desens Northwest Technical Institute Chad Bryant Bevill State Community College Christiaan B. Desmond Ferris State University Daniel Kolasinski Mercedes-Benz of Milwaukee North Dave Crowley College of Western Idaho

David M. Sitchler Burlington County Institute of Technology Westampton Campus James P. Clarke Lincoln Tech Jerry Clemons Elizabethtown Community and Technical College Patrick LaDue Tennessee College of Applied Technology at Oak Ridge High School Peter Eastwood Alpena Community College Ronald Strzalkowski Baker College Scott Hadzik Weber State University Simeon Gammage Somerset Community College

CHAPTER 1

Strategy-Based Diagnostics NATEF Tasks ■■ ■■

■■

N01001 Review vehicle service history. N01002 Demonstrate use of the three C’s (concern, cause, and correction). N01003 Identify information needed and the service requested on a repair order.

■■

N01004 Research vehicle service information including fluid type, vehicle service history, service precautions, and technical service bulletins.

■■

■■

K01005 Describe step two of the strategy-based diagnosis. K01006 Describe step three of the strategy-based diagnosis. K01007 Describe step four of the strategy-based diagnosis. K01008 Describe step five of the strategy-based diagnosis. K01009 Explain how the three Cs are applied in repairing and servicing vehicles. K01010 Describe the information and its use within a repair order.

■■

S01002 Complete a repair order.

Knowledge Objectives After reading this chapter, you will be able to: ■■

■■

■■

■■

K01001 Describe the purpose and use of vehicle service history. K01002 Demonstrate an understanding of the active listening process. K01003 Demonstrate an understanding of the strategy-based diagnosis process. K01004 Describe step one of the strategy-based diagnosis.

■■ ■■ ■■ ■■

Skills Objectives After reading this chapter, you will be able to: ■■

S01001 Use service history in the repair and service of vehicles.

You Are the Automotive Technician A regular customer brings his 2014 Toyota Sienna into your shop, complaining of a “clicking” noise when he turns the steering wheel.You ask the customer further questions and learn that the clicking happens whenever he turns the wheel, especially when accelerating. He tells you he has just returned from vacation with his family and has probably put 300 miles (482 kms) on the car during their trip.

1. What additional questions should you ask the customer about his concern, the clicking noise he hears when turning? 2. How would you verify this customer’s concern? 3. What sources would you use to begin gathering information to address this customer’s concern? 4. Based on what you know this far about the customer’s concern, what systems might be possibly related to this customer’s concern?





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Chapter 1  Strategy-Based Diagnostics

▶▶ Introduction The overall vehicle service involves three major components. Those pieces are gathering information from the customer, the strategy-based diagnostic process, and documenting the repair. The flow of the overall service can be seen below. 1. Initial information gathering is often completed by a service advisor (consultant) and should contain details about the customer concern and pertinent history. 2. Verifying the customer concern begins the strategy-based diagnostic process. Technicians will complete this step to ensure that a problem exists and that their repair eliminated it. 3. Researching the possible cause will provide a list of possible faults. The technician will expand this list as testing continues. 4. Testing will focus on the list of possibilities. Technicians will start with broad, simple tests that look at an entire system or group of components. Testing will progressively become more narrowly focused as it pinpoints an exact cause. 5. Repairs will be made using suggested tools and recommended procedures. This is done to ensure a reliable repair and that manufacturer requirements are met. 6. Repairs must always be verified. This confirms that the technician has completed the diagnosis accurately and completely. The repair must be documented. The technician has been doing this all along. When the customer concern is recorded, the tests are recorded, and the final repair procedure recorded, the repair has been documented.

▶▶ Vehicle

Service History

Service history is a complete list of all the servicing and repairs that have been performed on a vehicle (FIGURE 1-1). The scheduled service history can be recorded in a service booklet or owner’s manual that is kept in the glove compartment. The service history can provide N01001 Review vehicle service valuable information to technicians conducting repairs. It also can provide potential new history. owners of used vehicles an indication of how well the vehicle was maintained. A vehicle with S01001 Use service history in the a regular service history is a good indication that all of the vehicle’s systems have been well repair and service of vehicles. maintained, and the vehicle will often be worth more during resale. Most manufacturers store all service history performed in their dealerships (based on the VIN) on a corporate server that is accessible from any of their dealerships. They also use this vehicle service history when it comes to evaluating warranty claims. A vehicle that does not have a complete service history may not be eligible for warranty claims. Independent shops generally keep records of the repairs they perform. However, if a vehicle is repaired at multiple shops, repair history is much more difficult to track and, again, may result in a denial of warranty claims. Vehicle service history can be very valuable to the technician. This history is typically retrieved from service records kept by the shop, dealer network, original equipment manufacturer (OEM), or aftermarket service center. This information often contains a list of services performed on a vehicle and the date and mileage at which they were completed. Not all service history contains the same information. Some histories may only contain repair information, while others include every customer concern and maintenance task performed. This information can be very helpful when diagnosing a concern. Service history may help technicians diagnose a vehicle and can also be used to prevent costly duplicated repairs. Service history can also be used to guide repairs. Records of vehicle service history may indicate that the customer has recently been in for service and now has returned with a new concern. This all-too-common situation is usually found to be caused by error during the previous service. When working on a vehicle that has returned after a recent repair, the previous technician’s FIGURE 1-1  Print outs of completed repair order as saved in the work (whoever that may be) should be inspected meticulously. online repair order system. K01001 Describe the purpose and use of vehicle service history.



Vehicle Service History

3

The service history may also show that the customer is returning for the same issue due to a component failure. The ­history might indicate when the component was installed, help the customer get their vehicle repaired, and help the shop to get paid under the component warranty. A vehicle that returns more than once for the same repair could be an indicator that an undiagnosed problem is causing these failures. The service history allows technicians to determine if the vehicle has been well maintained. This can be extremely useful when a technician suspects that lack of maintenance may be the cause of the problem. The vehicle’s service history helps technicians determine what maintenance needs to be performed, and therefore helps customers save money over time by preventing future costly repairs. Routine maintenance is essential on today’s modern automobile and prevents FIGURE 1-2  Recall notice example. premature failures due to contamination and component wear. Today’s vehicles also require regular software updates. There are many advanced com▶▶TECHNICIAN TIP puter systems on modern vehicles. From time to time, updates will be available to fix a bug Technicians and service advisors should or glitch in the computer programming. These updates are often designed to eliminate a check the vehicle service history against customer concern, improve owner satisfaction, or increase vehicle life. This is very similar the manufacturer’s service maintenance to an update for your PC or mobile device. Service history will indicate to the technician schedule to determine if the vehicle is that the vehicle may need an update. The technician will inspect the vehicle’s computer due for scheduled maintenance. The system and perform any needed updates as necessary. maintenance schedule is a guide that indiService history can also be used to keep customers safe. Occasionally, manufacturers cates what service is due when; it can be may need to recall a vehicle for service due to a safety concern that has been identified found in the manufacturer’s service inforfor a vehicle (FIGURE 1-2). This means that the manufacturer has found that the potential mation and often in the owner’s manual. exists for a dangerous situation to occur, and the vehicle must be serviced to eliminate it. Keeping the vehicle well maintained can Depending on the nature of the problem, recalls can be mandatory and required by law, or avoid a failure that strands the customer on the roadside. manufacturers may voluntarily choose to conduct a recall to ensure the safe operation of the vehicle or minimize damage to their business or product image. The service history would be used to verify that the vehicle is subject to the recall and has or has not had the recall service completed. The technician would perform the service, update the service history, and return the vehicle to the customer.

Applied Science AS-11 Information Processing: The technician can use computer databases to input and retrieve customer information for billing, warranty work, and other record-keeping purposes. Dealership service departments have access to databases run by manufacturers for the purposes of accessing warranty information, tracking vehicle servicing and warranty repair history, and logging warranty repair jobs for payment by the manufacturer. When a customer presents their vehicle for a warranty repair, the customer service department staff begin by consulting the database to confirm that the vehicle is within its warranty period and that the warranty has not been invalidated for any reason. Once it is confirmed that the vehicle is still under a valid warranty, the repair order will be passed to the workshop for diagnosis and repair. Any parts required for the warranty repair must be labeled by the technician and stored for possible recall by the manufacturer.

For example, a young man comes in complaining that his vehicle is “running rough.” The customer service staff confirms that the vehicle is nine months old and only has 14,500 miles (approx. 23,000 km), so it  is within the manufacturer’s 3-year/100,000 mile (160,000 km) warranty period. They check the manufacturer’s database to confirm that the vehicle’s warranty has not been invalidated before handing the repair order onto the workshop. Then a technician diagnoses the fault as a defective ignition coil and fills out a warranty parts form. Once the repair has been completed and the parts labeled, the warranty parts form and any repair order paperwork is passed back to administrative staff for processing. Processing will include billing the manufacturer for the correct, pre-approved amount of time, logging the repair on the database for payment, and ensuring that all documentation is correct for auditing purposes.

Warranty Parts Form Customer concern: Vehicle running rough

Vehicle Information

Cause: #6 ignition coil open circuit on primary winding

VIN: IG112345678910111

Correction: Replaced #6 ignition coil

RO Number: 123456

Parts description: #6 ignition coil

Date of repair: 10/04/2016

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Chapter 1  Strategy-Based Diagnostics

SKILL DRILL 1-1 Reviewing Service History 1. Locate the service history for the vehicle. This may be in shop records or in the service history booklet within the vehicle glove compartment. Some shops may keep the vehicle’s service history on a computer. 2. Familiarize yourself with the service history of the vehicle. a. On what date was the vehicle first serviced? b. On what date was the vehicle last serviced? c. What was the most major service performed? d. Was the vehicle ever serviced for the same problem more than once?

3. Compare the vehicle service history to the manufacturer’s scheduled maintenance requirements, and list any discrepancies. a. Have all the services been performed? b. Have all the items been checked? c. Are there any outstanding items?

To review the vehicle service history, follow the steps in SKILL DRILL 1-1.

Active Listening Skills Depending on the size of a shop, the first point of contact for the customer is the service advisor or consultant. This person answers the phone, books customer work into the shop, fills out repair orders, prices repairs, invoices, keeps track of work being performed, and builds customer relations with the goal of providing a high level of ▶▶TECHNICIAN TIP customer support. The service advisor also serves as a liaison between the customer and the technician who is working on the vehicle. A service advisor or consultant may A vehicle’s service history is valuable for advance to become a service manager. In smaller shops, a technician may perform several reasons: these duties. ■■ It can provide helpful informaWhen the customer brings his or her vehicle in for service, the service advisor tion to the technician when peror technician should ask for more information than just the customer concern. It is forming repairs. important to let the customer speak while you use active listening skills to gather as ■■ It allows potential new owners many pertinent details as possible. Active listening means paying close attention to not of the vehicle to know how only the customer’s words, but also to their tone of voice and body language. Maintain well the vehicle and its systems were maintained. eye contact with the customer throughout your conversation and nod to show you ■■ Manufacturers use the history understand and are paying attention. Do not interrupt. Wait for the customer to finish to evaluate warranty claims. speaking before responding, then ask open-ended questions to verify that you have heard the complaint clearly and understand the problem. An openended question is one that cannot be answered with a yes or no, but Pay attention to instead requires the customer to provide you with more informanonverbal messages tion about the problem (FIGURE 1-3). If the shop is noisy, try to find (e.g. tone of voice, a quieter location in which to speak with the customer. Excellent body language) Give individual attention communication helps ensure that all relevant information is colMaintain eye contact lected. It also makes a good first impression with customers; they are likely to feel that they were listened to and cared for. Avoid interrupting Politely use open-ended questions to ask about any symptoms the customer may have noticed, such as: K01002 Demonstrate an understanding of the active listening process.

Ask questions to verify understanding

■■ ■■

■■ ■■

■■

FIGURE 1-3  The active listening process.

■■

Under what circumstances does the concern occur or not occur? What unusual noises do you hear (e.g., squeaks, rattles, clunks, and other noises)? What odd smells or fluid leaks have you noticed? What recent work, service, or accessories have been added to the vehicle? What other recent changes or experiences have you had with the vehicle? What other systems seem to be operating improperly?



Strategy-Based Diagnostic Process

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Although problems may seem unrelated initially, when multiple systems fail at the same time, the issues are frequently related. Open-ended questions can provide valuable information to the technician who is performing the diagnosis.

▶▶ Strategy-Based

Diagnostic Process

Diagnostic problems can be very challenging to identify and correct in a timely and efficient manner. Technicians will find that having a plan in place ahead of time will vastly simplify the process of logically and systematically (strategically) solving problems. The plan should be simple to remember and consistent in its approach; yet it must work for the entire range of diagnostic problems that technicians will encounter. In this way, technicians will have one single plan to approach any diagnostic situation they may encounter, and will be confident in their ability to resolve it. This problem solving plan is called the Strategy-Based Diagnostic Process. The strategy-based diagnostic process is focused on fixing problems correctly the first time. It is a scientific process of elimination, which is much the same process as a medical doctor uses for their diagnosis. It begins with identifying the customer’s concern and ends with confirming that the problem has been resolved. The purpose of the problem-solving process is twofold: to provide a consistent road map for technicians as they address customer concerns that require diagnosis, and to ensure that customer concerns are resolved with certainty. This process simplifies the problem-solving portion of the repair, making the job easier for the technician; it prevents technicians from having to work on the same job more than once; and it all but eliminates customer comebacks. While repeat customers are good for business, a customer coming back with the same problem is not. The customer is likely to be upset and the technician is likely to be working for free. In order to avoid this scenario, it is imperative to address customer concerns correctly the first time. Proper diagnosis is important to consumers and to the federal government. F ­ ederal and state law protects consumers against the purchase of vehicles with significant persistent defects. Technicians are held to a standard of reasonable repair times and limited visits for the same concern. Although the law varies from state to state, this means technicians must not return a vehicle to a customer without addressing the customer’s original concern. Also, technicians cannot make the vehicle unavailable to the customer for a long period while the vehicle is being repaired. The purpose of the state and federal laws is to protect consumers buying new vehicles. Failure to comply with the state and federal law can be very expensive for the dealership and manufacturer. Although most state laws hold the manufacturer directly responsible, dealerships are also hurt by a loss in sales revenue, a loss in repair revenue, and irreparable damage to their customer and sometimes manufacturer relationships. Many state laws hold the manufacturer responsible for full purchase price, incurred loan fees, installed accessories, and registration and similar government charges. This can be a heavy cost on top of the value of the vehicle itself.

K01003 Describe each step in strategy-based diagnosis.

▶▶TECHNICIAN TIP Technicians need to do their best to find the issue and resolve it; otherwise, the vehicle may be required to be bought back from the customer, costing the dealership and manufacturer significant money.

Need for the Strategy-Based Diagnostic Process Finding the source of every customer concern can prove to be a challenge. Novice technicians frequently struggle with diagnostics situations. Even some veteran technicians have difficulty tackling diagnosis on some new technologies. However, if the strategy for solving a problem is generally the same every time, this greatly simplifies the process. Hopefully, by applying a strategy-based diagnostic process, technicians will resolve challenging customer concerns 100% of the time in an efficient manner. Customer comebacks occur when the customer picks up the vehicle after service, only to bring it back shortly thereafter with the same concern. This situation is understandably upsetting to the customer. Typically, the end result is wasted labor time and a loss in shop productivity. The customer is left with one of the following impressions: ■■ ■■ ■■

The work was not performed; The shop is incompetent; Or, worse yet, the shop was trying to scam the customer.

▶▶TECHNICIAN TIP The diagnostic process makes the technician’s job easier by providing a step-bystep strategy to solving the problem. It also answers the question: “Now what do I do?” As even the toughest job becomes easier, technicians will find their rate of diagnostic success increasing.

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Customer comebacks are usually caused by one of two avoidable reasons: 1. The customer concern is misinterpreted or misunderstood. This results in the technician “fixing” a problem that does not exist or missing a problem altogether. 2. The technician failed to verify that the original concern was resolved. Technicians are often hurried; some will forget to ensure that the repair they had performed actually fixed the original customer concern. Use of the strategy-based diagnostic process enables the technician and shop to make more money and satisfy more customers. This is a win-win situation for all involved. Using the strategy-based diagnostic process requires starting at the beginning and following it through to the end every time (FIGURE 1-4). This systematic approach will ensure the best results for each diagnostic situation.

Step 1:Verify the Customer’s Concern K01004 Describe step one of the strategy-based diagnosis.

The first step in the diagnostic process is to verify the customer’s concern. This step is ­completed for two main purposes: ■■ ■■

SAFETY TIP There should be limits to recreating the customer concern. Technicians need to be careful when riding as a passenger with the customer or driving an unfamiliar customer vehicle. Technicians have died during test drives due to customers’ driving or their own driving of unfamiliar vehicles. The purpose of the test drive is to verify the concern or its repair. It is not an opportunity for a thrill ride. Customers and their vehicles should be treated with respect. Additionally, customers should respect the technician. If a customer asks a technician to verify a concern in an unsafe situation, such as a high rate of speed, the technician should decline. This is for both safety and liability reasons.

To verify that the vehicle is not operating as designed To guarantee that the customer’s concern is addressed

Failure to complete this step may result in wasted time, wasted money, and, worst of all, an unhappy customer. The customer is probably not an experienced automotive technician. For this reason, the customer does not always accurately verbalize the problem that may be occurring. Therefore, it is very important that you have a complete understanding of the customer’s concern before beginning the diagnosis. This will enable you to know with certainty that you have actually resolved the original concern after repairing the vehicle and before returning it to the customer. During this step, you may perform several of the following tasks, depending on the customer concern. First, ask the customer to demonstrate the concern, if possible. This may necessitate a test drive (FIGURE 1-5). The customer should be encouraged to drive the vehicle while you ride along as a passenger and gather symptoms and details about the concern. Seeing the customer recreate the concern in real time will often provide some much needed context to the problem. Having the customer demonstrate the concern is ideal in most situations, though not always possible. In the event that the customer is not present, you must do your best to recreate the concern on your own based on the information obtained from the customer. With or without the customer present, be sure to document in writing any details about the scenario in which the concern arises. Next, make sure that the customer concern doesn’t fall outside the range of normal operation of the component or system. The manufacturer’s service information provides

Step 1 Verify the customer's concern Step 2 Research possible faults and gather information Step 3 Focused testing

Step 4 Perform the repair

Step 5 Verify the repair FIGURE 1-4  The strategy-based diagnostic process.

FIGURE 1-5  Ask the customer to describe the concern.



Strategy-Based Diagnostic Process

system descriptions and expected operations; technicians can use these details, provided in the owner’s manual or in the vehicle service manual, to become familiar with the system and then explain its operation to the customer. Especially on new cars with many amenities, customers may not be familiar with the controls and subsequent operation. This can cause a customer to bring a vehicle in for service unnecessarily, due to unfamiliarity with the system controls. Many shops use online service (shop) manuals where you can quickly access any information related to the customer’s concern (FIGURE 1-6). Checking to make sure that the concern is really a fault, and not a normal operation, will avoid unnecessary diagnosis time. This is also an opportunity to provide excellent customer service by demonstrating the features and their controls to the customer. Conducting a quick visual inspection to look for obvious faults can be very helpful (FIGURE 1-7). However, it does not replace the need for testing and is absolutely not intended as a shortcut to the diagnostic process. With that said, the visual inspection can provide valuable information that may speed up the testing processes. The visual inspection provides an opportunity for a quick safety check by the technician and may help to avoid some potentially dangerous situations during service. While visual inspections can be very valuable, technicians must be careful not to jump to conclusions based on what they see. For example, a customer comes in with an illuminated and flashing overdrive light on the control panel. The technician has seen this p ­ roblem before and it was caused by a bad solenoid pack in the transmission. If the technician decides that this problem is also caused by a bad solenoid pack, this determination is one that was reached solely on conjecture; no actual test was performed. Although the flashing light might indicate a fault with the solenoid pack, steps in the diagnostic process should never be skipped. This guess can lead to a very costly mistake when it is discovered that the new solenoid pack does not, in fact, fix the problem. In reality, the wiring harness to the transmission is frayed and shorting out. Had the technician performed a test, the cause of the customer concern could have been confirmed or denied before a solenoid pack was put in unnecessarily. While the visual inspection is very valuable, tests must always be performed to compare suspected faults against the expectations and specifications defined in the service information. When recreating the customer concern, the technician should operate the system in question in all practically available modes. System operation should be checked to see if there are other symptoms that may have gone unnoticed by the customer. These other symptoms can be very valuable when determining which tests to perform; they could save the technician significant time during the diagnosis. When recreating the customer concern, it is important to check the entire system for symptoms and related faults. Recreating intermittent faults can be a challenge. Intermittent symptoms often stem from a component or system that is failing or one where the nature of the fault is not yet clear. In these situations, the aforementioned check of system operation can prove to be highly valuable, as it may uncover previously unnoticed but consistent symptoms. Attempting to repair

FIGURE 1-6  A technician researching service information.

FIGURE 1-7  Performing a visual inspection.

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▶▶TECHNICIAN TIP All too often, the customer does not have symptoms to share and their only concern is that the malfunction indicator is illuminated. In this situation, the data stored in the computer is invaluable. Record it and do not clear it out unless directed to do so in the manufacturer’s service procedure. Even then, you should capture the information before clearing the memory.

an intermittent fault without consistent symptoms, data, or diagnostic trouble codes (DTCs) is a gamble, because a technician cannot be certain that the actual problem is isolated. This means that there would be no way to confirm with certainty that a repair was effective. The fault could appear again as soon as the vehicle is returned to the customer. To avoid such a situation, look for symptoms, data, or DTCs that are repeatable or consistent. Intermittent diagnosis may require the use of an oscilloscope (a specialized tool for looking at electrical waveforms), or a “wiggle” test (as the name implies, a test instrument is monitored as the electrical or vacuum harness is manipulated by hand). This can verify the customer concern and remove some of the challenge from the diagnosis of an intermittent fault. Lastly, but notably, save DTCs and freeze frame data. Freeze frame data refers to snapshots that are automatically stored in a vehicle’s power train control module (PCM) when a fault occurs; this is only available on vehicles model year 1996 and newer. Intermittent faults may be found by reviewing data stored just before, during, and after the fault occurred, similar to an instant replay. When working with computer controlled systems, it is very important to save the recorded data. It may become necessary to erase this information from the computer, though that should generally be avoided. This information is absolutely critical when the technician is trying to answer the questions, “When did this happen?” and “What was going on at the time?” What will step one look like? When information is gathered and recorded for step one, it should contain the customer concern, any symptoms, and any retrieved DTCs. View the following example from a vehicle that has no reverse. The technician verified the customer concern and recorded: 1. Vehicle will not move when shifted into reverse 2. Vehicle operates normally in all forward gears in OD, D, L2, and L1 3. Current code P0868 Notice that the technician in this example verified and recorded the customer concern. The technician also tried other functions in the system. Specifically, the technician drove the vehicle and tested the other gears in each of the gear ranges and then recorded the results. The DTC data was also retrieved from the control module and recorded. Although it was short and concise, the information will be very useful in the next step.

Step 2: Researching Possible Faults and Gathering Information K01005 Describe step two of the strategy-based diagnosis.

N01004 Research vehicle service information including fluid type, vehicle service history, service precautions, and technical service bulletins.

The second step in the diagnostic process is to research possible faults that may be related to the customer’s concern. The goal of this step is to create a list of possible faults. The list is created based on the information gathered in step one. The list will later be narrowed down by the tests performed in step three until the cause of the concern has been confirmed. Before testing can begin, a technician must know what possible faults need to be tested. Researching possible faults should begin broadly. Especially when diagnosing electrical and electronic systems, this step should begin at the system level and work down to individual components. For example, if a vehicle engine cranks, but will not start, a technician would list these familiar possible faults: Air, Fuel, Ignition, Compression, and Security. These possible faults are not single components, but rather they are systems. This is where a diagnosis should begin. Starting a diagnosis by listing the dozens of components for each system will make the job unreasonably time intensive. However, once a test determines that there is a fault within a specific system, the list should be expanded to encompass that particular system’s subsystems and components. This systematic elimination starts broadly and narrows, allowing technicians to work more efficiently. In the second step of the diagnostic process, the technician creates a list to help focus their tests. The list may aid in a simple process of elimination by testing one possibility after the next. The list can also start broadly and narrow as testing continues. When starting a list, it may look similar to the following: 1. Air 2. Fuel



Strategy-Based Diagnostic Process

3. Ignition 4. Compression 5. Security This list is broad and starts at the system level. As you’ll soon see in the next step, the technician would eliminate possible faults with a test that is focused on analyzing the whole system. When a system is located with a fault, in the ignition system for example, the list would become more specific: 1. Spark Plug 2. Coil on Plug 3. CKP 4. CMP 5. Sensor Triggers 6. Harness 7. Control Module The technician would again focus his or her testing on the list, seeking to eliminate possible faults until one is confirmed, repaired, and verified. Several great sources of information are available for researching possible faults, although the best source of information is usually the manufacturer’s service information system. These systems are typically found online; however, some manufactures still publish paper service manuals. The manufacturer’s service information contains definitions for diagnostic trouble codes, system description and operation, electrical wiring diagrams, diagnostic steps, repair procedures, and much more. Fault diagnosis should almost always begin with the factory service information. Other resources for identifying faults can be used in conjunction with the factory service information. As previously discussed, the vehicle service history can provide valuable insight into the past maintenance or lack thereof. It can also provide information about recent or repeated repairs. Technical service bulletins (TSBs) are service notifications and procedures sent out by the manufacturers to dealer groups alerting technicians about common issues with a particular vehicle or group of vehicles (FIGURE 1-8). Some aftermarket sources also exist for the pattern failures addressed by TSBs (FIGURE 1-9). Additionally, both original equipment manufacturers (OEM) and aftermarket technician support services offer hotlines, or call-in support, that specifically provide technical support to professional technicians. Some of these hotlines offer subscriptions to searchable web-based components. These resources do not guarantee a repair; that is still the responsibility of the technician. However, all of the sources mentioned here can be a huge help as technicians research possible faults.

FIGURE 1-8  Technical service bulletin.

FIGURE 1-9  Aftermarket source.

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While these resources are essential, the list of possible faults is just that: a list of possible faults. A technician must always be aware that steps in the diagnostic process cannot be skipped.

Step 3: Focused Testing K01006 Describe step three of the strategy-based diagnosis.

▶▶TECHNICIAN TIP A repair should never be performed unless the possible fault has been verified through testing. Do not let a possible fault become a possible mistake. In some cases, the list of possible faults can be found in the service information, but many times the technician will need to produce the list based on the concern, the information gathered, and the results of the research.

▶▶TECHNICIAN TIP The test description must provide enough information that someone could repeat the test with the same result. This is very important!

Step three of the diagnostic process involves focused testing. In this step technicians use their testing skills to eliminate possible faults from the list they created in step two. Steps two and three work together; testing will start at a system level and work down to subsystems, then finally to individual components. The idea of focused testing should be to eliminate as many potential faults as possible with each test. Focused testing is intended to eliminate possible causes with certainty. Each time a test is performed, the following three pieces of information must be recorded: ■■ ■■ ■■

a test description an expectation a result

These can be recorded on the repair order, electronic service record, or on an extra sheet of paper. Test records must be kept handy because they will become part of the documented record for this repair. The three pieces of test information are recorded carefully for several reasons. ­Having an expectation before a test is performed makes each test objective and effective. The expectation is what the result is compared against, in order to determine if the vehicle passed or failed. Many manufacturers, both original equipment and aftermarket, require that documented test results be submitted with each warranty claim. If the technician fails to document his or her work, the manufacturer will not pay the claim. The result is that the shop is out money for the parts and service, and the technician will not be paid for their work. Be sure to document the work properly (FIGURE 1-10).

Well the line pressure is within specifications I'll just record that on the repair order as follows

Test Description - Line Pressure Expectation - 65 to 108 psi @ idle - 285 to 321 psi @ stall Result - 80 psi @ idle & 300 psi @ stall

FIGURE 1-10  The test record should include the test description, expectation or specification, and the result of measurement.



Strategy-Based Diagnostic Process

1. The test description is not long, or even a complete sentence; it is simply a brief description. It allows the reader to know what test was performed and on what component or system. The test description should be accurate enough that the reader could repeat the test with the same result. 2. The expectation should describe the expected result as if the system is operating ­normally. The expectation could come directly from the system specifications listed in the manufacturer’s service information or from system description and operation. 3. The result is the third part that must be recorded for each test. This information should accurately reflect what happened when the test was performed. In summary, the testing is focused on isolating a fault or faults from the list of potential faults, and the results are compared to the expectation. Testing should begin broadly and simply. Consider the following example: A light bulb circuit is suspected of having a fault. If the light bulb is easily accessible, the first test might be to check the voltage drop (i.e., voltage used to push current through the bulb). If the result of the voltage drop measurement is as expected (i.e., within specification), then the problem is in the bulb or socket. In this test, the technician is able to check the integrity of the entire electrical circuit with one test. If the result of the measurement is outside of the expectation (i.e., out of specification), the technician would know that the bulb is not the source of the problem. Further testing would isolate the problem to the ground or power side of the circuit. The technician in the example performed a simple test with an easy expectation. The test allowed the technician to quickly determine the state of operation for the entire system/circuit and move on. If a fault had been found, then the technician would have isolated the cause of the customer’s concern to that particular system/circuit and would need to perform further testing to isolate the cause to a particular component. To do that, the technician would use the service information to determine what components comprise the system and adjust the list from step two to take into account the new information. Then testing would continue. The next test might measure voltage supply at the bulb (i.e., available voltage). In this way, the technician would be testing the power supply, the conductors, and the switch (assuming a power-side switched circuit). The technician would have an expectation for the circuit voltage and compare his or her result to this expected voltage. As we saw earlier, the technician is testing more than one component with a single test, thereby operating in an efficient manner. This strategy—starting with broad, simple tests and moving to more complicated, pinpoint tests—makes efficient use of the technician’s time while still effectively testing the possible faults. 1. A technician is investigating a customer concern of “no heat from the dash.” The technician’s investigation might begin with a simple list. a. Engine cooling system b. HVAC duct and controls 2. The technician would then eliminate one or the other and expand the list. The technician might verify coolant level and temperature at the inlet and outlet of the heater core. The HVAC components controlling and delivering warm air could then be used to expand the list for the next round of testing. a. Doors and ducts b. Cables c. Servos d. HVAC control head e. Blower motor f. Harness g. In-cabin filter or debris 3. Notice that the technician has moved from broad system tests to individual components or component groups. The technician’s test continues to become more specific as the possibilities are narrowed down.

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Technicians commonly encounter vehicles with more than one customer concern. When these concerns both originate from the same or companion systems, technicians are inclined to search for one cause to both problems. Unfortunately, trying to diagnose two faults at once can quickly become problematic and confusing. Instead, select the easier customer concern and follow it through to the end. If both problems were caused by the same faults, then both were fixed. If they were caused by two separate faults, the technician is no worse off for having fixed one concern. When selecting tests to perform, remember that they should be simple and easy (FIGURE 1-11). Except when following service procedures, you should select tests that have simple expectations, are easy to perform, and provide you with the maximum amount of information. This means simple tests that inspect an entire system or circuit are ideal ways to begin testFIGURE 1-11  Select tests that have simple expectations and are easy ing. Simple tests have expectations and results that are quickly to perform. understood and interpreted. They are short and involve basic tools and access to areas that are comfortable to reach. ▶▶TECHNICIAN TIP When selecting tests, prioritize your testing. First choose tests that can be performed quickly and simply, even if they do not test an entire circuit. If a preferred test is in a diffiWhen selecting tests, it is not a bad idea cult place to access, move to another test and come back to it, if needed. The answer may be to choose those tests that might look at found in the meantime and the time-consuming test can be avoided. Simple and easy tests components of both systems (e.g., voltare ideal, but they must be measurable or objective. age drop on a shared electrical ground), but DO NOT attempt to test for both Yes, a visual inspection is a simple and valuable test, but a technician must determine faults at once. While multiple faults within what the issue is in an objective manner, with help from the service information. A guess a companion or the same system often based only on appearance is insufficient. If the service information says, “cracks in the turn out to be related, they should be serpentine belt indicate that it needs replaced,” the belt can be visually and objectively isolated and tested separately. (yes or no) tested. The belt will either have the indicated wear or it will not. If the service information states, “Chain deflection cannot exceed 0.75,” then the deflection can be measured and compared to the specification. As testing continues, it may become necessary to use advanced tests, sophisticated equipment, additional time, or tests in areas difficult to access. Keeping initial testing simple and easy will produce the quickest, most reliable, and effective results. When testing, use the recommended procedures and equipment. Manufacturers fre▶▶TECHNICIAN TIP quently recommend a particular procedure when testing one of their systems. When performing tests for an inspection Failure to follow the specified service procedure can result in the warranty claim under warranty, it is absolutely necessary being denied by the manufacturer. In that case, both the shop and the technician lose to follow the manufacturers’ guidelines. money. Manufacturers may recommend a certain procedure because of the way their system is designed or monitored. Technicians must also be very careful to perform tests safely (FIGURE 1-12). Beyond the mechanical dangers posed by automobiles, many of today’s vehicles have dangerously high fluid pressures and deadly high voltage. It is of the utmost importance for the safety of the technician, and those working in the area, that safety procedures are always followed. Proper test equipment and procedures are intended to test a particular component or system without causing any damage. Improper equipment or test procedures can create a second fault in the system being tested; making the technician’s job even more difficult. For example, front probing an electrical terminal with the lead of a DMM can cause the terminal to spread or deform. This can create an intermittent high resistance or open within the circuit that was not there prior to the technician’s test. Using the recommended equipment and procedures will help to ensure warranty claims are approved, people are safe, and testing goes smoothly. When performing repairs, look beyond the obvious for the root cause. This simple suggestion can avoid customer comebacks. Novice technicians frequently have problems with misdiagnosing fuse-related issues. For example, a technician diagnoses a blown fuse as the cause of the customer concern. While replacing the fuse may have fixed the immediate fault, the technician did not look beyond the obvious. What causes a fuse to blow? Low



Strategy-Based Diagnostic Process

Ok Safety Check - When was this hoist last certified? - Am I using the hoist correctly? - Are the arm locks functioning? - Is the center of gravity right for this vehicle? - Are the lift pads positioned properly? - Is my PPE appropriate? - I think I am ready to start work now.

FIGURE 1-12  Always perform all tests safely.

resistance and increased amperage cause a fuse to blow. However, the technician did not test for one of these faults and the vehicle is likely to return with the same customer concern and the same blown fuse. In another example of incomplete reasoning, a technician diagnoses a leaky transmission cooler line. The line is chaffed and leaking. This cooler line runs along the frame rail; the inner and outer tie rods are immediately below. The technician diagnoses the vehicle while it is on a lift and the suspension is unloaded (increasing the distance between the hose and steering linkage). The technician should have looked for the root cause of the chaffing, but instead the vehicle and customer come back some time later for the same concern. The technician notices several broken clips that held the flexible line into place on the frame rail. In both cases, the technician will work for free to repair the same vehicle, because time was not taken to ask the question: “Did something else cause this failure?” Testing must be focused beyond the obvious to identify the root cause of the problem and consequently avoid customer comebacks. In summary, focused testing has several key elements. It picks up the possible faults identified in step two and begins testing each one broadly, narrowing down to more specific tests. Focused testing requires accurately documenting the tests performed, including a test description, expectation, and result, each and every time a test is performed. It should also be performed in a safe and proper manner, following manufacturers’ guidelines and safety protocols. Focused testing is a safe, accurate, and repeatable method for isolating possible faults.

Step 4: Performing the Repair The fourth step of the diagnostic process is to perform the repair. Although performing the repair is often the most straightforward step in the process, technicians must still avoid making several common mistakes. The following tips will help you to perform an effective and reliable repair.

K01007 Describe step four of the strategy-based diagnosis.

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Use Proper Service Procedures Manufacturers will often indicate what procedures are appropriate for their vehicles and components. Many design features and component materials require certain procedures be used and others avoided. Following the manufacturer’s service information can prevent premature failure of the repair (FIGURE 1-13). For example, repair methods that are safe around the home may be unacceptable in the automotive industry. The use of twist-on wire connectors can create an unreliable and potentially dangerous electrical situation when used in a vehicle. Additionally, warranties, both original equipment and aftermarket, rely on the technicians’ adherence to the manufacturer’s service information. If technicians fail to do so, the warranty claim can go unpaid and the shop will lose money. Therefore, it is important for reliable repairs and warranty reimbursements that technicians follow the service information when performing repairs.

Use the Correct Tool for the Job Failure to use the correct tool can lead to a customer comeback and injury to the technician. Proper tool selection is essential. If you are ever in doubt, refer to the manufacturer’s service information. Improper tool use or selection can damage the component being installed or other components around it. For example, a technician may choose to install a pump busing with a hammer instead of using the recommended press and bushing driver. This incorrect tool selection can easily lead to misalignment, or damage to the bushing, pump, or torque converter. Using the wrong tool (or the right tool in the wrong manner) can also damage the tool and potentially injure those in the area. For example, if a technician is using a hardened chrome socket on an impact wrench, the socket may shatter, sending shrapnel flying. Using the correct tool for the job will produce better work and ensure the safety of the technician.

MAINTENANCE/SPECIFICATIONS CHANGING YOUR WIPERS The wiper arms can be manually moved when the ignition is disabled. This allows for ease of blade replacement and cleaning under the blades.

1. Disable the ignition before removing the blade. 2. Pull the arm away from the glass. 3. Left leading edge retaining block to release the blade. Swing the blade, away from with the arm, to remove it. 4. Swing the new blade toward the arm and snap it into place. Replace the

retaining block at the leading edge of the wiper arm. Lower the wiper arm back to the windshield. The wiper arms will automatically return to

their normal position the next time the ignition is enabled. Refresh wiper blades at least twice a year for premium performance. Poor preforming wipers quality can be improved by cleaning the blades and the windshield. See Windows and wiper blades in the Cleaning chapter. To extend the life of wiper blades, scrape off the ice on the windshield BEFORE turning on the wipers. The ice has many sharp edges and will damage and shred the cleaning edge of your wiper blade.

FIGURE 1-13  A typical shop manual page has a task description broken into steps and diagrams or pictures to aid the technician.



Strategy-Based Diagnostic Process

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Take Time to Perform the Repair Properly Because technicians are frequently paid by the job, or a flat rate, rather than paid hourly, it is possible for technicians to feel a rush to complete their current job. Rushing increases the likelihood of a mistake. If a mistake occurs, the customer will come back with the vehicle and the technician will work for free to repair the mistake. For example, if a technician replaces a water pump and fills the coolant without bleeding the system, a potentially damaging situation can occur. The trapped gas can affect the flow of coolant and create a hot spot in the cylinder head. This can lead to warning lights, poor performance, and possible engine damage. Take a little extra time to ensure that the work is performed correctly, with the right tools and the proper service procedures. Taking time to perform the repair will ensure fewer “comebacks” and more satisfied customers.

Make Sure the Customer Approves of the Repair This may seem trivial, but it is extremely important. Most states’ laws protect consumers by preventing unauthorized services from being charged or performed. This means that technicians cannot just repair a vehicle and charge the customer for the cost incurred. If the customer is paying, shops must receive a customer’s approval prior to performing repairs.

▶▶TECHNICIAN TIP It is very important to quote accurately and wait for approval before performing repairs on a customer’s vehicle.

Check for Updates Prior to the Repair It is also good practice to check for updated parts and software/firmware before performing a repair. It is possible that manufacturers have become aware of a problem with a particular component or software version and issued a software update or produced an updated component. When performing repairs it is a good idea to check for these sorts of updates (often found in TSBs), because it may prevent a customer comeback. Software updates are often downloaded from the manufacturer’s website. For hard parts, the best resource is frequently the respective dealership’s parts department. Technical service bulletins also provide information related to unexpected problems, updated parts, or changes to a repair procedure on a particular vehicle system, part, or component. The typical TSB contains step-by-step procedures and diagrams on how to identify if there is a fault and perform an effective repair. Shops typically keep TSBs in a central location, or you may look them up online. Compare the information contained in the TSB with that of the shop manual. Note the differences and, if necessary, copy the TSB to take with you to perform the repair.

Pay Attention to Details Performing the repair is straightforward but requires attention to detail. There are several things to keep in mind. Proper service procedures can be located in the manufacturer’s service information. The correct tool for the job will lessen injury and ensure reliability. Use the necessary time to make sure that the repair was completed correctly. Document your work. These tips can greatly improve the likelihood of a successful repair, but the process does not stop with the repair.

Step 5:Verify the Repair The most important step of the strategy-based diagnostic process is verifying the repair. The reason that this is the most important step is straightforward. The vehicle would never have been in the shop if the customer did not have a concern. If the technician fails to address the original concern, the customer may view the trip as ineffective, a waste of their time and money. Even when a valid repair that makes the vehicle safer and more reliable was performed, the customer will still be unsatisfied if his or her original concern was not addressed. For example, a customer brings the vehicle into the shop for a sticky glove box latch. The technician identifies and repairs a dangerous brake line leak, but fails to fix the glove box. Some customers may view this trip to the shop as unsuccessful because it failed to fix their original issue. When verifying the repair, technicians must always double check their work. This is a valuable confirmation that the repair performed did fix the identified problem.

K01008 Describe step five of the strategy-based diagnosis.

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Chapter 1  Strategy-Based Diagnostics

▶▶TECHNICIAN TIP The job is not complete until you have verified that the repair resolved the customer’s concern.

There are several ways to verify a repair, but generally, the simplest method is the best method. For example, a customer is concerned that the wipers stop moving when the switch is moved into the high position. In step one, the technician will verify that the customer concern and fault exist by turning the wiper switch to all positions. Then the technician uses the wiring diagram (step 2) to diagnose a fault (step 3) within the wiring harness and repairs it (step 4). The technician could then verify the repair by performing the last diagnostic test (from step 3) again. In most cases, the repair would be confirmed if the results had changed and were within expectation/specification. But what if there was a second problem affecting the wipers such as worn brushes in the motor, or the wiper linkage fell off of the pivot on one side? The customer would still have issues with the wipers and would likely to be unhappy with the repair. So while performing the last diagnostic test (step 3) is a valid verification method, it is not foolproof. An easier method exists: simply return to the process used in step one to verify the customer concern. If the repair has eliminated the problem, the technician should now be able to turn the wiper switch to all positions (step 1) and confirm normal operation. Be certain to perform the same inspections used to verify the customer concern in step one after the repair is performed. This may include checking the entire system operation, not just a single function. This method of verifying that the customer concern is resolved is usually best in most scenarios because it is simple and it is exactly what the customer will do to check your work. However, sometimes verifying the repair requires a more complicated means of verification. A common concern that falls into this scenario is as follows: The customer brings their vehicle in with a concern that the MIL (malfunction indicator lamp) or “check engine” light is illuminated. In this scenario, NEVER verify the repair by simply checking to see that the light is off. While this is what the customer will do to check your work, the failure of the MIL to light can often be misleading and result in a comeback for the exact same problem. This can occur because the MIL is illuminated when tests run by the computers in the vehicle fail. The computers are constantly running tests, but some tests require very specific conditions before they can be run and, hence, fail. Due to the requirements for the conditions to be right, simply checking to see if the light is illuminated is an inadequate method of verification. For more complicated computer-controlled systems, the best method of verification is checking the test results stored on the vehicle’s computer. This option will require an electronic scan tool that communicates with the vehicle’s computer, along with a high level of diagnostic experience and service information to verify that the concern has been fully resolved. If the communication option is not available, the second best method of verification is repeating the last diagnostic test performed (in step 3) and confirming that the result has changed to now match the expectation/specification. Complicated computer-controlled systems require that the technician do more than verify the customer concern is eliminated. The technician will have to repeat a diagnostic test (step 3) or view test results stored on the vehicle’s computer (this is the preferred method) in order to verify that the repair was effective. Step five of the strategy-based diagnostic process is the most important. A vehicle should never be returned to a customer without this step completed.

▶▶ Documenting

the Repair

The first two components, gathering information from the customer and the strategy-based diagnostic process, have already been described; this section discusses documentation. The repair is documented for several reasons: accurate vehicle history, returns or comebacks, and OEM or aftermarket warranties. Keeping accurate service records will help technicians to know what services and repairs have been performed on a vehicle when it needs any future services. This can be invaluable during the diagnostic process and can also help service advisors and technicians identify what maintenance or recall work still needs to be completed. Documenting the repair also helps technicians in the event that a vehicle returns, now or in the future, with the same customer concern or fault. This can help to identify defective parts or common problems. Warranty work is another reason that all repairs must be documented. Whether the repair is submitted to an original equipment manufacturer or to an aftermarket warranty



Documenting the Repair

17

company for reimbursement, the repair must be well documented. Warranty clerks will review the repair order to ensure that proper testing and repair procedures have been followed. Technicians must document their work to ensure that the shop, and in turn the technician, get paid for the work performed. Finally, documenting the work provides the shop with a record that the work was initiated and completed. This is important in case the vehicle is later involved in an accident or other mishap and the shop is involved in a lawsuit. It is important to have the customer sign or initial, depending on shop policy, the repair order, to verify that the customer accepted the repair.

The Three Cs of Documentation When documenting a repair, technicians need to remember the 3 Cs: concern, cause, and correction (FIGURE 1-14).

Concern The main focus of the 3 Cs is the customer concern, which is also the focus of step one of the diagnostic process. Often the concern is documented on the repair order prior to the technician receiving the vehicle. If this is the case, the technician who works on the vehicle should take time to fully understand the concern, read the repair order, and possibly talk further with the customer to understand the nature of the problem. Think through the problem and develop a strategy to attack it. Other symptoms and diagnostic troubles codes are some examples of other information that should be included in the “concern.”

K01009 Explain how the three Cs are applied in repairing and servicing vehicles. N01002 Demonstrate use of the three C’s (concern, cause, and correction).

Cause The second C in the 3 Cs is cause, which details the cause of the customer concern. This correlates to the documentation done in step three of the diagnostic process. The technician should document any tests that they perform with enough detail that they can be repeated, as well as specifications/expectations, and results. This goes for all tests, even the simple ones.

Correction The technician should then document the last C, the correction. This must include the procedure used as well as a brief description of the correction. This information comes from the fourth step of the diagnostic process. When documenting the repair order, technicians should include the customer concern and symptoms (DTCs are symptoms); brief Your concern was the check engine light on. descriptions of tests; expectations; and results, along with The cause was a faulty pressure sensor. the procedure and repair that were performed. The techni- To correct the fault we have replaced the cian should also include all parts that were replaced as well, pressure sensor, cleared the code and road tested your car which is now performing normally. and noted if they were new or used, OEM, or aftermarket.

Other Parts of Documentation Additional service recommendations should also be documented on the repair order. While working on the vehicle, technicians should also be mindful of other work that may need to be performed. Technicians are obligated to make the customer aware of safety concerns that require attention. Customers may be unaware of a potential hazard or lack of maintenance. Bringing this to the attention of the customer right away can help the technician, as well as the customer. For example, if the technician is already working on the vehicle, they would not have to remove the vehicle from the service bay, bring in a new vehicle, and start all over. Repairing multiple issues in one trip to the service bay makes good use of the technician’s time. It also improves customer relations by bringing the customer’s attention to problems and thereby preventing possible failures.

FIGURE 1-14  The 3Cs of documenting the repair.

Thank you so much for your explanation

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Chapter 1  Strategy-Based Diagnostics

For example, a technician may be changing the fluid in a transmission and notice that the brake friction pads are extremely low. Bringing this to the attention of the customer can result in additional work for the technician and save the customer from a potentially more costly repair. For this reason, technicians should also note safety issues and maintenance items on the repair order.

Repair Order K01010 Describe the information and its use within a repair order. N01003 Identify information needed and the service requested on a repair order. S01002 Complete a repair order.

A repair order is a key document used to communicate with both your customers and coworkers. Thoroughly document the information provided by the customer on the repair order; every bit of it may be helpful during the diagnosis (FIGURE 1-15). If you are not typing this information, make sure your handwriting is clear and easy for others to read. Unfortunately, if documentation of a complaint is not done well, the technician could be led on a much longer diagnostic path, wasting everyone’s time. It can also be a time-consuming process for the diagnosing technician to make contact with the customer in order to get more information that was missed the first time. From time to time, it may be inevitable that the customer will need to be contacted for further inquiry after the initial visit. However, carefully gathering information from the customer on their initial visit will save time, prevent inconveniencing the customer, and aid in the diagnostic process. To complete a repair order using the 3 Cs, follow the steps in SKILL DRILL 1-2.

▶▶TECHNICIAN TIP A repair order is a legal document that can be used as evidence in the event of a lawsuit. Always make sure the information you enter on a repair order is complete and accurate. The information required on a repair order includes: date; customer’s name, address, and phone number; vehicle’s year, make, model, color, odometer reading, and VIN; and description of the customer’s concern. Store repair orders in a safe place, such as in a fireproof filing cabinet or electronically on a secure computer network. Finally, to prevent future complications, it is a good idea to have the customer sign or initial the repair order, indicating that they understand and agree to the needed repair. Having the customer’s signature will help prevent the shop from being held liable in an accident involving the vehicle later.

FIGURE 1-15  A repair

order.

Applied Communications AC-23: Repair Orders: The technician writes a repair order ­containing customer vehicle information, customer complaints, parts and materials used (including prices), services performed, labor hours, and suggested repairs/maintenance.

A repair order is a legal contract between the service provider and the customer. It contains details of the services to be provided by you and the authorization from the customer. To make sure everyone understands clearly what is involved, a repair order should contain information about the following aspects of the repair.



Documenting the Repair

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Your company or service providers: The service provider section contains the company name, address, and contact details; the name of a service advisor who is overseeing the job; and the amount of time the service technician will have to service the vehicle. The customer:The customer section contains the customer’s name, address, and contact phone numbers. The customer’s vehicle: The vehicle section includes details about the vehicle to be serviced. Check the vehicle’s license plate before starting work. The license plate numbers are usually unique within a country.You should also record information about the vehicle’s make, model, and color.This information will make it easier for you to locate the vehicle on the parking lot. You may also need to know the manufacture date of the vehicle to be able to order the right parts. The odometer reading and the date will help keep track of how much distance the vehicle travels and the time period between each visit to the shop.The VIN is designed to be unique worldwide and contains specific information about the vehicle. Many shops do a “walkaround” with the customer to note any previous damage to the vehicle and to look for any obvious faults such as worn tires, rusted-out exhaust pipes, or torn wiper blades. The service operations: This section contains the details of the service operations and parts.

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The first part is the service operation details. For example, the vehicle is in for a 150,000 mile (240,000-km) service, which can be done in 3 hours and results in approximately $300 of labor costs. The information about the chargeable labor time to complete a specific task can be found in a labor guide manual. In some workplaces, this information is built into the computer system and will be automatically displayed. The second part of this section is the details of parts used in the service, including the descriptions, quantities, codes, and prices. The codes for each service and part are normally abbreviations that are used for easy reference in the shop. Some shops may have their own reference code system. As you do the vehicle inspection, you may discover other things that need replaced or repaired. These additional services can be recorded in another section. It is essential that you check with the customer and obtain their approval before carrying out any additional services. The parts requirements: This section lists the parts required to perform the repair.

Some repair orders also contain accounting information so they can be used as invoices.

SKILL DRILL 1-2 Completing a Repair Order 1. Greet the customer. 2. Locate a repair order used in your shop and obtain or verify the customer’s name, address, and phone number. 3. Obtain details about the vehicle, including the year, make, model, color, odometer reading, and VIN. 4. Ask the customer to tell you more about the concern by using open-ended questions, such as “When does problem occur?” “At what speed(s)?” “How do you experience the problem?” “How long has this been occurring?” “How many passengers do you typically carry?” Type or clearly write the customer’s responses on the repair order. 5. Ask the customer about other changes with the vehicle, such as recent work, or recent travel. Type or clearly write the customer’s responses on the repair order. 6. Remembering the lessons learned regarding the proper diagnostic process, begin to verify the customer’s concern by first performing a visual inspection. 7. If you see nothing unusual during your visual inspection, continue to verify the customer’s concern by conducting a road test of the vehicle. The customer may ride along, if possible, to help identify the issue as it occurs, or you may conduct the test by yourself. Following the test drive, after verifying the customer’s concern, record it on the repair order. 8. The second step of the diagnostic process is to research the possible faults, and gather information. Access the vehicle service history to determine if the vehicle has experienced a similar problem in the past, requires a routine service maintenance, or has been serviced recently. Document this information, if applicable, on the repair order. 9. Conduct research by accessing various sources of information related to the vehicle, such as the vehicle service

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manual or the owner’s manual. Check to see if a TSB related to the issue exists. As part of the process, rule out the possibility that the customer’s concern is a normal operation of the vehicle. Now that you have your broad list of possible faults related to the concern, begin step three, focused testing. Choose one of the possible broad faults you identified in step two. Now refer to the service manual to locate information that matches the concern. Service manuals usually contain diagnostic charts to aid in the focused testing process. Conduct a test and record its description, your expectation, and the result on the repair order or another piece of paper. Continue to check each possible fault until you identify the cause of the concern. Once you have identified the fault, you’re ready for step four, performing the repair. You would inform the customer of your finding and obtain his or her approval to make the repair. Pending customer approval, you would then follow proper safety procedures and use the manufacturer’s guidelines to correct the problem, being sure to use the correct tools and taking the time to complete the job properly. Once you’ve made the repair, you are ready for step five, verifying the repair. The simplest way to verify that you have addressed and corrected the customer’s concern is to repeat the test drive. Take the vehicle for a test drive and repeat the tests you initially performed. Is the issue gone? If so, you have verified the repair and can return the vehicle to the customer. Document the correction on the repair order. If the issue is not resolved, you must return to your list of possible faults and continue testing after first alerting the customer that additional work and time will be necessary.

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Chapter 1  Strategy-Based Diagnostics

▶▶Wrap-Up Ready for Review ▶▶

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Service history is typically retrieved from service records kept by the shop, dealer network, original equipment manufacturer (OEM), or aftermarket service center and contains a list of services performed on a vehicle and the date and mileage at which they were completed. The service history allows technicians to determine if the vehicle has been well maintained. This can be extremely useful when a technician suspects that lack of maintenance may be the cause of the problem. Failure to comply with the state and federal law can be very expensive for the dealership and manufacturer. Today’s vehicles also require regular software updates made available to fix a bug or glitch in the computer programming. These updates are often designed to ­eliminate a customer concern, improve owner satisfaction, or increase vehicle life. The strategy-based diagnostic process is focused on fixing problems correctly the first time. It begins with identifying the customer’s concern and ends with confirming that the problem has been resolved. The problem-solving process provides a consistent road map for technicians as they address customer concerns that require diagnosis and to make sure that customer concerns are resolved with certainty. Strategy-based diagnosis simplifies the problem-solving portion of the repair, making the job easier for the technician; it prevents technicians from having to work on the same job more than once; and it all but eliminates customer comebacks. Customer comebacks are usually caused by the customer concern being misinterpreted or misunderstood or failing to verify that the original concern was resolved. The strategy-based diagnostic process begins by gathering preliminary information from the customer and by reviewing the vehicle’s service history. The first step in the diagnostic process is to verify the customer concern. This step is completed for two main purposes: verify that there is an actual problem present, and guarantee that the customer’s concern is addressed. Visual inspections can be very valuable, but technicians need to be careful not to jump to conclusions. DTC’s (Diagnostic Trouble Codes) and freeze frame data should always be saved and recorded on the repair order. Freeze-frame data provide a snapshot of the entire engine data when the DTC occurs, which allows for duplication of the condition so that the DTC can be replicated. The second step in the diagnostic process is to research possible faults. The goal of this step is to create a list of possible faults. The list will be created based on the information gathered in step 1 and narrowed down by the

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tests performed in step 3 until the cause of the concern has been confirmed. The best source of information is usually the manufacturer’s service information system. Technical service bulletins (TSBs) are service notifications and procedures sent out by the manufacturers to dealer groups, alerting technicians to common issues with a particular vehicle or group of vehicles. Some aftermarket sources also exist for the pattern failures addressed by TSBs A technician must always be aware that steps in the diagnostic process cannot be skipped. A repair should never be performed unless the possible fault has been verified through testing. Step 3 of the diagnostic process involves focused testing, where technicians use their testing skills to eliminate possible faults from the list they created in step two. Steps 2 and 3 work together, because testing starts at a system level and works down to subsystems, then finally to individual components. When selecting tests prioritize your testing. First choose tests that can be performed quickly and simply, even if they do not test an entire circuit. If a preferred test is in a very difficult place to access, move to another test and come back to it, if needed. Following manufacturers’ guidelines and safety protocols keeps technicians safe. Focused testing is a safe, accurate, and repeatable method for isolating possible faults. Once the fault has been isolated, it is time to perform the repair. The fourth step of the diagnostic process is to perform the repair. Performing the repair is often the most straightforward step in the process. Use proper service procedures when performing a repair. Manufacturers often indicate what procedures are appropriate for their vehicles and components. Use the correct tool for the job when performing a repair. Failure to use the correct tool for the job can lead to a customer comeback and injury to the technician. Take time to perform the repair properly. Technicians are frequently paid by the job, or flat rate, rather than paid hourly, it is possible for technicians to feel a rush to complete their current job. Rushing increases the likelihood of a mistake and the next time you may pay for it. The most important step of the strategy-based diagnostic process is verifying the repair. The reason that this is the most important step is straightforward. The vehicle would never have been in the shop if the customer did not have a concern. Verifying the original concern is the best method of double-checking your work and meeting your ­customers’ ­expectations. The job is not complete until you have ­verified that the repair resolved the customer’s concern.

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Wrap-Up

Documentation is key to effective and efficient repairs. Keeping all the information available to the service advisor, technician, and the customer allows for a more open dialogue which can limit the confusion of the repair process. The repair is documented for several reasons: accurate vehicle history, returns or comebacks, and warranties. Keeping accurate service records will help technicians to know what services and repairs have been performed on a vehicle when it needs any future services. When documenting a repair, technicians need to remember the three Cs: concern, cause, and correction. When documenting the repair order, technicians should include the customer concern and symptoms (Diagnostic Trouble Codes are symptoms) and a brief description of tests, expectations, and results, along with the procedure and repair that were processed.

Key Terms Strategy-Based Diagnostic Process  A systematic process used to diagnose faults in a vehicle. service advisor  The person at a repair facility that is in charge of communicating with the customer. service history  A complete listing of all the servicing and repairs that have been performed on that vehicle. repair order  The document that is given to the repair technician that details the customer concern and any needed information. freeze frame data  Refers to snapshots that are automatically stored in a vehicle’s power train control module (PCM) when a fault occurs (only available on model year 1996 and newer). technical service bulletin (TSB)  Service notifications and procedures sent out by the manufacturers to dealer groups alerting technicians about common issues with a particular vehicle or group of vehicles. original equipment manufacturer (OEM)  The company that manufactured the vehicle. aftermarket  A company other than the original manufacturer that produces equipment or provides services. intermittent faults  A fault or customer concern that you can not detect all of the time and only occurs sometimes. 3 Cs  A term used to describe the repair documentation process of 1st documenting the customer concern, 2nd documenting the cause of the problem, and 3rd documenting the correction. concern  Part of the 3Cs, documenting the original concern that the customer came into the shop with. This documentation will go on the repair order, invoice, and service history. cause  Part of the 3Cs, documenting the cause of the problem. This documentation will go on the repair order, invoice, and service history. correction  Part of the 3Cs, documenting the repair that solved the vehicle fault. This documentation will go on the repair order, invoice, and service history.

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Review Questions 1. When a vehicle comes in for repair, detailed information regarding the vehicle should be recorded in the: a. service booklet. b. repair order. c. vehicle information label. d. shop manual. 2. The service history of the vehicle gives information on whether: a. the vehicle was serviced for the same problem more than once. b. an odometer rollback has occurred. c. the vehicle meets federal standards. d. the vehicle has Vehicle Safety Certification. 3. Which of the following steps is the last step in a ­strategybased diagnostic process? a. Verifying the customer’s concern b. Researching possible faults c. Performing the repair d. Verifying the repair 4. When possible, which of the following is the best way to understand the customer’s concern? a. Asking the customer to guess the cause of the problem. b. Asking the customer to suggest a solution to the problem. c. Encouraging the customer to demonstrate the problem. d. Encouraging the customer to help you fix the problem. 5. The best way to address intermittent faults is to: a. look for symptoms, data, or DTCs that are repeatable or consistent. b. reverse the steps in the diagnostic process. c. ask the customer to bring back the vehicle when the fault occurs. d. take it up only when it is covered by warranty. 6. When the technician encounters a vehicle with more than one customer concern, and both originate from companion systems, the technician: a. should attempt to test for both faults at once. b. need not attempt to fix the second fault. c. should never choose those tests that might look at components of both systems. d. should isolate the faults and test them separately. 7. Choose the correct statement. a. When performing tests for an inspection under warranty, follow your intuition rather than the manufacturers’ guidelines. b. Researching possible faults should begin with a specific cause in mind. c. For hard parts, the best resource is frequently the respective dealership’s parts department. d. DTCs and freeze frame data need not be captured before clearing the memory. 8. All of the following will happen if the technician fails to document test results except: a. The manufacturer will not pay the claim. b. The shop is out money for the parts and service.

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Chapter 1  Strategy-Based Diagnostics

c. The technician will be unable to diagnose the fault. d. The technician will not be paid for his or her work. 9. All of the following statements with respect to the 3 Cs are true except: a. Customer concern is documented on the repair order prior to the technician receiving the vehicle. b. The second C in the 3 Cs refers to the cause of the customer’s concern. c. Technicians should note safety issues and maintenance items on the repair order. d. Additional service recommendations should never be documented on the repair order. 10. Which of the following is not one of the 3 Cs of vehicle repair? a. Cause b. Cost c. Concern d. Correction

ASE Technician A/Technician B Style Questions 1. Tech A says that when diagnosing a transmission problem, it is important to first verify the customer concern by taking the vehicle on a road test if possible. Tech B says that you should check for TSBs during the diagnostic process. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 2. Tech A says that additional service recommendations should be documented on the repair order. Tech B says technicians are obligated to make the customer aware of safety concerns that require attention. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 3. Tech A says the strategy-based diagnostic process is a scientific process of elimination. Tech B says the strategy-based diagnostic process begins with scanning the vehicle for DTCs. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 4. Tech A says that manufacturers will often indicate what procedures are appropriate for their vehicles and components. Tech B says that the manufacturer’s service information can avoid premature failure of the repair. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 5. Tech A says that technicians are frequently paid by the job, or flat rate, rather than paid hourly. Tech B says rushing the

repair is best for the customer, so they get their vehicle back quickly. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 6. Tech A says that it is only necessary for dealerships to check for updated parts and software/firmware before performing a repair. Tech B says that it is possible that manufacturers have become aware of a problem with a particular component or software version and have issued a software update. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 7. Tech A says that the customer concern is the focus of step 1 of the diagnostic process. Often the concern is documented on the repair order prior to the technician receiving the vehicle. Tech B says the technician who works on the vehicle should take time to fully understand the concern, read the repair order, and possibly talk further with the customer to understand the nature of the problem. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 8. Tech A says that a repair order is only used in the shop and will be discarded when the vehicle is complete. Tech B says that the repair order is a legal document and could be used in a court. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 9. Two technicians are discussing a transmission problem. Tech A says that it is important to test drive the vehicle because there may actually be no issue with the vehicle and the customer complaint is actually a normal operational characteristic of the transmission. Tech B says you should always check TSBs before performing any service of the transmission because the manufacturer may have updated a component. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B 10. Tech A says that experience will allow you to skip many of the steps of the diagnostic process because you will be familiar with the transmission. Tech B says that skipping steps of the diagnostic process can cause issues to be missed, or misdiagnosis of the problem. Who is correct? a. Tech A only b. Tech B only c. Both A and B d. Neither A nor B

& actuuators)

SENSORS

ABS WARNING LIGHT PUSHROD BRAKE LINES

PEDAL

RELAYS

CHAPTER 2

SPEED SENSORS CALIPER

ABS COMPUTER

ROTOR OR DISC

Principles of Braking

SPEED SENSOR

HUB

BACKING PLATE ROTOR OR DISC TOOTHED RING

Learning Objectives ■■ ■■ ■■ ■■

2-1 Explain braking system fundamentals. 2-2 Explain the physics behind braking system application. 2-3 Examine how energy is transformed. 2-4 Explain how brake fade affects vehicle stopping.

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2-5 Describe rotational force and its effect. 2-6 Differentiate between the applications of the various types of braking systems.

You Are the Automotive Technician A vehicle that has been modified from the factory-equipped braking system is having issues with the distance it takes to stop.You as the technician must be able to diagnose why this is the case. What is your thought process? In order from first to last, answer the following questions:

1. What was done to the vehicle’s braking system? 2. Did the vehicle have a stopping issue before this modification was implemented? 3. Has the modified braking system failed?





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24

Chapter 2  Principles of Braking

▶▶ Introduction

Oh No..........

Yikes, brakes don't fail me now!!

The brake system is one of the most critical systems on a ­vehicle. It allows the driver to slow down or stop the vehicle as needed. In ideal situations, the driver will have enough time to anticipate the need to slow down well in advance of an event, allowing the vehicle to slow down gradually. However, many situations require the quick use of a very efficient braking s­ystem to avoid an accident (FIGURE 2-1). This chapter ­discusses the history, theory, and operation of modern ­braking systems.

▶▶ Braking

FIGURE 2-1  Brakes must be fully functional in case of an emergency.

Systems Fundamentals

Before work can commence on repairing any braking s­ ystem issue, the technician must be able to understand how the braking system should operate. Throughout this chapter, ­ the  reader will be introduced to the differences between service and parking brakes. The science behind the braking system will be explained as well as applied to the modern automobile.

Service and Parking Brakes 2-1 Explain braking system fundamentals.

There are two brake systems on all vehicles: a service brake and a parking brake. The ­service brake is used for slowing down or stopping a vehicle when it is in motion; it is operated by a foot pedal (FIGURE 2-2). Service brakes consist of drum and/or disc brakes. Some have disc brakes on the front wheels and drum brakes on the rear wheels; others have disc brakes on all four wheels. The parking brake is used for holding the vehicle in place when it is stationary. The parking brake is usually operated by hand, but some vehicles use a foot-activated pedal (FIGURE 2-3). Modern braking systems are hydraulically operated and have two main sections: the brake assemblies at the wheels and the hydraulic system that applies them. The driver pushes the brake pedal, which applies mechanical force to the pistons in the master cylinder. The pistons apply hydraulic pressure to the fluid in the cylinders. The lines transfer the pressure, which is applied equally in all directions within the confines of the brake lines to the hydraulic cylinders. The hydraulic cylinders at the wheel assemblies apply the brakes (FIGURE 2-4). Force is transmitted hydraulically through the fluid. For cylinders of the same size, the force transmitted from one is the same value as the force applied to the other. By using

FIGURE 2-2  A vehicle service brake is used for slowing down or

FIGURE 2-3  A vehicle parking brake is used to hold the vehicle in

stopping a moving vehicle.

place when it is stationary.



Braking Systems Fundamentals

25

200 lb 50 lb 400 lb FIGURE 2-4  A braking system in a typical modern vehicle.

cylinders of different sizes between the front and rear brakes, forces can be increased or decreased, allowing designers to obtain the desired braking force for each wheel (FIGURE 2-5). The pistons FIGURE 2-5  By using cylinders of different sizes, hydraulic forces can in the cylinders force friction linings into contact with the brak- be increased or decreased, allowing designers to obtain the desired ing surfaces. The resulting friction between the surfaces converts braking force for each wheel. kinetic energy to heat energy and slows the vehicle. In disc brakes, pads are forced against the outside of a brake disc (FIGURE 2-6). In both systems, heat spreads into other parts and the atmosphere, so brake linings and drums, pads and discs, and brake fluid must withstand high temperatures and high pressures. On modern vehicles, the basic brake system has some refinements, such as a power booster and electronic brake control (EBC) systems. These help the driver apply the brakes, prevent skidding, and maintain directional control of the vehicle under various driving situations, such as when ABS (antilock brake system) brakes are needed (FIGURE 2-7).

Rotational Force When brakes are operated on a moving vehicle, a rotational force is generated. As the wheel rotates, the friction between the brake components tends to twist the brake support in the direction of wheel rotation (FIGURE 2-8). Because the brake support is ultimately connected to the suspension of the vehicle, the suspension tends to rotate in the same direction. A good example of rotational force is when a motorcycle rider applies the front brake hard enough that the rear wheel is lifted completely off the ground. Rotational forces are usually controlled by the suspension components, but they can become worn and allow movement, which can be felt as a clunk or pop during brake application.

FIGURE 2-6  In disc brakes, pads are forced against the outside of a

FIGURE 2-7  ABS brakes help to prevent skidding and maintain

brake disc.

directional control of the vehicle.

26

Chapter 2  Principles of Braking

Vehicle Momentum Caliper

Braking G-Force

Rotor Downward Force

Upward Force

FIGURE 2-8  Braking rotational force occurs when the friction

between the brake components twists the brake support in the direction of the wheel rotation.

FIGURE 2-9  Weight transfer during braking.

Another result of rotational force is weight transfer. The rotational force tends to push the nose of the vehicle down and lift the rear of the vehicle, transferring weight to the front wheels. Rotational force and weight transfer also happen because the centerline of the vehicle is higher than the centerline of the axles; thus, the center of gravity tends to move forward when the brakes are applied firmly (FIGURE 2-9). Weight transfer causes the front wheels to have increased traction, allowing them to bear more of the stopping load, and causes the rear wheels to have less traction, ­reducing the amount of load they can bear. Engineers take this into account when designing the brakes; otherwise, the front wheels will not get enough stopping power, and the rear wheels will have too much, resulting in rear-wheel lockup and loss of control of the v­ ehicle. Lockup is avoided by engineering the system with the proper-sized master cylinder, wheel cylinders, and ­valving that modifies the hydraulic pressure to the rear wheels under hard braking.

▶▶ Physics 2-2 Explain the physics behind braking system application.

Behind Braking System Application

Science has been prevalent in all of the systems of the automobile and in understanding how science influences the designs of braking systems and what would happen if unapproved components that can compromise the braking of the vehicle were used. Understanding all the factors that affect the braking system will allow a technician to fix the issue causing the problem.

Factors That Affect Braking A number of factors can influence vehicle braking. An effective braking system takes all of the following factors into account: ■■

■■

■■

■■

■■

Road surface: Generally asphalt and concrete road surfaces allow for good braking while gravel surfaces or dirt roads do not. Road conditions: Roads that are wet or icy, covered in snow, or have loose gravel reduce the tires’ traction and result in longer stopping distances. Extremely hot temperatures on asphalt roads can soften the asphalt, making it slippery. Weight of the vehicle: Heavier vehicles require more braking force to stop than lighter vehicles, and therefore, they usually have larger wheel brake units. Also, loading down a vehicle increases its stopping distance due to the vehicle’s extra mass. Load on the wheel during stopping: Heavier loads increase the downward force on the wheels, thereby increasing tire traction (FIGURE 2-10). Height of the vehicle: Stopping power is exerted at the point where the tire and the road connect. The centerline of the vehicle’s weight is above this tire-to-road contact point. The taller the vehicle, the greater the leverage on the contact point. During braking, this shifts some of the vehicle’s weight from the rear wheels to the front wheels. Thus, controlling the vehicle in a panic situation becomes much more difficult.



Physics Behind Braking System Application

A

B

FIGURE 2-10  Tires with differing loads. A. The empty pickup truck has low traction on its rear tires.

B. A heavily loaded truck has higher traction on its rear wheels.

■■

■■

Driving method: Aggressive driving causes the tires to become hot and possibly overheated, thus reducing the tire’s ability to obtain maximum traction. Also, increased speed and aggressive handling force the brakes to work under extreme conditions. The tires on the vehicle: A tire’s composition, tread style, tread condition, and inflation pressure all affect its traction (FIGURE 2-11). Manufacturers design tires with different qualities based on a vehicle’s needs. Tires are rated for their traction ability. Using the wrong tire will affect the vehicle’s stopping power. For example, a tire with tread designed to channel water away from the tire-to-road contact point has greater traction when the road surface is wet than the tires used on drag race cars, which have a slick tread which provides best traction on a dry road.

Kinetic Energy Kinetic energy is the energy of an object in motion. All moving objects have kinetic energy. Heavier objects at a given speed have more kinetic energy than lighter objects moving at the same speed. If the weight doubles, the kinetic energy doubles. Faster moving objects have more kinetic energy than slower moving objects of the same weight. Kinetic energy increases by the square of the speed. This means that if we double the speed of an object, the kinetic energy will increase by four times. If we triple the speed, the kinetic energy will increase by nine times. Thus, the heavier and faster an object is, the greater its kinetic energy (FIGURE 2-12). During braking, all of the kinetic energy in the moving vehicle must be converted to another form of energy (in most cases, heat) for the vehicle to stop moving; this is the function of the braking system.

A

B

FIGURE 2-11  The condition of the tires affects braking performance. A. Tire in excellent condition. B. Worn-out tire.

27

28

Chapter 2  Principles of Braking Kinetic Energy = 53,486 ft/lb

Kinetic Energy = 26,743 ft/lb

20 mph

20 mph

Kinetic Energy = 53,486 ft/lb

4,000 lb

2,000 lb

4,000 lb

Kinetic Energy = 855,600 ft/lb

20 mph

4,000 lb

80 mph

B

A

FIGURE 2-12  A. If weight doubles, kinetic energy doubles. B. If speed increases, kinetic energy increases by the square.

Acceleration and Deceleration Newton’s first law of motion states that an object will stay at rest or uniform speed unless it is acted upon by an outside force. Acceleration refers to an increase in an object’s speed. In an automobile, acceleration—i.e., an increase in kinetic energy—is caused by the power from the engine. When the driver steps down on the throttle pedal, the engine’s power output is increased, and thus the vehicle accelerates (FIGURE 2-13). This acceleration requires a certain amount of energy. The heavier the vehicle, the more energy required to accelerate it to a given speed. A lighter vehicle requires less energy to accelerate; this is why race cars are stripped of all unnecessary weight. Deceleration refers to a decrease in an object’s speed. Remember that an outside force is needed for the speed of an object to change. So we need an outside force to act upon the vehicle to cause it to decelerate. That force comes from the mass of the Earth. The thought that it comes from the brakes is only partially correct. Imagine traveling at a high speed in a four-wheel-drive vehicle and hitting a bit of a jump. Stepping on the brakes in mid-air to slow the vehicle wouldn’t do much good, would it? So the brakes only function when they connect the vehicle to the ground or roadway. In fact, that is what they do; they connect the vehicle to the ground through the rolling wheel and tire assembly. In doing so, they apply a varying amount of force from the ground to the vehicle, thereby causing the vehicle to decelerate. The force of the brakes absorbs the kinetic energy of the vehicle (FIGURE 2-14). The heavier the vehicle and the faster it is going, the more kinetic energy must be dissipated and the harder the brakes must work.

FIGURE 2-13  The engine’s horsepower is accelerating this vehicle hard.

FIGURE 2-14  The vehicle’s brakes are decelerating this vehicle hard.



How Energy Is Transformed

29

Applied Science AS-1: Kinetic Energy : The technician can develop an understanding on how kinetic energy affects the vehicles braking system. Kinetic energy is the energy of an object in motion. All moving objects have kinetic energy. Heavier objects have more kinetic energy than lighter objects moving at the same speed. If the weight doubles, the kinetic energy doubles. Energy is needed to start a vehicle. Chemical energy from a battery is converted into electrical current and turns the starter motor starting motion within the engine. Heat energy is generated in the engine via chemical energy (air and fuel); it is then

▶▶ How

converted via mechanical energy to kinetic energy, setting the vehicle in motion. Kinetic energy is converted back into heat energy once again, through the operation of the brakes. This heat energy is then dissipated in the surrounding air through the brake system, bringing the vehicle to rest. Newton’s first law of motion states that the greater the weight, the greater the energy needed to accelerate and maintain speed. For example, it is easier for three people to push a two-door hatchback than to push an SUV (sport utility vehicle). Also, the more the vehicle weighs, the more kinetic energy must be converted to decelerate.

Energy Is Transformed

The law of conservation of energy states that energy cannot be created or destroyed. This means that the energy used to cause a vehicle to accelerate and decelerate must be transformed from one form of energy to another. This section will follow the cycle of energy transformation in a typical vehicle. Gasoline or diesel fuels are potential energy in chemical form. A portion of the fuel’s chemical energy is transformed inside the engine, first into heat energy and then into mechanical energy. The mechanical energy is used to accelerate the vehicle, thus converting the mechanical energy into kinetic energy. Once the vehicle is up to speed, the engine only needs to transform enough chemical energy into kinetic energy to overcome wind friction, climb hills, and power the vehicle’s accessories. This is why most vehicles get better fuel economy while operating at steady speeds than in stop-and-go traffic: it takes a lot more energy to accelerate a vehicle than it does to maintain a particular speed. Deceleration also requires energy transformation. The kinetic energy has to be removed from the vehicle for it to decelerate. In other words, the kinetic energy must be transformed into another form of energy for the vehicle to slow down. In a standard vehicle, the braking system transforms the kinetic energy into heat energy (FIGURE 2-15). In essence, it takes the same amount of energy to slow a vehicle down as it does to speed it up. For safety’s sake, we expect a vehicle to stop from a given speed faster than the time it took to accelerate to that speed. For this reason, the braking system can transform energy faster than the engine. Or in other words, the brake system can absorb energy at a faster rate than the engine can create it.

2-3 Examine how energy is transformed.

Friction and Friction Brakes Brakes transform kinetic energy to another form of energy. Standard brakes do this through the principle of friction. Friction is the resistance created by surfaces in contact. Static friction is resistance between non-moving surfaces and is present in parking brakes. Kinetic friction is resistance between moving surfaces and is present in standard brakes. Just as rubbing sandpaper over a block of wood produces heat, operating the brakes causes the moving friction surfaces to come into contact with each other and generate heat. This transformation of energy converts kinetic energy into heat energy and slows the vehicle. The amount of friction between two moving surfaces in contact with each other is expressed as a ratio called the coefficient of friction. It can be found by comparing the amount of force pushing the two surfaces together to the amount of resistive force generated between the two surfaces sliding against each other. For example, a stationary steel surface pushed Brake Heat against a moving steel surface with 100 lb (45.36 kg) of force might generate 20 lb (9.07 kg) of resistive force. This is expressed as 20/100 (9.07/45.36), which equals a coefficient of friction of 0.20 (FIGURE 2-16A). A stationary block of rubber that is pushed Kinetic Energy Heat Energy against a moving steel surface with 100 lb of force might generate Potential Energy 125 lb (56.7 kg) of friction. This would be 125/100 (56.7/45.36), FIGURE 2-15  During deceleration, kinetic energy is transformed into heat energy. which equals a coefficient of friction of 1.25 (FIGURE 2-16B).

30

Chapter 2  Principles of Braking

Coefficient of Friction (

) = 0.2

Load = 100 lb (45.36 kg)

Resistive Force = Load x = 100 lb x 0.2 = 20 lb (9.07 kg)

Steel Block

A

Steel Plate

▶▶TECHNICIAN TIP A higher coefficient of friction usually results in a faster wearing of the softer material, such as rubber. This is one reason why the brake lining is designed to be made of materials that are softer than the drum or rotor, which leads to wearing out the brake lining more than the drum or rotor.

Coefficient of Friction (

Effort (kinetic energy)

Load = 100 lb (45.36 kg)

) = 0.80

Resistive Force = Load x = 100 lb x 0.80 = 80 lb (36.3 kg) B

Rubber Steel Plate

Effort (kinetic energy)

FIGURE 2-16  A. Coefficient of friction = 0.20. B. Coefficient of friction = 0.80.

Applied Science AS2: Friction:The technician can demonstrate an understanding of friction and its effects on linear and rotational motion. When adjusting drum brakes, the common method is to adjust the brake and turn the wheel at the same time in order to feel how hard it is to turn the wheel while adjusting the brake. As the brake becomes tighter, the friction between the brake drum and the brake shoe increases; therefore, the rotational motion requires more torque or force to produce the same speed of movement.

Linear friction is demonstrated when a heavy object is pulled across a flat surface. The more surface area that is in contact with the object, the harder it is to pull.The larger the surface area that the object is covering, the greater the force required to move it. For example, try to pull a tool box across a bench. Now lift one end of the toolbox and pull. It is far easier to pull the toolbox with one end lifted because the lifted tool box is in contact with less surface area, reducing the force required to move the box.

Applied Science AS3: Friction: The technician can explain the role that friction plays in acceleration and deceleration. Friction is very important in acceleration. A vehicle’s tires must be able to keep friction between the tire and the ground in order to propel the car forward. If the tires are not in contact with the road, they will have nothing to push against. If no friction is present, the car will not move forward.

Friction is also very important in deceleration since the tire again has to maintain friction between the road surface and tire. The braking force on the vehicle also uses friction in order to slow or decelerate the vehicle. As the brake pads press against the brake disc, friction is created between the two surfaces. This friction creates heat and slows the vehicle.

Heat Transfer Heat transfers from a hot area to a cool area. Because there is a lot of kinetic energy converted to heat during the braking process, the brakes must be able to effectively dissipate that heat to the atmosphere. Heat transfer is critical to this process. Heat must be continuously transferred away from the friction materials so that the brakes can perform their job of transforming the kinetic energy into heat energy. Ultimately, most of the heat generated by the braking process radiates into the atmosphere. How it radiates into the atmosphere depends on the type of braking system (FIGURE 2-17). In drum brakes, the heat is created inside of the drum and transfers through the drum to the outside surface where it radiates into the atmosphere. In disc brakes, the heat is created on the outer surfaces of the rotors, which are in contact with the atmosphere. Disc brakes also may have internal ventilation to help dissipate the heat transferred from the outer surface even faster. Both drums and rotors must have enough mass to act as heat sinks. This is the primary reason that rotors have a minimum thickness specification and that drums have a maximum diameter specification.



How Brake Fade Affects Vehicle Stopping

Heat Passes into Tire

Heat Passes into Tire

Friction generates heat which passes into the drum.

Heat Passes into the Wheel

Heat Radiates to Atmosphere

Friction generates heat which passes into the disc surface.

Heat Radiates to Atmosphere

Heat Passes into the Wheel

Heat Radiates to Atmosphere

Heat radiates to Atmosphere

A

B

FIGURE 2-17  A. Heat transfer in a drum brake. B. Heat transfer in a disc brake.

▶▶ How

Brake Fade Affects Vehicle Stopping

In automobiles, brake fade is the reduction in stopping power caused by a change in the brake system caused by three factors. The first and most common is heat fade. Heat fade is caused by the buildup of heat in the braking surfaces, which get so hot that they cannot create any additional heat, leading to a loss of friction (FIGURE 2-18). Remember that the brakes must transform kinetic energy into heat energy to decelerate the vehicle. If heat energy cannot be generated, then the kinetic energy cannot be reduced and the vehicle will not decelerate. Heat transfer is used to move heat away from the friction surfaces and allow them to continue generating heat. Once the temperature of the friction materials become so hot that they cannot transfer any additional heat, the coefficient of friction drops, and the brakes cannot generate stopping power until some of the heat dissipates. A driver may experience heat fade after using the brakes too much, such as during high-performance driving or when going down a long, steep hill, particularly when towing. The brake pedal will be hard, but the braking effect “fades” away, and the vehicle’s rate of deceleration decreases. This is a dangerous condition and is why many long hills on freeways have truck escape ramps made of sand or some other soft material to slow a vehicle by absorbing the truck’s kinetic energy in the soft material. A. Cold Disc Brake

Normal Pedal Feel

= 0.35 Normal Stopping Distance

2-4 Explain how brake fade affects vehicle stopping.

B. Overheated Disc Brake

Normal Heat Generation and Dissipation

Pedal Harder and Slightly Higher

Excessive heat can't be dissipated fast enough.

= 0.02 Excessive Stopping Distance

FIGURE 2-18  Heat fade—caused by the brake system reaching the temperature generated by the friction of the brake pads.

31

32

Chapter 2  Principles of Braking

Minimal friction, heat generation, and stopping power

▶▶TECHNICIAN TIP Excessive heat can cause warpage of disc brake rotors and brake drums. Brake fade and rotor warping can be reduced through proper braking technique. When traveling on a long downgrade requiring braking, the driver should simply select a lower transmission gear. Periodic rather than continuous application of the brakes allows the brakes to cool between applications. Continuous light application of the brakes, sometimes referred to as riding the brakes, can be particularly destructive in both wear and overheating of the brake components.

▶▶TECHNICIAN TIP Some owners raise their vehicles for better off-road clearance by installing a lift kit and/or large-diameter tires. Doing so raises the vehicle’s center of gravity and increases the amount of weight transfer the vehicle experiences, making it more prone to rear-wheel lockup and vehicle rollovers. It is important to use only those lift kits that are engineered for the particular vehicle. After installing such a kit, inform the driver of the effect that a higher center of gravity will have on vehicle operation.

To Front Brakes Pedal is high and hard.

Water inside drum and between brake drum and brake shoes.

FIGURE 2-19  Water fade—water reduces the friction in brakes, causing a hard pedal with minimal stopping power until the brakes dry out.

The second type of brake fade is called water fade and is caused by water-soaked brake linings. The water acts like a lubricant and lessens the coefficient of friction between the braking surfaces (FIGURE 2-19). Disc brakes are very resistant to water fade. Water fade was much more common when drum brakes were used on front wheels. This leads to a hard brake pedal but very little braking power. Once the water is removed from the friction surfaces through evaporation, the normal coefficient of friction will be restored. The third kind of brake fade is called hydraulic fade and is caused by the brake fluid becoming so hot that it boils. Once it boils, it is no longer only a liquid, converting in part into a vapor, which can be compressed. The brake fluid can no longer transfer force effectively to the wheel brake units and apply them firmly enough to create friction. Because the boiling fluid can be compressed, hydraulic fade can be recognized by the brake pedal becoming soft and having increased pedal travel during heavy brake usage. Water absorbed by brake fluid lowers its boiling point. This is the primary reason for testing brake fluid for water content and flushing brake fluid when required (FIGURE 2-20).

Boiling Brake Fluid

To Front Brakes

Pedal drops and feels spongy as bubbles in the brake fluid compress.

Excessive Heat Generation

FIGURE 2-20  Hydraulic fade—when the brake fluid boils and can no longer transmit movement in the brake system to apply the brakes. The brake pedal is squishy and goes to the floor.



Rotational Force

▶▶ Rotational

Force

Brake systems use levers and mechanical advantage to apply service and parking brakes. A simple example of a lever is a bar. The point around which a lever rotates and that supports the lever and the load is called the fulcrum. A lever allows the user to lift a large load over a small distance at one end by applying a small force over a greater distance from the other end. The effort distance is from the fulcrum to the point where effort is applied. The load distance is from the fulcrum to the point where the load is applied. The effort required to move a load depends on the relative distance of the load and the effort from the fulcrum. The ratio of load and effort is called mechanical advantage. If the effort distance from the fulcrum is greater than the load distance, then the effort required will be less than the load being moved. If the load distance is greater than the effort distance, then the effort required is greater than the load being moved. This situation is known as a negative mechanical advantage, or mechanical disadvantage. Using the right kind of lever in the right way allows a user to move larger loads with less effort. There are three basic types of levers:

2-5 Describe rotational force and its effect.

1. Lever of the first order: The fulcrum is in the middle, between the load and the effort (FIGURE 2-21). Examples are a pry bar or a seesaw. The force applied in this situation is in the opposite direction of the load. 2. Lever of the second order: The load is in the middle, between the effort and the fulcrum (FIGURE 2-22). An example is a wheelbarrow. The force applied in this situation is in the direction of the load. Brake pedals are usually of the second order. They pivot at the top end (fulcrum). The foot pressure (effort) is applied to the bottom end. And the master cylinder (load) is applied between the two. Mechanical advantage is engineered into the brake pedal to provide the proper brake pedal application and feel (FIGURE 2-23). 3. Lever of the third order: The effort is in the middle, between the load and the fulcrum (FIGURE 2-24). An example is an oar when paddling a canoe, where the hand holding the top of the oar is the fulcrum; the other hand holding the middle of the oar provides the effort; and the water is the load. The force in this situation is in the direction of the load. F Second

First F W FIGURE 2-21  Lever of the first order.

W FIGURE 2-22  Lever of the second order.

Fulcrum

100 lb 50 lb

F Third

W FIGURE 2-23  The brake pedal uses leverage to multiply the force applied to the master cylinder.

FIGURE 2-24  Lever of the third order.

33

34

Chapter 2  Principles of Braking

Applied Science AS4: Levers: The technician can explain how levers can be used to increase an applied force over distance. The lever action is applied many times on a daily basis. The simplest example is the use of a wrench. If a bolt or nut is tight, then instead of

using a wrench with a short handle, a wrench with a longer handle can be used. The longer handle improves the lever’s mechanical advantage by increasing the ratio of the effort distance to the load distance, thus providing more torque to undo the nut or bolt.

▶▶ Different Applications

of Braking Systems

This section explores the different types of brakes used on various vehicles. Each vehicle application lends itself to a particular type of brake system. For example, conventional passenger vehicles use hydraulic brakes, which are fairly simple and reliable. Trailers are another example: Because they must be able to be hooked up to multiple tow vehicles, connecting a hydraulic brake system becomes difficult because of the leaking of hydraulic fluid as well as air entering the lines. Thus, some heavy trucks use air-operated brakes on both the truck and trailer. That way, when connecting and disconnecting the brake air line, only air will leak, not brake fluid. On passenger-type tow vehicles, trailers are typically equipped with electric brakes.

2-6 Differentiate between the applications of the various types of braking systems.

▶▶TECHNICIAN TIP Brake Repair Legal Standards and Technician Liability Brake repair is right up with steering and suspension repair on the liability scale. Improperly repaired brakes can function reasonably well under normal driving situations but can fail during a panic situation when they are needed the most.The likelihood of accidents, injury, or death goes up drastically in those situations. Shops and technicians have been successfully sued for improper brake repairs, resulting in large cash settlements. Technicians also risk being found criminally negligent if they are determined to have acted maliciously. Always follow the manufacturer’s procedures when servicing brake systems. Research service information, precautions, and technical service bulletins. Never take shortcuts, which could cause the vehicle to be unsafe. Remember: safety first!

Hydraulic Brakes Hydraulically operated friction brakes use two kinds of wheel brake units. Drum brakes have a drum attached to the wheel hub and rotate with the tire. Braking occurs by means of stationary brake shoes expanding against the inside of the drum, which creates f­riction and slows the vehicle. Disc brakes have a disc brake rotor attached to the wheel hub and rotate with the tire. The braking occurs by means of stationary pads clamping against the outside of the rotor, creating friction to slow the vehicle. On light vehicles, both of these systems are hydraulically operated, meaning they use hydraulic fluid to transfer the force from the driver. The brake pedal operates a master ­c ylinder. Hydraulic lines and hoses connect the master cylinder to the wheel brake units ­(FIGURE 2-25). Disc brakes require greater force to operate than drum brakes and usually include a power brake booster to assist the driver by increasing the force applied to the master cylinder when the brake is operated (FIGURE 2-26). Modern drum and disc brake systems are regularly fitted with an ABS that monitors the speed of each wheel and prevents wheel lockup or skidding, no matter how hard brakes are applied or how slippery the road surface. This allows the driver to better maintain directional control of the vehicle. ABS also generally reduces stopping distances. The system consists of a brake pedal, power booster, master cylinder, wheel speed sensors, the electronic control unit (ECU), and the hydraulic control unit, also called a hydraulic modulator.

Air Brakes

Master Cylinder Lines

Wheel Brake Units

Pedal

Hoses

FIGURE 2-25  The hydraulic brake system.

Air-operated braking systems are used on heavy vehicles and are commonly called air brakes (FIGURE 2-27). Compressed air, operating on large-diameter diaphragms, provides the large forces at the brake assembly that are needed to apply the brakes and slow the vehicle. The wheel brake units can be either drum or disc style. Some brake canisters incorporate a separate air chamber and large spring assembly (FIGURE 2-28) and are referred to as spring brakes. The spring applies the brakes when air pressure is released from the spring brake chamber. Air pressure applied to this chamber compresses the spring and releases the brakes. This system acts as a parking brake when the vehicle or trailer is parked and as a safety measure by applying the brakes in the case of an inadvertent



Different Applications of Braking Systems

35

Master Cylinder Booster Pedal Secondary Primary Piston Piston

FIGURE 2-26  A power brake

booster.

Pressure Gauge Front Brake Chambers

Mv-3 Module

150 75

Psi 0

Tp-3dc Valve

Tc-7 Valve

Gladhands

75

Dc-4 Valve Foot Valve

Sr7 Valve

Quick Release Valve

Spring Brake Chambers Relay Valve

Secondary

Air Drier

Primary Supply

Compressor

Governor

FIGURE 2-27  The air brake system.

loss of air pressure in the brake system. A vehicle or trailer with this type of air-operated brakes must have a minimum amount of air pressure in the system to compress the spring in order to keep the brakes released. The air pressure for air brake systems comes from an engine-driven air compressor that pumps air into storage tanks. The tanks provide a reservoir of pressurized air to operate the system. The driver-controlled foot valves then direct the compressed air to different wheel units to operate the wheel brake units. When the driver releases the brakes, the control valve releases the air in the service brake chambers, and the brakes retract. On articulated vehicles, such as tractor/trailers, any delays in applying the trailer brakes should be minimized. This is achieved by using a relay valve and a separate air tank on the trailer (FIGURE 2-29). This arrangement also applies the brakes if the trailer becomes disconnected from the tractor.

Exhaust Brakes

Anchor Point

Brake Shoe

Service Brake Chamber

S-Cam Backing Plate

Brake Drum Brake Shoe Lining

FIGURE 2-28  An air brake canister.

Medium-duty and heavy-duty vehicles often require increased braking in situations where friction brakes overheat and fail. This can be achieved by using an exhaust brake.

Spring Brake Chamber

Spring Brake Air Supply Service Brake Air Supply

36

Chapter 2  Principles of Braking

Relay Valve

Tank

FIGURE 2-29  The relay valve and separate air reservoir.

FIGURE 2-30  Exhaust brake butterfly operated by a pneumatic cylinder.

An exhaust brake works by restricting the flow of exhaust gases through the engine by closing a butterfly valve located in the exhaust manifold. Restricting the exhaust flow results in high pressure in the exhaust manifold and the engine’s cylinders. Because the engine acts as an air compressor, restricting the exhaust flow causes the engine speed to slow down, which in turn slows the road wheels through the transmission or powertrain (FIGURE 2-30).

Compression Brakes In heavy diesel vehicles, engine braking is less effective because more of the compression energy in the cylinders is returned to the crankshaft after the piston reaches top dead center. This results in an engine that wants to freewheel and does not create much force to slow the vehicle. A jake brake (or compression brake) uses an extra lobe on the camshaft to control an auxiliary exhaust valve at the top of each cylinder. When engaged, the jake brake releases the compression stroke pressure before it can be transmitted back to the power stroke of the piston. This causes a substantial amount of energy to be used on the compression stroke, which slows the crankshaft and increases the braking effectiveness of the engine. The disadvantage with this form of braking is that it is very noisy, producing a chattering machine-gun sound that usually requires additional muffling. It is loud enough that some municipalities ban the use of compression brakes in residential areas or other areas where noise would be an issue. Reactor Spring

Electric Brakes Hold Down Springs

Lever Arm

Drum Primary Shoe Wires

Secondary Shoe Electromagnet Adjuster Adjuster Spring

FIGURE 2-31  An electric braking system.

Trailers towed by light vehicles must have a braking system if their gross weight exceeds a certain value. An electric braking system is commonly used to activate drum-type friction brakes on the trailer (FIGURE 2-31). The driver can increase or reduce the braking effect by adjusting a control unit to suit the load on the trailer. This manual override can be used in certain driving conditions, to dampen a trailer’s tendency to sway. When the brakes in the towing vehicle are applied, the brake light circuit sends the signal to the control unit. The control unit then sends an appropriate current to the trailer brake actuators to operate the trailer brakes at the selected braking level. The electric current is supplied to the brake units at the wheels. The brake assembly uses lever-operated brake shoes. The lever incorporates an electromagnet on the end. The electric current flows through the electromagnet, creating magnetism that draws the magnet toward the spinning brake drum. The magnet contacts the drum and is pulled around, applying the brake shoes. If



Different Applications of Braking Systems

the driver steps harder on the brake pedal, the trailer controller sends a stronger electric current to the electromagnet, causing it to be held tighter to the spinning drum, which increases the braking force. In some places, trailers above a specified weight must also have an auxiliary battery-powered braking system that automatically applies the brakes and keeps them applied if the trailer ever breaks away from its towing vehicle.

Brake-by-Wire Braking Systems

37

Hydraulic Fluid Reservoir

Pedal Travel Simulator Pedal Displacement Sensor

Hydraulic Unit Block Valves

Electronic Control

Hydraulic Power Supply Accumulator

Motor M

“Brake by wire” means that there is no mechanical or hydraulic connection between the brake pedal and the wheel brake units. Pump P P P P There are two types of brake-by-wire systems: electric and elecU U U U trohydraulic (FIGURE 2-32). The electric type uses wheel brake P P U U P P P P units with an integrated motor system that applies clamping force to the disc brake pads. The electrohydraulic type uses a hydraulic controller to apply hydraulic pressure to standard Dividing Pistons Wheel Pressure Modulator with Pressure Sensor wheel brake units. The brakes, in either case, are controlled by Brake Pressure Sensor a computer. They use the brake pedal position and pedal application speed as indicators of the driver’s intentions. The comL R L R puter evaluates that information in light of other data, such as Front Brakes Rear Brakes vehicle speed and traction conditions. It then compares those data to preloaded information in memory and commands the FIGURE 2-32  Electrohydraulic brake-by-wire system. brake controller to create a specific amount of brake pressure at the appropriate wheel brake units. The driver can modify the programmed braking effort by depressing or releasing the brake pedal an additional amount. Most newer vehicles use this same principle to control the engine’s throttle (drive by wire), meaning that there is no mechanical connection between the throttle pedal and the throttle plate. On a throttle, it is easy to replicate throttle pedal feel because the pedal only works against spring tension. Integrating a spring with similar characteristics results in a throttle pedal that has a similar feel. The feel of a brake pedal is not as easily replicated because in addition to spring pressure, the driver is also pushing against hydraulic pressure, giving the pedal a much firmer feel. This problem can be overcome by using a brake pedal emulator, which is a sealed cylinder that builds pressure in a manner similar to that of a master cylinder. It also incorporates the sensors that send brake pedal data to the ECU. There are several benefits to brake by wire, including the following: ■■ ■■

■■

Weight is reduced and space is saved. Drivers do not feel ABS brake pulsations, making them less likely to remove their foot from the brake pedal during a panic stop. The brakes are applied more quickly in an emergency situation.

There are also several drawbacks, such as the risk of a system failure and the initial cost. Some current hybrid vehicles use brake-by-wire technology during regeneration, but with a hydraulic backup brake system. In this case, the brake-by-wire system initiates braking first, by increasing generator output, and once that is at its maximum output, it can also apply the hydraulic brakes to get additional stopping power, if needed.

Regenerative Brake Systems With the pressure to improve fuel economy, some manufacturers have equipped their hybrid vehicles with regenerative braking. Regenerative braking takes brake-by-wire technology to the next level. Instead of applying friction brakes and losing energy as heat, the brake-by-wire system uses the electric motor as a generator, which slows the vehicle by converting the vehicle’s kinetic energy into electrical energy. The amount of stopping power is controlled by how much electricity is being generated (FIGURE 2-33). The more stopping power needed, the more electrical output is demanded of the generator (up to its maximum

38

Chapter 2  Principles of Braking

FIGURE 2-33  A vehicle

display showing regeneration charging the high-voltage battery.

rated output) by the control system. The electricity is stored in the vehicle’s high-voltage battery and can then be used later by the electric motor to drive the vehicle. This type of system increases brake component life since the generator is doing most of the work of slowing the vehicle down, as well as it allows the driver to control the stop more efficiently. The regeneration process makes the vehicle more fuel-efficient, especially in stop-and-go traffic. At very low speeds, regenerative braking is disabled and hydraulic brakes provide all the braking.

▶▶Wrap-Up Ready for Review ▶▶

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Electronic brake systems use computer technology to assist with braking by monitoring speed and the force needed to stop. The development of electronic brake systems has led to improvements in consumer safety and fuel economy. Brake-by-wire systems use computer technology in place of the mechanical or hydraulic connection between the brake pedal and wheel brake units. Regenerative brake systems (used in hybrid vehicles) convert a vehicle’s energy into electrical energy, which is stored in the battery for later use. Factors affecting the effectiveness of braking systems include road surface, road conditions, vehicle weight and height, load on wheels, type of tire, and aggressive versus defensive driving. Every vehicle has two brake systems: the service brake (for stopping the vehicle in motion) and the parking brake (for holding the vehicle when stationary). Hydraulically operated braking systems use a system of cylinders to transfer pressure from the brake pedal to each wheel. The braking system converts a vehicle’s kinetic energy (the energy of an object in motion) into an alternate form of energy (e.g., heat). Acceleration and deceleration determine whether the vehicle’s speed is increasing or decreasing. Both require an

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outside force for action, such as the vehicle’s engine (acceleration) or braking system (deceleration). The law of conservation of energy states that energy must be transformed from one form to another; it cannot be created or destroyed. Standard brakes use friction to create resistance, thus transforming kinetic energy into heat energy. The amount of force pushing two surfaces together compared to the amount of resistive force generated between the two surfaces sliding against each other is called the coefficient of friction. Heat energy created by the braking process gets dissipated into the atmosphere. Reduction in a vehicle’s stopping power (brake fade) can be caused by heat fade, water fade, or hydraulic fade. Braking systems create rotational force on the vehicle’s suspension, resulting in weight transfer to the front wheels. There are three types of levers: lever of the first order (fulcrum in the middle), lever of the second order (load in the middle), and lever of the third order (effort in the middle). Disc brakes and drum brakes are two types of friction brakes that are used on lighter vehicles; both can be equipped with an ABS. Air-operated braking systems, used on heavier ­vehicles, use air pressure to apply the brakes (drum or disc style).

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Wrap-Up

An exhaust brake may be used in addition to friction brakes in medium and heavy vehicles, which creates ­engine braking by restricting the exhaust flow. Jake brakes are used in heavier diesel vehicles and rely on compression to slow the crankshaft and increase braking effectiveness. Trailers over a certain weight often use an electric braking system so that the driver can control braking.

Key Terms acceleration  An increase in a vehicle’s speed. air-operated braking system, also called air brakes  A braking system that uses compressed air operating on large-diameter diaphragms to provide force to the braking assembly. antilock brake system (ABS)  A safety measure for the braking system that uses a computer to monitor the speed of each wheel and control the hydraulic pressure to each wheel to prevent wheel lockup. band brake  A braking system that uses a metal band lined with friction material to clamp around the outside of a wheel or drum. brake-by-wire system  A braking system that uses no mechanical connection between the brake pedal and each brake unit. The system uses electrically actuated motors or a separate hydraulic system to apply brake force. brake fade  The reduction in stopping power caused by a change in the brake system, such as overheating, water, or overheated brake fluid. brake pedal emulator  A brake pedal assembly used in electronically controlled braking systems to send the driver’s braking intention to the computer; it mimics the feel of a standard brake pedal. brakes  A system made up of hydraulic and mechanical components designed to slow down or stop a vehicle. coefficient of friction  The amount of friction between two moving surfaces in contact with each other. conservation of energy  A physical law that states that energy cannot be created or destroyed. deceleration  The process of decreasing a vehicle’s speed. disc brakes  A type of brake system that forces stationary brake pads against the outside of a rotating brake rotor. drum brakes  A type of brake system that forces brake shoes against the inside of a brake drum. drum-style parking brake  A mechanically operated drum brake that can be set while the vehicle is not moving, to serve as a parking brake. electric braking system  A braking system used to provide braking to trailers; the drum brakes are electrically activated in the trailer when the driver applies the brakes on the tow vehicle. exhaust brake  A brake system that restricts the flow of exhaust gases through the engine by closing a butterfly valve located in

39

the exhaust manifold. Restricting the exhaust flow causes the engine speed to slow down, slowing the vehicle. friction  The resistance created by surfaces in contact. Kinetic friction is resistance to motion when one surface moves over another. Static friction is resistance to motion between two surfaces that are not moving. fulcrum  The point around which a lever rotates and that supports the lever and the load. heat fade  Brake fade caused by the buildup of heat in braking surfaces, which get so hot that they cannot create any additional heat, leading to a loss of friction. hydraulic fade  Brake fade caused by boiling brake fluid. This causes a spongy brake pedal. jake brake, also called a compression brake  A brake system that consists of an extra exhaust valve on a diesel engine, which releases compressed gases from the combustion chamber at the top of the compression stroke. kinetic energy  The energy of an object in motion. It doubles with weight and increases by the square of the speed. lever  A tool that allows the user to move a large load over a small distance at one end by applying a small force over a greater distance from the other end. master cylinder  Converts the brake pedal force into hydraulic pressure, which is then transmitted via brake lines and hoses to one or more pistons at each wheel brake unit. mechanical disadvantage  A situation in which the load distance on a lever is greater than the effort distance, which means the effort required to move the load is greater than the load itself. Newton’s first law of motion  A physical law that states that an object will stay at rest or uniform speed unless it is acted upon by an outside force. parking brake  A brake system used for holding the vehicle when it is stationary. parking brake cable  A mechanism used to transmit force from the parking brake actuating lever to the brake unit. rotational force  The force created by the rotating wheel when the brakes are applied; it causes the brake components to twist the brake support, and ultimately the vehicle, in the direction of wheel rotation. scrub brakes  A brake system that uses leverage to force a friction block against one or more wheels. service brake  A brake system that is operated while the vehicle is moving, in order to slow down or stop the vehicle. top-hat parking brake  A drum brake that is located inside a disc brake rotor in order to act as a parking brake. transmission-mounted parking brake  A drum brake that is mounted on the drive shaft, just after the transmission, to serve as a parking brake. water fade  Brake fade caused by water-soaked brake linings. weight transfer  Weight moving from one set of wheels to the other set of wheels during braking, acceleration, or cornering.

40

Chapter 2  Principles of Braking

Review Questions 1. Which of these is a simple mechanical system that uses leverage to force a friction block against one or more wheels? a. Drum brake. b. Band brake. c. Scrub brake. d. Disc brake. 2. All of the below statements are true except: a. Service brakes consist of drum and/or disc brakes and are operated by hand. b. The parking brake is used for holding the vehicle in place when it is stationary. c. In disc brakes, pads are forced against the outside of a brake disc. d. Modern braking systems are hydraulically operated. 3. If we triple the speed of an object, the kinetic energy will ____________. a. increase by three times b. increase by nine times c. decrease by three times d. increase by six times 4. The cycle of energy transformation in a typical vehicle is which of the following? a. Chemical energy → Heat energy → Mechanical energy → Kinetic energy b. Kinetic energy → Heat energy → Mechanical energy → Chemical energy c. Chemical energy → Mechanical energy → Heat energy → Kinetic energy d. Mechanical energy → Heat energy → Chemical energy → Kinetic energy 5. All of the following statements are true except: a. Heat transfers from a hot area to a cool area. b. Friction is the resistance created by surfaces in contact. c. Kinetic friction is resistance between non-moving surfaces. d. In drum brakes, the heat is created outside of the drum and transferred to the inner surface. 6. The kind of brake fade caused by the brake fluid becoming so hot that it boils is _________. a. heat fade b. water fade c. hydraulic fade d. hard fade 7. The brake pedal uses leverage __________. a. for weight transfer b. to multiply the force applied to the master cylinder c. to increase traction d. to decrease heat generation 8. Which of the following is used on heavy vehicles and is commonly called an air brake? a. A hydraulic control unit. b. A hydraulic brake. c. An air-operated braking system. d. An electric brake.

9. Medium-duty and heavy-duty vehicles often require increased braking in situations where friction brakes overheat and fail. This can be addressed by using a(n) _________. a. compression brake b. jake brake c. electric brake d. exhaust brake 10. Vehicles equipped with disc brakes incorporate a mechanically operated drum-style parking brake in the center of the rear disc brake rotors, commonly called a(n) _________. a. drum-style parking brake b. top-hat parking brake c. electric braking system d. transmission-mounted parking brake

ASE Technician A/Technician B Style Questions 1. Technician A says that regenerative braking converts brake heat from friction into electricity. Technician B says that regenerative braking converts electrical energy from the battery into braking energy. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that an antilock brake system (ABS) allows the front wheels to be steered during a panic stop. Technician B says that ABS sensors apply or release hydraulic pressure to the wheel brake units during ABS brake events. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that water-soaked brake shoes can be a cause of momentary brake fade. Technician B says that disc brakes dissipate heat faster than drum brakes. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that a disc brake operates by clamping friction materials to the outside of a disc. Technician B says that a drum brake operates by clamping friction materials to the outside of a drum. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that tire pressure does not affect braking. Technician B says that heavy vehicle loads increase stopping distance. Who is correct? a. Technician A b. Technician B



c. Both A and B d. Neither A nor B 6. Technician A says that service brakes on light-duty vehicle are typically applied hydraulically. Technician B says that parking brakes on light-duty vehicle are typically applied hydraulically. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that the heavier the vehicle, the more stopping power is needed. Technician B says that the faster a vehicle is moving, the more braking power is needed. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says that kinetic energy is created during braking to stop the vehicle. Technician B says that kinetic energy is converted to heat energy during braking. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

Wrap-Up

41

9. Technician A says that the brake pedal uses leverage to multiply foot pressure. Technician B says that when braking hard while moving forward, the vehicle’s weight transfers to the rear wheels, increasing their traction. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that friction brakes can fade due to overheating of the brake lining. Technician B says that friction brakes can fade due to overheating of the brake fluid. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

CHAPTER 3

Hydraulic Brake Systems Learning Objectives ■■ ■■ ■■ ■■

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3-1 Analyze the principles behind the hydraulic braking system. 3-2 Classify the hydraulic components in braking systems. 3-3 Explain master cylinder service. 3-4 Develop an understanding of how brake pedals help the operator of the vehicle. 3-5 Discuss brake line and hose usage.

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3-6 Summarize the purpose and function of hydraulic braking system controls. 3-7 Inspect brake lines, hardware, and hoses. 3-8 Examine the brake-warning-light system. 3-9 Diagnose stop-light operation.

You Are the Automotive Technician A customer brings in their 2011 Chevrolet Tahoe with 96,000 miles to the shop to have a brake concern addressed. The vehicle is regularly driven on the beach and has been pulling to the left, especially after braking. It has become worse over the last few weeks. You check the service history and see that the front brake pads were replaced, the rotors refinished, and the brake fluid flushed four years ago, at 54,000 miles. Your mentor technician wants to find out how much you know about braking systems before allowing you to start working on the vehicle. How will you answer their questions?

1. What is Pascal’s law, and how can it be used to help you to diagnose this problem? 2. What are the different types of brake fluid, and when should they be used? When should they not be used? 3. What are the purposes of the proportioning, metering, and pressure-differential valves? 4. What is the purpose of a tandem master cylinder, and how does it operate? 5. How does a vacuum brake booster operate?





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44

Chapter 3  Hydraulic Brake Systems

▶▶ Hydraulic 3-1 Analyze the principles behind the hydraulic braking system.

Braking System

Most automotive applications employ a hydraulic braking system because of the simplicity, adaptability, and reliability within the environment that the vehicle operates in. The abilities of hydraulics allow for the multiplication of force applied to the braking system so that even the smallest person is able to control a vehicle with ease. Developing an understanding of how hydraulics are applied to a vehicle will help with diagnosing hydraulic faults.

Pascal’s Law(s) In the 1600s, Blaise Pascal observed the effects of pressure applied to a fluid in a closed system. Pascal’s law states that pressure applied to a fluid in one part of a closed system will be transmitted without loss to all other areas of the system (FIGURE 3-1). This law is the principle behind hydraulic brakes. Pressure created in the master cylinder is transmitted through the hydraulic braking system as long as the system remains closed and has no leaks. In a closed system, hydraulic pressure is transmitted equally in all directions throughout the system. What happens to the pressure levels if there is a leak in the system? According to Pascal’s law, a substantial leak prevents the pressure from building up, and therefore, the pressure within the system will be equally low. This means that the vehicle may lose some or all of its braking ability if a leak develops. Pascal’s law helps in diagnosing problems with the hydraulic braking system. For example, if the brake pedal is squishy (soft or spongy), there is a good chance that the hydraulic braking system has air in it and has to be bled. If the brake pedal slowly sinks to the floor, there is likely a small leak in the system that must be found. If the vehicle pulls to one side, it could be that a brake hose is plugged up and is not transmitting pressure to one of the brake units. Knowing that pressure should be equal throughout the system helps identify the cause, based on the reaction of the braking system.

Hydraulic Pressure and Force Varying amounts of mechanical force can be extracted from a single source of hydraulic pressure. Because pressure is force per unit area (e.g., 50 psi, or 344.7 kPa), the same pressure applied over different-sized surface areas will produce different levels of force (FIGURE 3-2). This principle allows engineers to design brakes to have a precise amount of braking force at each wheel. For example, the front wheels on some front-wheel drive vehicles can produce up to 80% of the vehicle’s stopping power because of the weight distribution and weight transfer. For these vehicles to brake smoothly, more pressure must be applied to the front brake units than the rear brake units. This is accomplished through

Drum Brake Assembly

Master Cylinder

Booster Brake Pedal

Proportioning Valve Caliper

Disc FIGURE 3-1  A schematic view of a hydraulic brake system.



Hydraulic Braking System

45

100 lb

600 lb

200 lb

FIGURE 3-2  Engineers apply 300 lb

hydraulic principles to create varying amounts of mechanical force in hydraulic braking systems.

the front and rear brake pistons. The larger brake pistons on the front wheels give greater mechanical force and braking power to the front wheels.

Input Force, Working Pressure, and Output Force Figure 3-2 illustrates a hydraulic system that has cylinders of different sizes. When the brake pedal is pressed, the force against the piston in the master cylinder applies pressure to the brake fluid. This same pressure is transmitted equally throughout the fluid, but each output piston develops a certain amount of output force, depending on its diameter (surface area). The top cylinder is smaller than the master cylinder, so the amount of output force it exerts will be less than the force applied to the master cylinder piston. The middle cylinder is the same size as the master cylinder, so the output force will be the same. The bottom cylinder is larger than the master cylinder, and so its output force will be greater. There are three variables to consider when talking about pressure and force in hydraulic systems (FIGURE 3-3): ■■

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Input force: The force applied to the input piston is measured in pounds (lb), newtons (N), or kilograms (kg). For example, if 100 lb (45.36 kg) of force were applied to the input piston, this force would be labeled as 100 lb (45.36 kg). Working pressure: The working pressure of the hydraulic fluid is expressed as the amount of force per specified area. For example, 100 lb of force per square inch is labeled as 100 psi. It could also be expressed as 689.5 kilopascals (kPa). Note that 1 pascal = 1 newton per square meter, or 1 N·m2. To find the working pressure, divide the input force by the area of the input piston. The example of 100 lb (45.36 kg) of force applied to a 1 sq in. piston creates 100 psi of working pressure. The same 100 lb of force applied to a 0.5 sq in. piston creates 200 psi (1,379 kPa) of working pressure (100/0.5 = 200 psi). Conversely, the 100 lb of force applied to a 2 sq in. piston creates 50 psi (344.7 kPa) of working pressure.

Working Pressure = Input Force/Input Piston Working Pressure = 100lb/1 in2 Working Pressure = 100 psi

▶▶SAFETY TIP It is critical that the hydraulic portion of the brake system not have any leaks or weak spots that could fail and cause leaks. Failure of the braking system could occur, putting the driver, passengers, and others in danger.Vehicles operated in corrosive environments, such as in areas where salt and certain de-icers are used, are susceptible to brake-line corrosion. Inspect all vehicles carefully for potential brake-fluid leaks.

▶▶TECHNICIAN TIP Input force, output force, and working pressure are optimized during the design of the hydraulic system, based on a specific vehicle application.This is one reason why it is never acceptable to arbitrarily substitute hydraulic components from another vehicle.

Output Pistons S/A = 3 in2 300 lb S/A = 2 in2 200 lb

Input Force

Output Force

S/A = 1 in2 100 lb

100 lb S/A = 0.5 in2 Input Piston Surface Area (S/A)= 1 square inch (in2)

50 lb S/A = 0.25 in2 25 lb Output Force = Working Pressure x Output Piston

FIGURE 3-3  Working pressure and output force depend on the size of the pistons and the input force.

46

Chapter 3  Hydraulic Brake Systems ■■

▶▶TECHNICIAN TIP Even though DOT 3 and DOT 4 have similar base composition, brake fluids with different Department of Transportation (DOT) ratings should not be mixed.

▶▶SAFETY TIP Because silicone-based fluid tends to aerate (tendency to create air bubbles) when forced at high pressure through small passages, it is not to be used in any vehicle equipped with an antilock brake system (ABS). The control valves in ABS brakes would cause DOT 5 brake fluid to aerate under an active ABS stop. This would lead to a spongy pedal and poor brake application.

▶▶TECHNICIAN TIP If the brake system needs to be bled, then the brake fluid should be tested to determine whether it needs to be flushed out and replaced with new brake fluid.

Output force: Output force is exerted by the output piston and is expressed as pounds, newtons, or kilograms. Finding this measurement is fairly simple: Multiply the working pressure by the surface area of the output piston. For example, 200 psi (1,379 kPa) of working pressure pushing on a 1 sq in. piston exerts 200 lb (90.72 kg) of force. The same 200 psi of working pressure acting on a 0.5 sq in. output piston creates 100 lb (45.36 kg) of output force. And if 200 psi of working pressure is applied to a 2 sq in. output piston, 400 lb (181.44 kg) of force will be created.

Brake-Fluid Types and Characteristics Brake fluid is hydraulic fluid that has specific properties designed for mobile applications. It is used to transfer force while under pressure through hydraulic lines to the wheel braking units. Braking applications produce heat, so the brake fluid used must have a high boiling point to remain effective under extreme temperatures. If brake fluid boils, it turns from a liquid to a vapor, which is compressible. This causes a spongy brake pedal and a loss of braking ability. Brake fluid must also have a low freezing point so that it will not freeze or thicken in cold conditions. If this were to happen, the force from the brake pedal would not be transferred to the wheel brake units. Standard brake fluid is harmful to painted surfaces because it tends to soften paint; it must therefore be kept off all painted surfaces. Standard brake fluid is also hygroscopic, which means it absorbs moisture. It can absorb moisture from the atmosphere when it comes into contact with the air in the master cylinder reservoir. Over time, it can even absorb moisture through the flexible brake hoses. Because water boils at a lower temperature than brake fluid does, this will gradually reduce the brake fluid’s boiling point, making the fluid more likely to boil and cause a hydraulic braking failure. Brake fluid must be flushed periodically to replace old contaminated fluid with new brake fluid to ensure the continued effectiveness of the hydraulic braking system. Brake fluids are graded against compliance standards set by the DOT (TABLE 3-1). Brake fluids that meet these standards qualify for the DOT rating and are considered to be quality brake fluids. Brake fluids are tested to ensure they meet the standards for the following: ■■ ■■ ■■ ■■ ■■

pH value viscosity resistance to oxidation stability boiling point.

▶▶ Hydraulic

Components in Braking Systems

The hydraulic system is made up of a number of components that work together to transmit the driver’s effort to the brake pads or shoes. These components must be able to withstand the force, pressure, and temperatures generated in the brake system. They must also be protected from the elements, because they are generally exposed to weather and road hazards. This section explores each of the basic components of the hydraulic ­braking system.

Master Cylinder 3-2 Classify the hydraulic components in braking systems.

The master cylinder converts the brake pedal force into hydraulic pressure that is transferred to the wheel brake units (FIGURE 3-4). Its mounting ears allow it to be mounted to a power booster or the bulkhead. The cylinder itself has threaded passageways to which the brake lines firmly connect, and a brake-fluid reservoir supplies the brake system with an adequate amount of brake-fluid. The master cylinder piston is operated by a pushrod from the power booster or the brake pedal. Although all vehicles manufactured for sale in the United States are required to use tandem master cylinders for safety purposes, we start our discussion with the less complicated single-piston master cylinder.



Hydraulic Components in Braking Systems

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TABLE 3-1  DOT Ratings for Brake Fluid Type

Materials

Minimum Dry Boiling Point

Minimum Wet Boiling Point

Additional Specifications

DOT 2

Castor oil based

Not specified

Not specified

• Outdated type of brake fluid that should not be used in any modern vehicles

DOT 3

Various glycol esters and ethers

401°F (205°C)

284°F (140°C)

• Good all-around brake fluid • Harms paint • Absorbs water

DOT 4

Various glycol esters and ethers

446°F (230°C)

311°F (155°C)

• Similar to DOT 3, but higher boiling point • Compatible with DOT 3 and 5.1

DOT 5

Silicone based

500°F (260°C)

356°F (180°C)

• Not hygroscopic: will not absorb moisture • Less harmful to painted surfaces than glycol-based brake fluids • Provides better protection against corrosion • More suitable for use in wet driving conditions • Not to be used in any vehicle equipped with ABS

DOT 5.1

Contains polyalkylene glycol ether

500°F (260°C)

375°F (190.6°C)

• Suitable for ABS-equipped vehicles because of its high boiling point • More expensive than other brake fluids • Compatible with DOT 3, 4, and 5.1

Source: United States Department of Transportation, Federal Motor Carrier Safety Administration: S5.1.2 Wet ERBP

Single-Piston Master Cylinder Single-piston master cylinders have one piston with two cups: a primary cup and a secondary cup (FIGURE 3-5). These cups are also known as seals because they keep the brake fluid from leaking past the piston. When force is applied to the piston by the pushrod, the primary cup seals the pressure in the cylinder while the secondary cup prevents loss of fluid past the rear end of the piston. An outlet port links the cylinder to the brake lines. An inlet port connects the reservoir with the space around the piston and between the piston cups. A compensating port connects the reservoir to the cylinder, just barely ahead of the primary cup. With the brake pedal in the released position, the compensating port connects the brake system with the reservoir. The compensating port adjusts for changes in the volume of the brake fluid ahead of the piston. This occurs due to the expansion or contraction of the brake fluid as it heats up or cools down. It can also compensate for brake fluid that does not return to the master cylinder, due to worn disc brake pads moving the caliper pistons outward, increasing the brake-fluid volume in the caliper. In this way, brake fluid can move as needed between the reservoir and the master cylinder bore. Compensating Port

Compensating Port

Inlet Port

Inlet Port

Master Cylinder Outlet Port Primary Seal

Residual Pressure Valve

Piston

Secondary Cup

Outlet Port Primary Seal

Residual Pressure Valve

Piston

Secondary Cup

FIGURE 3-4  The master cylinder converts the driver’s effort into

FIGURE 3-5  A single-piston master cylinder with primary and

hydraulic pressure.

secondary cups.

48

Chapter 3  Hydraulic Brake Systems

As the pushrod from the brake pedal or power booster moves the piston forward, the compensating port is closed off, trapping brake fluid ahead of the primary cup. Fluid can no longer return to the reservoir. Fluid trapped in the Housing cylinder is then forced from the master cylinder outlet port into the brake lines. Boot When the brakes are released, the master cylinder piston returns to its original position by action of the spring. When the piston fully returns against its stop, the primary cup uncovers the compensating port. Fluid ahead of the primary cup can now return to the reservoir as needed. Return When the brake pedal is released quickly, a spring in the brake pedal pushes the Spring piston back quickly. However, because of the restrictions in the hydraulic system, the brake fluid cannot return as quickly to the cylinder, creating a low-pressure area ahead of the primary cup. As a result, air can be drawn into the system at the wheel Piston cylinders on a drum brake system. To prevent this, small holes are drilled into the piston so that brake fluid from the reservoir can pass through the inlet port and past FIGURE 3-6  A single-piston master cylinder with the edge of the primary cup, thus preventing a vacuum from being created. This is small holes into the piston to allow for recuperation. called recuperation (FIGURE 3-6). On a drum brake system, when the brake fluid in the lines returns to the master cylinder, brake-fluid pressure is held slightly above atmospheric pressure by a valve called the residual-pressure valve (or residual-check valve). The residual pressure helps to stop air from entering at the wheel cylinder cups when the brakes are not being applied. The residual-pressure valve is located at the outlet end of the master cylinder on single-piston master cylinders or under the tube seats on tandem master cylinders. Residual-pressure valves are not used on disc brake circuits, because the caliper piston seal seals tightly between the piston and bore so that air cannot be easily drawn past it into the hydraulic system. Also, the small residual pressure would keep the brake pads slightly applied against the brake rotors, causing brake drag, premature wear, and reduced fuel economy. Filler Cap

Reservoir

Tandem Master Cylinder With a single-piston master cylinder in the braking system, any fluid leak could mean that the whole braking system will fail. To reduce this risk, modern vehicles must have at least two separate hydraulic braking systems—hence the development of the t­ andem master cylinder. If one system fails, the other system can still provide a measure of ­braking ability, though it will not be as effective. For example, the pressure can be 0 psi (0 kPa) in one circuit and normal in the other circuit. Tandem master cylinders combine two master cylinders within a common housing that share a common cylinder bore (FIGURE 3-7). Like two single-piston cylinders built end to end, a tandem cylinder has a primary piston and a secondary piston. The primary piston is in the rear of the cylinder. It is called primary because it is pushed directly by the pushrod. The secondary piston is in the front of the cylinder. The secondary piston has a rear-facing seal that seals fluid in the primary chamber and is what ultimately pushes the secondary piston during normal operation. Each half of the cylinder has an inlet port, an outlet port, and a compensating port. There can be two separate reservoirs feeding each half of the cylinder or just one reservoir divided into separate ­sections. Dividing the reservoir prevents all of the fluid from draining out in the event of a leak. This keeps a reserve amount of fluid for the working half of the master cylinder in case of a leak. When the brakes are applied, the primary piston moves forward and closes its compensating port. Continued movement of Secondary Primary the piston causes fluid pressure in front of the primary piston to Piston Piston rise, which acts upon the secondary piston, moving it forward and closing its compensating port (FIGURE 3-8). Pressure now builds up equally in both circuits of the master cylinder. Both pisMaster Cylinder tons continue moving forward, displace fluid into their separate circuits, and apply the brake units on each wheel. FIGURE 3-7  A tandem master cylinder showing a common cylinder.



Hydraulic Components in Braking Systems

49

Compensating Port (closed) Compensating Port

Secondary To Piston To Rear (moved by primary Front Brakes pressure not yet building secondary Brakes pressure)

Primary Piston (past compensation port and just starting to build pressure)

To Rear Brakes

Secondary Piston (bottomed on housing)

To Front Brakes

Primary Piston

FIGURE 3-8  Moving the primary piston forward closes the

FIGURE 3-9  If there is a leak in the secondary circuit,

compensating port and pushes the secondary piston forward, closing its compensating port.

the secondary piston travels to its stop, and then pressure builds in the primary circuit.

Just like the single-piston master cylinder, the tandem master cylinder can have problems with a low-pressure area developing when the piston returns quickly but the brake fluid lags. The tandem master cylinder overcomes this by using holes in the piston and grooves in the side of the primary cup. These primary cup grooves allow brake fluid to flow from the inlet port into the low-pressure area, preventing air from entering the system. If there is a failure in the secondary circuit, the primary system pushes the secondary piston until it contacts the end of the cylinder bore (FIGURE 3-9). Once that happens, the primary circuit can start building pressure and operate its brake units. In this situation, it operates, but with increased pedal travel. If the primary circuit fails, no pressure is generated to move the secondary piston; thus a rod attached to the front of the primary piston pushes the secondary piston directly so that it is still able to generate pressure to operate its brake units (FIGURE 3-10). This also results in a lower-than-normal brake pedal. A differential pressure switch in the master cylinder or hydraulic system can illuminate the brake warning light on the instrument panel, alerting the driver to a brake system loss of pressure between the two hydraulic circuits.

Quick Take-Up Master Cylinders

To Rear Brakes

Secondary Piston

To Front Brakes

Primary Piston (bottomed out on secondary piston)

FIGURE 3-10  If there is a leak in the primary circuit, the primary piston travels until it contacts the secondary piston, moving the secondary piston and building pressure in the secondary circuit.

Quick take-up master cylinders are used on disc brake systems that are equipped with low-drag brake calipers. These calipers are designed to maintain a larger running clearance between the disc brake pads and rotor. If a standard master cylinder were used, the brake pedal would have to be pushed much farther down before the running clearance would be overcome and the pads would contact the rotor. Hence, a quick take-up master cylinder was designed. It uses a relatively large-diameter piston in the rear of the cylinder to push a large volume of fluid into the hydraulic system at low pressure (FIGURE 3-11). This moves the brake pads into contact with the rotor. Once the pressure rises above a predetermined point, a quick take-up valve opens and bleeds off any extra pressure created by the large piston. At this point, a smaller diameter piston takes over and builds pressure within the hydraulic system to apply the brakes normally.

ABS Master Cylinders The ABS master cylinder is a tandem master cylinder used in divided systems. It has a primary piston and a secondary piston. It may also incorporate the quick take-up principles of operation. In some applications, the compensating port in the secondary chamber

50

Chapter 3  Hydraulic Brake Systems

Atmospheric Seal

Reservoir

Fast Fill Valve

Fill Port Compensating Port Primary Seal

Secondary Seal To Secondary To Rear Piston Front Brakes Brakes

Secondary Seal Primary Piston Primary Seal

FIGURE 3-11  A quick take-up master cylinder with a larger diameter on the rear of the primary piston.

▶▶TECHNICIAN TIP The ABS pedal pulsation has been blamed for actually causing some accidents. Because the ABS system is normally activated only in a panic stop situation, drivers who are unfamiliar with the pedal pulsation have been known to lift their foot off the brake pedal, causing an accident. It is good for drivers to familiarize themselves with the feel of the ABS pedal pulsation by activating the ABS in a safe location, such as an abandoned parking lot. To address this issue, manufacturers have started to move toward electronic braking (brake by wire), which prevents hydraulic pulsations from being transmitted to the brake pedal.

FIGURE 3-12  An ABS master cylinder.

is removed, which means in this case that there is only an inlet port on the secondary piston. The primary chamber still uses a compensating port and an inlet port. The secondary piston incorporates a center valve that controls the opening and closing of a supply port drilled into the piston. At rest, the supply port is open and connects the reservoir with the front brake circuits. This supply port replaces the compensating port in a normal master cylinder (FIGURE 3-12). The primary piston still has an inlet port and a compensating port. When the brake pedal is applied, the primary piston moves and closes its compensating port. Brake-fluid pressure in the primary circuit rises and acts with the primary piston spring to move the secondary piston forward, closing the center valve. The pressure builds in the secondary circuit. Pressure keeps building in both circuits and applies force in both circuits. If there is a leak in either circuit, the master cylinder acts like a standard master cylinder and builds pressure in the working circuit. When the brake pedal is released quickly, recuperation happens, similar to what ­happens with a tandem master cylinder. When the primary piston is returned fully, any extra brake fluid returning from the wheel brake units displaces brake fluid into the reservoir through the compensating port. In the secondary circuit, the inlet port connects with the supply port drilling in the piston. Any difference in pressure lifts the center valve from its seat and lets the brake fluid enter the chamber ahead of the secondary seal, thus preventing low pressures from developing. When the piston has returned to the “rest” position, the seal is pulled off its seat by the action of the link and spring. This lets the brake fluid still returning from the wheel brake units displace the brake fluid back to the reservoir. If braking conditions are such that the hydraulic modulator must return brake fluid to the master cylinder, then for the front brake circuits, brake fluid is returned to the front section. This forces the secondary piston back against the force of the primary piston spring and the rear brake pressure and pushes both pistons rearward. If enough brake fluid returns, the center valve opens and allows brake fluid to return to the reservoir. If brake fluid is returned from the rear brake circuit, the secondary and primary pistons tend to be forced apart, causing the primary piston to be driven rearward. If enough brake fluid returns, the compensating port is uncovered and allows brake fluid to return to the reservoir. The amount of brake fluid that returns to the master cylinder is determined by the degree of ABS control. The driver may be aware of a rising brake pedal during this time.

Reservoirs and Float Switches Master cylinder reservoirs can be built into the master cylinder housing or can be a separate unit. Built-in reservoirs are made of the same material as the master cylinder, which is



Hydraulic Components in Braking Systems

FIGURE 3-13  Master cylinder with built-in reservoir.

FIGURE 3-14  Two-piece master reservoir.

usually aluminum or cast iron, and are formed on top of the master cylinder (FIGURE 3-13). The reservoir cover on these master cylinders must be removed to inspect the brake-fluid level. The cover uses a rubber diaphragm to isolate the brake fluid from the air. These c­ overs are usually held on by bail clips or tabs molded into the cover. On two-piece master cylinders, the reservoirs are usually made of a see-through ­plastic material and use grommets or O-rings to seal them to the master cylinder (FIGURE 3-14). Because they are see-through, it is usually unnecessary to remove the cover to check the brake-fluid level. The covers can be screw-on caps or clip-on covers. The caps usually ­incorporate a device to minimize contact of the brake fluid with air. Some systems use a disc that floats on the brake fluid to minimize the surface area of the brake fluid in contact with the air. To ensure that a leak in one brake circuit does not affect the other circuit, master cylinder reservoirs have two separate chambers. These reservoirs can be two totally separate chambers, or they can use a divider in a common reservoir, which keeps the brake-fluid level from falling below a minimum amount. Most modern master cylinder reservoirs are equipped with a float switch for the brake-fluid level, which turns on the red brake warning light on the instrument panel and/ or sets a notification on the driver information system when brake-fluid level is low. The warning system can be activated by the float directly, or a float with an embedded magnet may activate a switch when the float falls to a certain level (FIGURE 3-15). The brake-fluid From Ignition

Fluid Filler Cap

Fluid Filler Cap

▶▶TECHNICIAN TIP Master cylinder reservoirs should always have air space at the top of the reservoir to allow for the expansion of brake fluid as it heats up. Never fill master cylinder reservoirs all the way to the top.

From Ignition

Warning Lamp High

High

Low

Low Float

Float Reservoir

Magnet

Reservoir

Reed Switch (open) To Master Cylinder

51

Magnet Reed Switch (open)

To Master Cylinder

FIGURE 3-15  A magnetic switch is used to activate the brake warning light when the fluid level drops to a specified level.

52

Chapter 3  Hydraulic Brake Systems

level could be low due to worn brake pad linings or a leak in the system; both situations require further investigation. Adding brake fluid without further investigation into what is causing the low-fluid condition could lead to a brake failure, putting the driver and occupants in danger.

▶▶ Master 3-3 Explain master cylinder service.

Cylinder Service

Removing and bench bleeding the master cylinder are usually performed only when the master cylinder is being replaced. Bench bleeding makes bleeding the hydraulic system much easier by removing all of the air from the master cylinder. Failure to bench bleed the master cylinder prior to installing it into the hydraulic brake system will prevent the master cylinder from building much brake-fluid pressure. As a result, very little brake fluid will be bled out of each wheel cylinder or caliper, increasing the time it takes to bleed the hydraulic brake system. Also, it will push a lot of air through the brake system, making it more likely that air will get stuck in the system. To remove, bench bleed, and reinstall the master cylinder, follow the steps in SKILL DRILL 3-1.

SKILL DRILL 3-1 Performing Master Cylinder Service and Bench Bleeding 1. Compare the new unit to the old one to verify that it is the correct replacement. Remove all of the old brake fluid from the master cylinder reservoir. Remove the master cylinder brake lines, using a flare wrench and, if necessary, the double-wrench method.

2. Remove the nuts holding the master cylinder to the power brake booster. Remove the master cylinder. Mount the master cylinder in a vise with the reservoir facing up. Now is a good time to perform the next task: measure and adjust pushrod length.

Continued



Master Cylinder Service

3. Install the bleeder lines into the master cylinder outlet ports with the ends of the lines deep within the master cylinder reservoir. Fill the reservoir about half full with clean brake fluid. With an appropriate tool, slowly push the master cylinder piston into the bore. Allow the piston to return to its rest position. Repeat until all the air bubbles have been removed from the master cylinder.

4. Place the master cylinder on the power booster, and install the nuts just far enough to hold it from falling off.

5. Carefully line up the brake lines, and start them by using just fingers to thread them into the master cylinder outlet ports at least four or five threads from when they first catch. Do not use a wrench to start the threads.

6. Tighten the bolts connecting the master cylinder to the power booster to their proper torque. Tighten the brake-line fittings by using a flare nut or line wrench. Use the double-wrench method if the master cylinder is fitted with an adapter. Fill the master cylinder to the full mark, and bleed all wheel brake units to remove any remaining air. Start the engine, and check brake pedal feel to verify proper height and firmness. Visually inspect all fittings and bleeder screws for leaks, and rinse off all spilled brake fluid with water.

53

54

Chapter 3  Hydraulic Brake Systems

Measuring and Adjusting Master Cylinder Pushrod Length The master cylinder pushrod length is critical for proper brake operation. If it is too short, the driver will have to depress the brake pedal further than specified to operate the brakes. This reduces the amount of reserve pedal available in the case of a hydraulic leak. If the pushrod length is too long, it could prevent the master cylinder piston from returning far enough to uncover the compensating ports, trapping brake-fluid pressure in front of the pistons and holding the brakes in the applied position. As important as pushrod length is, normally it does not require adjustment, since it is locked in place. The situations that would call for adjusting it are as follows: Someone changed the adjustment setting; the brake pedal linkage has been repaired or adjusted; or the power booster is being replaced. Just changing the master cylinder does not normally require pushrod length adjustment. To measure and adjust master cylinder pushrod length, follow the steps in SKILL DRILL 3-2.

SKILL DRILL 3-2 Measuring and Adjusting Master Cylinder Pushrod Length 1. Remove the master cylinder, following the specified procedure if still installed on the power booster. Ensure the brake pedal is fully released and not bound up. Using the specified tool, measure the master cylinder pushrod length.

2. If the length is incorrect, loosen the locknut.

3. Adjust the pushrod to the proper length.

Continued



Brake Pedals Help the Driver

55

4. Retighten the locknut and recheck the pushrod length. Reinstall the master cylinder and check pedal free play, pedal height, and reserve pedal.

▶▶ Brake

Pedals Help the Driver

The brake pedal uses leverage to multiply the effort from the driver’s foot to the master cylinder. Different lever designs can be engineered to alter the brake pedal effort required of the driver by using different levels of mechanical advantage. Brake pedals should be mounted securely, free from any excessive sideways movement, and at a height and angle that will allow the driver to quickly move from pressing the accelerator (throttle pedal) to applying the brakes. Brake pedals are covered with a rubber nonslip cover to maintain sure footing. These covers can become worn and lead to slippage, so they need to be inspected periodically. The brake pedal is usually suspended from a bracket between the dash panel and the firewall (FIGURE 3-16). It works as a force-multiplying lever. The pushrod transmits the brake pedal force either directly to the master cylinder or to the power booster. If the power assist fails, the brake pedal’s leverage is designed to allow the driver to still generate a reasonable braking force at each wheel brake unit, but with substantially increased foot pressure. Brake pedals must be free to return to their starting position when pressure is removed. This allows the master cylinder piston and pushrod to return to their undepressed position. The pedal is enabled to return to its starting position by a return spring. The spring action also causes the brake pedal to push the brake-light switch open and stop current flow to the brake lights. When the brake pedal is applied, a lighter spring in the brake-light switch causes the brake-light switch contacts to close and activate the brake lights. Brake-light switches are adjustable on some vehicles. Misadjusted brake-light switches can cause the brake lights to stay on when they should be off or to not come on when they are supposed to. Both situations can be corrected by adjusting the switch to the proper position. If the switch is nonadjustable, it will likely have to be replaced if it is not operating correctly. ABS Master Cylinder Pivot

FIGURE 3-16  Brake pedal assembly.

3-4 Develop an understanding of how brake pedals help the operator of the vehicle.

▶▶TECHNICIAN TIP Changes to how far the pedal travels or to its resistance—whether it feels harder or softer than normal—can indicate problems such as a faulty power booster or air in the hydraulic system due to a leak.

▶▶TECHNICIAN TIP When ABS brakes are activated during heavy braking, the pulsations of the system can be felt by the driver, through the pedal. This is normal. However, a pulsating pedal during normal or light braking can indicate potential braking system problems, such as a rotor that has thickness variation beyond the manufacturer’s specifications or that is possibly warped.

56

Chapter 3  Hydraulic Brake Systems

A

B

C

FIGURE 3-17  An adjustable pedal system. A. Low position. B. High position. C. Activating switch.

Some vehicles come equipped with adjustable pedal assemblies. Such assemblies allow the driver to raise or lower the brake and throttle pedal assembly for personal comfort. These are usually adjusted by electrically driven motors that are operated by a switch on the steering column or dash (FIGURE 3-17).

Adjustable Brake Pedal System Some vehicles come equipped with adjustable pedal assemblies. Such assemblies allow the pedals to be moved closer or further away from the driver. These are usually adjusted by electrically driven motors that are operated by a switch on the steering column or dash. This type of feature is remembered by the body module and can be set to automatically be adjusted with the seat position; it can be automatically adjusted with the mirrors as well if the memory feature is used. The purpose of these types of comfort features is to allow the vehicle to adapt to the size of the driver. Along with driver comfort, it allows for safer operation of the braking system since the driver is not overexerting themselves to properly operate the brakes.

▶▶ Brake-Line 3-5 Discuss brake line and hose usage.

and Hose Usage

To transfer the pressure that has been created within the master cylinder to the calipers and wheel cylinders, using brake lines and brake hoses allows that pressure transfer happen. Hydraulic pressure is a result of the fact that liquids are incompressible, so they can transfer the pressure exerted on them to the surrounding contact area. This area is the brake line and brake hose, which then forces the pressure toward the caliper, and wheel cylinder causes them to move. Without these components, hydraulic brakes would not operate.



Brake-Line and Hose Usage

57

Brake Lines Brake lines and hoses carry brake fluid from the master cylinder to the brake units. They are basically the same on all brake systems and passenger vehicles. For most of their length, they are double-walled steel, coated to resist corrosion, and attached to the body with clips or brackets to minimize damage from vibration (FIGURE 3-18). In some vehicles, the brake lines are inside the vehicle to better protect them from corrosion and physical damage. Where the lines must move, flexible brake hoses allow for steering and suspension movement (FIGURE 3-19).

Brake-Line Materials The brake lines must be able to transmit considerable hydraulic pressure, 1,500 psi (10,342 kPa) or more during panic stops. They are made of seamless, double-walled steel rather than a softer, but less corrosive and easier-to-form, material such as ­copper. They also must conform to applicable standards, such as those set by the Society of Automotive Engineers. Only brake lines that meet those standards can be used on vehicles. If a brake line is damaged, it is common practice to replace the entire brake line rather than repair it. Also, universal brake lines are available in a variety of lengths that are already factory flared with the correct fitting installed. They just need to be formed with the proper tubing bender to the specified shape, following the original brake-line routing. Avoid kinks by using only the correct tubing bender. Kinked lines cannot be used or repaired.

Types of Brake-Line Flares Because brake lines connect all of the various hydraulic-braking-system components, their ends must make leak-proof connections. This is accomplished by using the following types of flared lines and matching fittings: ■■

■■

Inverted double flare: This type of flare is created by first flaring the end of the tube outward in a Y shape. Then about half of the flared end is folded inside of itself (inverted), leaving a double-thick section of brake line on the flared portion of the Y (FIGURE 3-20). The flared portion of the tube is clamped between the mating surfaces of the two fittings to provide a secure, leak-proof connection when performed properly. International Standards Organization (ISO) flare: This type of flare is sometimes called a “bubble flare.” The brake line is flared slightly out and then back in, leaving the brake line “bubbled” near the end (FIGURE 3-21). The bubble is then clamped between two matching fittings.

FIGURE 3-18  Steel brake line.

▶▶SAFETY TIP Never substitute a copper, aluminum, or non-approved line for the original; doing so could lead to a brake failure, which the technician could be held liable for.

▶▶TECHNICIAN TIP Many brake and fuel lines screw into an adapter. Always use a double-wrench method with flare nut wrenches when loosening and tightening brake and fuel lines. This technique will help to prevent twisting the steel lines (FIGURE 3-22).

▶▶TECHNICIAN TIP Flared fittings form a seal by tightly compressing the brake line between the two halves of the fitting. No sealer is needed—nor should it be used—on these types of fittings.

FIGURE 3-19  Flexible brake hose.

58

Chapter 3  Hydraulic Brake Systems

A

B

FIGURE 3-20  An inverted double-flared line and matching fitting.

A

B

FIGURE 3-21  A. An ISO flared line. B. A matching fitting.

FIGURE 3-22  Doublewrench method.

Brake Hoses A flexible section of the brake lines must be included between the body and suspension to allow for steering and suspension movement. This is accomplished by using flexible hoses made of tough, reinforced tubing. These flexible brake hoses transmit the brake system hydraulic pressures to the wheel units (FIGURE 3-23). They also must be tough so as not to



Brake-Line and Hose Usage

59

be damaged easily by objects thrown by the tires or other road hazards. When replacing brake hoses, always ensure that they are of the proper length. If they are too short, they can be damaged by being stretched. If they are too long, they can contact moving components such as tires or a suspension member, which could weaken the hose or wear a hole in it. This is especially common when a vehicle has a lift kit installed.

Brake Hose Materials Brake hoses are made of several layers of alternating materials. Inside it is a liner that helps seal the brake fluid in and any moisture out. The liner is wrapped with two or more layers of flexible webbing, which are usually embedded in a synthetic rubber material and provide reinforcement for the hose. These layers are covered with a tough flexible outer housing jacket designed to resist abrasion and damage (FIGURE 3-24). Although brake hoses are designed to be flexible, they should never be pinched, kinked, or bent tighter than a specified radius. Doing so reduces their life and can cause failure of the brake hose. Some technicians mistakenly use vise-grip pliers to crimp a brake hose while disconnected from the caliper or wheel cylinder to prevent brake fluid from leaking out. This practice can damage the brake hose and should be avoided. Brake hoses should be inspected periodically for damage or defects (FIGURE 3-25). Some possible issues include the following: ■■

■■

■■

■■

■■

Cracks: The outer layers become brittle and crack over time. Replacement is required. Bulges: The reinforcing layers become weak and break over time, resulting in FIGURE 3-23  A brake hose transmits bulges in the outer cover. Replacement is required. hydraulic pressure to the moveable brake unit. Abrasion or wear: This usually happens because the brake hose was routed incorrectly or was too long or short for the application, and it rubbed on a component, resulting, over time, in the abrasion. Replacement and rerouting are required. ▶▶TECHNICIAN TIP Kinks: Kinking usually happens when the brake hose has been pinched with vise grips or twisted on installation. Replacement is required. Never hang a disconnected brake caliper Internal deterioration: This causes blockage of the passageway. Replacement is required. by its flexible brake hose. Doing so can damage the brake hose. Always use a piece of wire or other material (e.g., cable or zip tie) to support the weight of the caliper assembly while it is hanging on a suspension or other suitable component.

Protective Layer Protective Layer Teflon Inner Core Kevlar Braid

A

Stainless Steel Braid

1 pc Crimp Design

FIGURE 3-24  Flexible brake hose construction.

B

FIGURE 3-25  Possible brake hose issues. A. Brake hose with abrasion/wear. B. Brake hose with kinks.

60

Chapter 3  Hydraulic Brake Systems

Sealing Washers and Fittings Many brake hoses use banjo fittings to connect the hose to the wheel unit. These fittings are composed of the banjo fitting, banjo bolt, and two copper or aluminum sealing washers (FIGURE 3-26). The banjo bolt, banjo fitting, and wheel unit usually have sealing ridges machined in them. These ridges dig into the softer sealing washers to ensure a leak-proof connection. The banjo bolt is hollow and allows the brake fluid through the middle to continue on to the wheel unit. It clamps the banjo fitting to the wheel unit. The sealing washers fit on both sides of the banjo fitting. One sealing washer is between the head of the banjo bolt and the banjo fitting, and the other sealing washer is between the banjo fitting and the wheel unit. Always use the proper torque when tightening banjo bolts so that they are not twisted off or left loose to leak.

FIGURE 3-26  Banjo-fitting assembly.

▶▶ Hydraulic

▶▶TECHNICIAN TIP Because the sealing washers are made of soft metal, they become crushed after use. It is good practice to replace them each time the banjo fittings are removed; otherwise, leaks could occur. 3-6 Summarize the purpose and function of hydraulic braking system controls.

Braking System Control

The hydraulic braking system must be accurately controlled to maintain adequate control of the vehicle during braking. As we learned earlier, hydraulic working pressure is equally applied throughout a sealed hydraulic braking system. We also learned that the hydraulic braking system can be designed to optimize the output force at each of the wheel brake units. In a perfect scenario, that would work just fine. But because machines are not perfect, the hydraulic pressure must have the capacity to be modified to accommodate different scenarios. Components such as proportioning valves, metering valves, pressure-differential valves, or antilock hydraulic control units are used to modify the pressures within the hydraulic braking system. Part of this section will show how they enhance the braking system.

Types of Divided Hydraulic Systems A wheel’s braking ability depends on the load it is carrying; therefore, the type of vehicle is a major factor in determining how its system should be divided (FIGURE 3-27). A front-­ engine, rear-wheel drive car has around 40% of its load on its rear wheels and 60% on its Front

Master Cylinder

60%

40%

A

Master Cylinder Master Cylinder

80%

B

20%

Proportioning Valves

Proportioning Valves C

FIGURE 3-27  Divided hydraulic systems. A. Vertical, or front–rear, split hydraulic system. B. Diagonal, or X, pattern

hydraulic system. C. L-split hydraulic system.



Hydraulic Braking System Control

front wheels. Its braking system can therefore be divided in a vertical, or front–rear, split. This design puts the front wheels on a different system than the rear wheels. If half of the system fails—either the front or the rear—there is still enough separate braking capability left in the other half to stop the vehicle. On a front-wheel drive vehicle, a load of about 20% on the rear wheels cannot provide enough braking force to adequately stop the vehicle. Therefore, front-engine, front-wheel drive vehicles use a braking system split in a diagonal, or X, pattern. The left-hand front brake unit is connected to the right-hand rear unit, and the left-hand rear unit is connected to the right-hand front unit. If one system fails, 50% braking capability is available in the other system.

Proportioning Valves Proportioning valves reduce brake pressure to the rear wheels when their load is reduced during moderate to severe braking. Proportioning valves can be pressure-sensitive or load-sensitive. The pressure-sensitive valve is in the master cylinder or in a separate unit in the rear brake circuit; the load-sensitive type is mounted on the body or on the rear axle, where it can respond to changes in the vehicle load. The effectiveness of braking force is determined by tire-to-road friction. The greater the load on the tire, the greater the friction; the greater the friction, the greater the stopping ability. When a vehicle stops abruptly, a portion of the weight on the rear wheels transfers to the front wheels, resulting in greater tire-to-road friction on the front tires and less on the rear. This is called load transfer, where weight is being transferred from the rear wheels to the front wheels. If equal braking force is applied to the front and rear wheels during this load transfer condition, the smaller load in the rear can cause the rear wheels to lock up. Skidding tires on the surface of the road (kinetic friction) do not have as much friction as rolling tires, and therefore the stopping distance is increased, which can lead to a collision. A pressure-sensitive proportioning valve reduces the hydraulic pressure applied to the rear brakes under heavy braking to prevent rear-wheel lockup and to help maintain traction. If the vehicle is equipped with ABS, it may not be equipped with a proportioning valve, as the ABS can make up for wheel slippage due to the changes in load transfer. This is ­covered in more detail in Chapter 11, which covers electronic brake control.

61

▶▶TECHNICIAN TIP When diagnosing and servicing brakes, it is helpful to know how the hydraulic system is divided. If the vehicle is pulling to one side due to a leak in half of the system, it will generally pull toward the side that is working in the front. Diagnosis and inspection of the other diagonal half usually leads to the leak. In the same way, if air is trapped in half of the hydraulic braking system, then bleeding (removing air) the corresponding diagonal half will make it easier to remove the air.

Return Spring Plunger Fluid Valve

OUT

IN

Pressure-Sensitive Proportioning Valve Operation The pressure-sensitive proportioning valve adjusts the braking force to allow for load transfer or variations in loads. During normal braking, the poppet piston of the pressure-sensitive proportioning valve is held in a relaxed position by a large pressure spring. The poppet valve is held against its retainer by a light return spring, and brake fluid passes freely through the pressure-sensitive proportioning valve to the rear brakes (FIGURE 3-28). In this condition, the rear brakes operate normally without any modification to the pressure. During heavy braking, master cylinder pressure can reach the poppet valve’s crack point. The pressure applied to the two different areas of the poppet piston creates unequal forces, which act on the pressure-sensitive proportioning valve. The higher hydraulic pressure moves the poppet piston against the large pressure spring to close the pressure-sensitive proportioning valve. At a specified inlet pressure, the conical section of the pressure-sensitive proportioning valve is held against the seat, which holds the pressure steady to the rear brakes until there is further change in inlet pressure (FIGURE 3-29). As greater pedal force increases pressure in the master cylinder, brake-fluid pressure rises on the smaller end of the poppet piston. This combines with the force of the pressure spring to overcome the lower

FIGURE 3-28  A pressure-sensitive proportioning valve in the

open position. Return Spring Plunger Fluid Valve

IN

OUT

FIGURE 3-29  A pressure-sensitive proportioning valve in the closed position.

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Chapter 3  Hydraulic Brake Systems Warning Lamp Switch IN Front Circuit

IN Rear Circuit

OUT Front Circuit

OUT Rear Circuit

Proportioning Valve Pressure Differential Valve OUT Front Circuit

Metering Valve

FIGURE 3-30  Proportioning valve action in a combination valve with a front brake circuit failure.

pressure now on the larger (output) end. As a result, the piston is forced back, opening the poppet valve and allowing pressure to rise to the rear brakes. The increased pressure now acts on the larger end of the poppet piston and again forces the piston forward to close the poppet valve and hold pressure steady. This repeated action causes a lowering of outlet pressure versus inlet pressure. When the brake pedal is released, the pressure of the rear brake fluid unseats the poppet valve, letting the brake fluid return to the master cylinder. The pressure spring now returns the poppet piston to its relaxed position. On vehicles that position the proportioning valve in a combination valve, should the front brake system fail, the warning lamp spool moves forward, taking the poppet valve with it (FIGURE 3-30). Pressure in the rear brakes rises, and the piston moves forward; however, it cannot close the poppet valve, so full system pressure remains available for the rear brakes in this situation. Should the rear brake system fail, the warning lamp spool will move backward to activate the warning light. The pressure-sensitive proportioning valve cannot operate in this situation. A diagonally divided system requires one pressure-sensitive proportioning valve for each rear wheel, so they are usually located in each rear brake line coming from the master cylinder (FIGURE 3-31). Each pressure-sensitive proportioning valve operates in a similar way to the pressure-sensitive proportioning valve in the master cylinder, but without the pressure-differential warning-light circuit.

Adjustable Proportioning Valves Aftermarket adjustable proportioning valves are available for performance applications. They have a method of adjusting the crack point of the proportioning valve and can be customized to the specific vehicle. Adjustable proportioning valves are popular with kit car builders, who use components from a variety of vehicles that were not originally designed to work together. Adjustable proportioning valves are not recommended for most applications, due to the amount of trial and error necessary to set them properly.

Load-Sensitive Proportioning Valve FIGURE 3-31  A pressure-sensitive proportioning valve on a diagonally

split system.

The load-sensitive proportioning valve reduces rear brake pressure when the vehicle is lightly loaded and allows higher pressure



Hydraulic Braking System Control Brakes Applied

Brakes Released Frame Mounted Valve Body

To Rear Brakes

Rear Brake Circuit

Proportioning Valve Failsafe Spring

Front Brake Circuit

Loaded

Suspension Connection Link

Load Spring

Unloaded

FIGURE 3-32  A load-sensitive proportioning valve adjusts rear braking force based on changes in rear-wheel load.

when it is heavily loaded. The load-sensitive proportioning valve is usually located on the chassis and has a lever that is connected to the rear axle (FIGURE 3-32). As the vehicle is loaded, the chassis squats on the rear suspension, and the lever is moved, applying more force on the load-sensitive proportioning valve. The lever increases or decreases the point at which pressure is limited by the load-sensitive proportioning valve. Heavier loads allow more pressure through the load-sensitive proportioning valve to the rear brakes, and lighter loads restrict pressure through the load-sensitive proportioning valve. A diagonally split system may have two load-sensitive proportioning valves, one for each rear brake unit. Each load-sensitive proportioning valve is mounted on the chassis, around the rear suspension.

Electronic Brake Proportioning Many ABS-equipped vehicles integrate an electronic brake proportioning function within the hydraulic control unit on vehicles that require it. This function reduces hydraulic pressure to

▶▶TECHNICIAN TIP There are very few adjustable components in the hydraulic brake system. One notable exception is the load-sensitive proportioning valve, which is used on some pickup trucks and other load-carrying v­ ehicles. Load-sensing proportioning valves operate so that as load weight increases, more brake p ­ ressure is applied to the rear wheels as required. In the case of a rear-wheel lockup on one of these vehicles, excessive pressure to the rear brakes may be suspected. To check the operation and adjustment of load-sensing proportioning valves, pressure gauges are fitted in line to the front and rear brakes. After the hydraulic brake system is opened to install the gauges, air is bled out, before accurate readings can be obtained. The weight on the rear axle must be set according to charts or graphs published by the manufacturer, to determine the correct relationship between front and rear pressure at a given load. For example, according to a graph in the service information, if a rear axle is loaded to 1,984 lb (900 kg), and front brake pressure is raised to 1,138 psi (7,846 kPa), then rear brake pressure should fall within the range of 569 to 711 psi (3,923 to 4,902 kPa). The rear axle is then loaded to 3,700 lb (1,678 kg). If the rear pressure is outside of the specified range, adjustment of the linkage between the proportioning valve and the rear suspension may be required. Rear Axle Load lb (kg)

Front Brake Pressure psi (kPa)

Rear Brake Pressure psi (kPa)

1,984 (900)

1,138 (7,846)

569–711 (3,923–4,902)

3,699 (1,678)

1,707 (11,769)

1,323–1,493 (9,122–10,294)

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Chapter 3  Hydraulic Brake Systems

the rear wheels under heavy braking, similar to the proportioning valve. However, in these vehicles the pressure reduction is handled electronically and can compensate for ­differences in traction by monitoring wheel slip.

Metering Valves Metering valves are used to hold off the application of the front brakes on vehicles with disc brakes on the front wheels and drum brakes on the rear wheels (FIGURE 3-33). Drum brakes use springs to return the brake shoes to their rest position. This means that it takes a certain amount of hydraulic pressure to overcome the tension of the return springs and move the shoes to contact the drums. Disc brakes use the much smaller force of the square-cut O-ring to return the caliper piston to its rest position. Thus, very little hydraulic pressure is needed to move the brake pads into contact with the rotor. Vehicles typically handle better when the rear brakes engage before the front brakes. This helps keep the vehicle tracking straight while the brakes are initially applied. The metering valve keeps the front disc brakes from being applied until the rear drum brakes have had a chance to overcome the tension of the return springs. Vehicles with four-wheel disc brakes don’t need metering valves, because the front and rear brakes apply at roughly the same time.

FIGURE 3-33  A metering valve.

Metering Valve Operation The metering valve operates like a radiator cap. It has a spring that holds the metering valve closed until a specified pressure is reached (FIGURE 3-34). Pressure rises in the hydraulic braking system, overcomes the rear brake return spring tension, and starts to apply the rear brakes. As pressure continues to rise above the metering valve’s crack point, brake fluid flows to the front disc brake calipers and starts to apply the front brakes. Because the metering valve has hydraulic pressure on only the inlet side, any pressure above its crack point holds it open. When the brake pedal is released, the metering valve is pushed closed by the spring, and a fluid return valve opens up, allowing brake fluid to flow freely back to the master cylinder (FIGURE 3-35). During bleeding, if there is air in the lines, it may be difficult to raise the brake-fluid pressure enough to open the metering valve. This makes it almost impossible to bleed any air out of the front part of the hydraulic braking system. Most metering valves are designed so that they can be manually held in the open position to allow bleeding to take place. Also, many pressure bleeders operate at an insufficient pressure to open the metering valve, and the valve has to be held open manually. Open Position

Closed Position IN Front Circuit

IN Front Circuit

OUT Front Circuit

OUT Front Circuit

OUT Front Circuit

Metering Valve

OUT Front Circuit

Metering Valve

FIGURE 3-34  A metering valve holds pressure from applying the front brakes, until a specified pressure is reached.



Hydraulic Braking System Control

65

Brakes Releasing IN Front Circuit OUT Front Circuit

OUT Front Circuit

Metering Valve

FIGURE 3-35  Brake fluid is free to flow through the fluid return valve back to the master cylinder when the brakes are released.

Vehicles equipped with a diagonally split system are generally front-wheel drive. They usually do not use metering valves, for two reasons. First, the torque of the spinning engine during braking compensates for the earlier application of the front brake pads and offsets not having a metering valve. Second, because up to 80% of the braking occurs at the front wheels, they need to be applied as quickly as possible to start effectively braking the vehicle.

Pressure-Differential Valve A pressure-differential valve monitors any pressure difference between the two separate hydraulic brake circuits. If there is a moderate leak anywhere in the system, it will illuminate the brake warning light on the instrument panel. Sometimes the light only flickers, or comes on when the brake pedal is pushed, indicating a small leak. The valve can be located in the master cylinder or in the combination valve.

Pressure-Differential Valve Operation The pressure-differential valve is connected between the two halves of the hydraulic braking system so that pressure is applied to each end of the pressure-differential valve. As long as the pressure stays the same in both circuits, the pressure-differential valve remains in the same position and the light stays off. A moderate leak in the hydraulic brake system lowers the pressure on that side of the circuit. This system allows the higher pressure in the non-leaking side to push the pressure-differential valve off center toward the side with the leak. The pressure-differential switch then closes and illuminates the brake warning light on the instrument panel, telling the driver that there is a serious leak in the hydraulic brake system that has to be diagnosed (FIGURE 3-36). During hydraulic-braking-system bleeding, the pressure-differential valve may need to be centered. Most pressure-differential valves have springs that help center them, along with a small amount of clearance between the valve and bore. Applying the brakes firmly causes the pressure to equalize, and the spring returns the pressure-­differential valve to center. On vehicles without this feature, once the hydraulic braking system has been bled, a small amount of brake fluid must be bled out of the opposite hydraulic circuit to allow the pressure-differential valve to move back to center. Follow the manufacturer’s procedure.

Combination Valve The combination valve can combine the pressure-differential valve, metering valve, and proportioning valve(s) in one unit (FIGURE 3-37). Some combination valves combine just the pressure-differential valve and proportioning valve(s). On others, just the pressure-­ differential valve and metering valve are combined. Each valve operates individually as it was designed and is contained in one unit. Combination valves are not serviceable. If they become faulty, they must be replaced.

▶▶SAFETY TIP After bleeding the hydraulic braking system, always remember to remove the tool that is holding open the metering valve. Failure to do so could cause the vehicle to brake improperly.

▶▶TECHNICIAN TIP On a front–rear split system, there can also be a pressure difference if the rear drum brake shoes are grossly under-­ adjusted. When the brake pedal is pushed, the shoes do not make contact. The primary piston moves so far that it contacts the ­secondary piston, which starts to apply the  front brakes. This raises the pressure in the ­secondary circuit higher than in the primary circuit, moving the pressure-­ differential valve toward the primary circuit and thus illuminating the brake warning light.

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Chapter 3  Hydraulic Brake Systems Pressure In from Master Cylinder

Brake Warning Lamp Brake Warning Lamp Switch

From Ignition Switch Pressure In from Master Cylinder

Leak

Washer

Piston

Seal

Seal

Low Pressure to Front Wheels

Connector

High Pressure to Rear Wheels

FIGURE 3-36  A pressure-differential valve with a leak in the hydraulic braking system.

Warning Lamp Switch

Out Front Circuit

In Front Circuit

In Rear Circuit Proportioning Valve Out Rear Circuit

Out Front Circuit

Metering Valve

FIGURE 3-37  A combination valve.

Pressure Differential Valve



Hydraulic Braking System Control

67

Diagnosing Hydraulic Concerns Control valves do not normally give technicians trouble. They tend to operate rather dependably. But like any other mechanical component, they can fail. Understanding how they operate will provides a head start on diagnosing concerns associated with them. Generally, the best way to test them is to connect a pressure gauge set designed for the high pressures within a brake system to the input and output of the valve. Operating the brake system and comparing the pressures to specifications will determine whether the control valve is operating correctly. To perform a metering valve test, follow the steps in SKILL DRILL 3-3. To perform a proportioning valve test, follow the steps in SKILL DRILL 3-4.

SKILL DRILL 3-3 Testing a Metering Valve 1. Research the testing procedure and specifications in the appropriate service information. Disconnect the inlet line and outlet line from the metering valve.

2. Connect the metering valve pressure tester to the inlet port and outlet port of the metering valve. Reconnect the inlet line to the metering valve, with the pressure gauge teed into it. Operate the brake pedal, and observe both pressure gauges; compare the findings to specifications.

SKILL DRILL 3-4 Testing a Proportioning Valve 1. Research the testing procedure and specifications in the appropriate service information. 2. Disconnect the inlet line and outlet line from the proportioning valve. Use a flare nut wrench to accomplish this. 3. Connect the proportioning valve pressure tester to the inlet port and outlet port of the proportioning valve. Reconnect the

inlet line to the proportioning valve, with the pressure gauge teed into it. 4. Operate the brake pedal as specified, and observe both pressure gauge readings; compare the findings to specifications.

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Chapter 3  Hydraulic Brake Systems

To perform a pressure-differential valve test, follow the steps in SKILL DRILL 3-5.

SKILL DRILL 3-5 Testing a Pressure-Differential Valve 1. Research the testing procedure and specifications in the appropriate service information. Disconnect the electrical connector from the pressure-differential valve, and connect an ohmmeter between the terminal on the valve and the body of the valve. Have an assistant lightly apply the brake pedal and hold until the bleeder screw is closed.

2. Open the front bleeder screw on the driver’s side of the vehicle and watch the ohmmeter reading. It should go to zero. Close the bleeder screw and move to the rear driver’s side wheel. Again, with the assistant applying light pressure to the pedal, open the driver’s side rear-wheel bleeder screw slightly and watch the meter reading. It should go to OL (out of limits) and then back to zero as the valve moves to the Off position and then to the On position.

▶▶ Brake 3-7 Inspect brake lines, hardware, and hoses.

Lines, Hardware, and Hoses

When evaluating the vehicles brake lines, hardware, and hoses, the technician must be aware of the properties of fluid transfer so that they are able to understand the issues that are present within a vehicle. Inspecting all of the components within a braking system will allow for a complete understanding of what problems are present within this vehicle. Each piece of the braking system relies on the other ones to operate correctly: if one piece fails, the braking system is not operating at 100% and must be serviced.

Inspecting Brake Lines and Hoses Inspecting brake lines and brake hoses can show where maintenance is needed to prevent vehicle breakdowns and unsafe vehicle operation. All manufacturers recommend regular inspection of these components. Be sure to take into account regional differences, such as regions that are highly susceptible to corrosion from de-icing chemicals or high humidity. Also, off-road vehicles may be more prone to dents, kinks, and cuts. Inspect all vehicles carefully. To inspect brake lines, brake hoses, and associated hardware, follow the steps in SKILL DRILL 3-6.



Brake Lines, Hardware, and Hoses

SKILL DRILL 3-6 Inspecting Brake Lines, Brake Hoses, and Associated Hardware 1. Safely raise the vehicle on a hoist. Trace all brake lines from the master cylinder to each wheel’s brake assembly. Inspect the steel brake lines for leaks, dents, kinks, rust, and cracks.

2. Inspect all flexible brake hoses for cracks, bulging, and wear.

3. Tighten any loose fittings and supports.

Replacing Brake Lines, Brake Hoses, Fittings, and Supports When brake lines and hoses have to be replaced, they must be disconnected at their fittings. Often, the hose or line fitting is connected very tightly to a matching fitting or adapter on another line, hose, or component. The best way to remove it is with the double-wrench

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Chapter 3  Hydraulic Brake Systems

method. In this method, place one flare nut wrench on each of the two fittings, and twist them apart. If only one flare nut wrench on just one fitting is used, the brake line or hose will likely kick because the line is much weaker than the fittings. So always use the double-wrench method when loosening brake-line fittings. To replace brake lines, brake hoses, fittings, and supports, follow the steps in SKILL DRILL 3-7.

SKILL DRILL 3-7 Replacing Brake Lines, Hoses, Fittings, and Supports 1. Depress the brake pedal slightly with a brake pedal depressor to prevent brake fluid from draining out of the vehicle. Remove the stop-light fuse to avoid draining the battery.

2. Safely raise the vehicle on a hoist. Using flare nut wrenches and the double-wrench method, carefully remove any brake lines, hoses, fittings, and supports that are to be replaced. Inspect all components for damage and wear.

3. Carefully reassemble the removed components by using fingers only. Once everything is assembled, tighten each fitting and support, using flare nut wrenches and the double-wrench method. Bleed any trapped air from the system. Start the vehicle, and verify proper brake pedal height, firmness, and feel. Reinstall the brakelight fuse.



Brake Lines, Hardware, and Hoses

71

Applied Science AS-1: Hydraulics: The technician can explain how fluid pressure transmits force from one location to another. Pascal’s law states that incompressible liquids transmit pressure in all directions. This is the basic principle underpinning the operation of hydraulic braking systems on motor vehicles. When braking, brake pedal pressure is applied, via the master cylinder, to the brake fluid. The hydraulic fluid under pressure pushes against the caliper or wheel cylinder pistons, forcing friction material against the brake discs or drums to stop the movement of the vehicle.

For the sake of simplicity, think of a hydraulic brake system as a tube of toothpaste. Start with a full tube (no air). If a hole in the tube is punched and then the tube squeezed, toothpaste (a thick liquid) will be displaced from the hole. If three holes are punched through various parts of the tube and the tube is squeezed in only one place, toothpaste will be displaced from all three holes. This illustrates that force applied to a liquid in one area is transferred to all areas within a container.

Brake-Line Flaring To fabricate brake lines, determine whether the vehicle being worked on uses the inverted double flare or ISO method. First, select new, double-walled steel brake line of the specified diameter. Ensure that the end to be flared is cut smooth and square, without dings or gouges. Clean it up with a file, or recut the end with a tubing cutter. Obtain the flaring tool for the type of flare to be fabricated: double flare or ISO. Also, ensure to place any fittings required to be in place on the tubing before the tubing is flared. To perform the inverted double-flare method, follow the steps in SKILL DRILL 3-8.

SKILL DRILL 3-8 Fabricating Brake Lines, Using the Inverted Double-Flare Method 1. Install the proper fitting onto the brake line. Use a bench vise to hold the brake-line clamping tool. Select the proper adapter to match the tubing size. Use it to set the height of the tube in the clamp. Tighten down the clamp securely.

2. Insert the adapter into the tubing, and install the flaring tool onto the clamp and over the adapter.

Continued

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Chapter 3  Hydraulic Brake Systems

3. Tighten down the flaring tool until the adapter touches the clamp. This forms a widening of the tube.

4. Remove the adapter and reinstall the flaring tool. Tighten down the flaring tool to fold the narrowed portion back into the widened portion of the flare. Stop when the tool starts to become difficult to tighten.

5. Remove the flaring tool, and inspect the flare to determine whether it has been formed correctly. Remove the flared line from the clamp and inspect it.

To perform the ISO flare method, follow the steps in SKILL DRILL 3-9.

Bleeding Brake Systems Bleeding the brakes means removing air from the hydraulic braking system so that only the brake fluid is left in the system. When pressure is applied to brake fluid in a hydraulic braking system, the brake fluid cannot be compressed into a smaller volume, and therefore, the pressure rises within the hydraulic braking system. Pressure is transmitted throughout the hydraulic braking system without loss. If air enters the hydraulic braking system, it can



Brake Lines, Hardware, and Hoses

73

SKILL DRILL 3-9 Performing the ISO Flare Method 1. Install the proper fitting onto the brake line. Use a bench vise to hold the brake-line clamping tool. Select the proper adapter to match the tubing size. Use it to set the height of the tube in the clamp. Tighten down the clamp securely.

2. Insert the adapter into the tubing, and install the flaring tool onto the clamp and over the adapter.

3. Tighten down the flaring tool until the adapter touches the clamp.

4. Remove the flaring tool, and inspect the flare to determine whether it has been formed correctly. Remove the flared line from the clamp and give it a final inspection.

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Chapter 3  Hydraulic Brake Systems

be dangerous. Unlike fluids, gases are compressible; when pressure is applied to air (a gas), the air decreases in volume and the pressure does not build up as quickly throughout the hydraulic braking system. There are a number of different brake bleeding methods. The three most common are described here: 1. Manual bleeding—using a helper to manually operate the brake pedal while opening and closing the bleeder screws on the wheel units to allow the air and old brake fluid to be pushed out of the hydraulic braking system. 2. Vacuum bleeding—using a vacuum bleeder to pull the air and old brake fluid from the hydraulic braking system. 3. Pressure bleeding—using clean brake fluid under pressure from an auxiliary tool or piece of equipment to force the air and old brake fluid from the hydraulic braking system. For all three methods, follow these general steps: 1. Remove as much of the old brake fluid from the reservoir as possible, using a suction gun, old antifreeze tester, or turkey baster. 2. Refill the master cylinder reservoir, using the specified fluid type. 3. Then determine which method of bleeding will be used. Also research the service information to obtain the bleeding sequence for the vehicle. Following the bleeding sequence will help perform the task more quickly and with less chance of air entering the system. The manual bleeding method requires the least amount of equipment and tools. It requires more time if a large percentage of the brake fluid has to be changed. It also is less effective at removing trapped air in systems that have high spots in the brake lines or in vehicles that have a large vertical drop between the master cylinder and the wheel brake units, such as a truck that has been lifted. This method is best when only a small amount of brake fluid has to be bled, such as after replacing front brake calipers or when more expensive equipment is not available. Using the manual bleeding method requires clear communication between the two technicians, to prevent air from entering the system. Be sure to not bleed one wheel so much that the reservoir runs dry and admits air into the hydraulic braking system. If this happens, it will be much harder to bleed the hydraulic braking system, because air is compressible, making it harder to build pressure within the hydraulic braking system. To perform the manual bleeding method, follow the steps in SKILL DRILL 3-10.

SKILL DRILL 3-10 Performing Manual Bleeding 1. Ask an assistant to slowly push the brake pedal down, keeping their left foot underneath the pedal to limit full pedal travel.

Continued



Brake Lines, Hardware, and Hoses

75

2. Install a clear hose on the farthest bleeder screw. Open the bleeder screw one-quarter to one-half turn. Observe any old brake fluid and air bubbles coming out. When the brake fluid stops, close the bleeder screw lightly, and then have the assistant slowly release the brake pedal. Repeat these steps until there are no more air bubbles or old fluid coming out of the hose.

3. Close off the bleeder screw, and tighten it to the manufacturer’s specifications. Check the level in the master cylinder reservoir, top it off, and reinstall the reservoir cap. Repeat this bleeding procedure for each of the brake units, moving closer to the master cylinder one wheel at a time.

Vacuum bleeding uses an air-operated or hand-operated vacuum pump to draw brake fluid out of the bleeder screws at each wheel unit. This method is much faster than using the manual bleeding procedure but easier to set up than the pressure-bleeding system, so it is commonly used in shops. Just be sure to watch how much fluid is collecting in the vacuum bottle so that the master cylinder doesn’t run dry. Also remember that tandem master cylinders use divided reservoirs, so the fluid level could be over half on one side and empty on the other side. Watch the reservoir carefully, and top it off frequently to avoid getting air in the system. Another issue when using a vacuum bleeder is that the tool draws air from around the threads of the bleeder screw while it is operating, which shows up as bubbles in the fluid being removed from the brake system. When using a vacuum bleeder, ignore the bubbles and focus more on the color of the fluid. Once the fluid is coming out clear, all air should be removed from the system. To perform vacuum bleeding, follow the steps in SKILL DRILL 3-11. The pressure-bleeding method uses equipment that can provide a continuous supply of brake fluid under pressure to the master cylinder reservoir. It may also include small-­ diameter hoses that attach to the bleeder screws and recover all of the old brake fluid. Because this method takes a bit of work to connect the equipment to the vehicle, it is best used when the hydraulic system needs a full flush or on systems that have a tendency to trap air, making manual bleeding much more difficult. To perform pressure bleeding, follow the steps in SKILL DRILL 3-12.

▶▶TECHNICIAN TIP One other method to know about is gravity bleeding. This method relies on the master cylinder being mounted higher than the wheel brake units. Because gravity tries to pull the brake fluid down, gravity bleeding can occur just by opening one or more bleeder screws. If the height difference is great enough, the brake fluid will slowly drain out of the system. A technician can use this method to start the bleeding process while working on other wheel brake units. However, gravity bleeding can have a disadvantage. If a technician inadvertently leaves a hose disconnected or a bleeder screw open, all of the brake fluid can drain out of the system, leaving it full of air and harder to bleed. Be very careful not to leave the hydraulic braking system open for too long.

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Chapter 3  Hydraulic Brake Systems

SKILL DRILL 3-11 Performing Vacuum Bleeding 1. Prepare the vacuum bleeder for use. Install the vacuum bleeder on the farthest bleeder screw, and open it one-quarter to one-half turn.

2. Operate the vacuum bleeder to pull brake fluid from the bleeder screw. Observe any old brake fluid and air bubbles coming out. Close off the bleeder screw and tighten it to the manufacturer’s specifications.

3. Check the level in the master cylinder reservoir and top it off. Repeat this procedure for each of the brake units, moving closer to the master cylinder, one wheel at a time.

After performing bleeding or flushing brakes, follow these steps: ▶▶TECHNICIAN TIP Never run the master cylinder dry during bleeding; doing so will allow air to enter the ABS hydraulic unit, making it much more difficult to bleed!

1. Double-check that all bleeder screws are properly tightened. 2. Replace all bleeder screw dust caps when finished. 3. Refill the master cylinder to the proper level, and reinstall the master cylinder reservoir cap. 4. Start the vehicle and check for proper brake pedal feel (it should be firm and high). 5. Verify that there are no leaks at each bleeder screw. 6. Dispose of any old brake fluid in an environmentally approved method.



Brake Lines, Hardware, and Hoses

SKILL DRILL 3-12 Performing Pressure Bleeding 1. Prepare the pressure bleeder and install it on the vehicle.

2. Install a clear hose on the farthest bleeder screw and open it one-quarter to one-half turn. Observe any old brake fluid and air bubbles coming out.

3. Close off the bleeder screw when the brake fluid is clear and has no bubbles. Tighten it to the manufacturer’s specifications. Repeat this procedure, moving closer to the master cylinder one wheel at a time.

Flushing Brake Systems Flushing brake fluid is similar to brake bleeding, except that it is designed not only to remove any trapped air but also to replace all of the old brake fluid with new brake fluid. A flush using clean brake fluid can be performed to remove any remaining residue and all air. Clean brake fluid of the proper type for the vehicle must be used for flushing a hydraulic braking system. However, if the hydraulic braking system has been

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Chapter 3  Hydraulic Brake Systems

▶▶SAFETY TIP Never use any mineral-based or petroleum products, such as gasoline or kerosene, to clean a hydraulic braking system or its components. They are not compatible with the seals used in the braking system and will result in a failure of the hydraulic braking system and its components. This failure may result in injury to the passengers or damage to the vehicle.

▶▶TECHNICIAN TIP Make sure to understand and observe all legislative and personal safety procedures when carrying out tasks. If a technician is ever unsure of what these procedures are, they should ask their supervisor.

contaminated with petroleum products or other improper fluids component replacement must be performed to remove all contaminates.

Inspecting the Brake Pedal Brake pedal height, free play, and travel are critical for proper brake operation. Having the proper brake pedal height helps to ensure that the brake pedal has enough starting height to fully apply force to the brakes, even if one-half of the hydraulic system is rendered useless by a leak. In other words, there has to be a specified distance that the brake pedal can travel before it contacts the floor or anything else. To measure brake pedal height, follow the steps in SKILL DRILL 3-13. Free play is the amount of clearance between the brake pedal linkage and the master cylinder piston. To measure it, apply very light hand pressure to the brake pedal, measuring how far the pedal travels before starting to feel resistance (FIGURE 3-38). To measure brake pedal free play, follow the steps in SKILL DRILL 3-14. Brake pedal travel is sometimes called reserve pedal. Travel is the distance that the brake pedal travels from its rest position to its applied height. Travel is measured by reading brake pedal height and subtracting the height from the floor (FIGURE 3-39). For example, if the brake pedal height is 8'' (203.2 mm) and the travel takes it down to 5" (127 mm) off the floor, then the travel is 3" (76.2 mm). Reserve pedal is the measurement from the floor to the height of the applied brake pedal and represents how much reserve is left for the brake pedal to travel if needed (FIGURE 3-40).

SKILL DRILL 3-13 Measuring Brake Pedal Height 1. Research the procedure and specifications for measuring brake pedal height, travel, and free play for the vehicle to be worked on. Some manufacturers specify how much travel the brake pedal should have, whereas others specify how much reserve pedal should remain when the brake pedal is fully applied. Know which process applies to the vehicle being worked on.

2. Remove any removable floor mats. 3. With the engine off, measure the brake pedal height between the two specified points, using a measuring stick. 4. Compare this reading to the specifications and determine any necessary actions.

FIGURE 3-38  To measure free play, apply light hand pressure to the brake pedal, and measure how far the pedal travels before resistance is felt.

SKILL DRILL 3-14 Measuring Brake Pedal Free Play 1. Research the specified procedure to measure the pedal free play. 2. Use light hand pressure to apply the brake pedal until all clearances are taken up, all the while measuring the distance the brake pedal moved.

3. Compare this reading to the specifications and determine any necessary actions.



Brake-Warning-Light System

FIGURE 3-39  Travel is measured by reading brake petal height and subtracting the height from the floor.

FIGURE 3-40  Reserve pedal is the measurement from the floor to the applied brake pedal.

SKILL DRILL 3-15 Measuring Brake Pedal Travel 1. Research the specified procedure to measure the brake pedal travel or reserve pedal height. 2. Start the engine. This allows the booster to operate normally. 3. Apply the brake pedal with the specified force.

4. Measure the brake pedal travel or reserve height. 5. Compare this reading to the specifications and determine any necessary actions.

To measure brake pedal travel, follow the steps in SKILL DRILL 3-15.

▶▶ Brake-Warning-Light

79

System

Red lights are used on vehicles as devices to warn drivers and others of specific conditions. When a red light comes on, the driver should take notice and respond appropriately. There are two general categories; the brake warning light and stop lights. The brake warning light is a single light located in the instrument panel, whereas the stop lights are made up of at least three lights at the rear of the vehicle.

Brake Warning Light The brake warning light is located on the instrument panel and designed to warn the driver of a condition in the brake system that needs attention (FIGURE 3-41). Usually, one of any of the following four causes can illuminate the light (FIGURE 3-42). 1. The first is when the parking brake is engaged and the light is turned on by the parking brake lever or pedal. This is to alert the driver that it is on so that the driver will release it before driving. 2. The second reason the brake warning light comes on is because the brake-fluid level is too low in the master cylinder reservoir. This could be caused by a leak in the hydraulic braking system or worn brake pads on the disc brakes. 3. On vehicles without ABS, the third cause of the brake warning light coming on is unequal pressure in the hydraulic brake system, which causes the pressure-differential valve to activate the brake-warning-light switch. 4. The fourth cause of the brake warning light illuminating is called a “prove out” or “proofing” circuit. On most vehicles designed without a controller area network bus (CANbus) system, turning the ignition switch to the Crank position causes the warning light to illuminate, so that the driver knows the bulb is good.

3-8 Examine the brake-warning-light system.

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Chapter 3  Hydraulic Brake Systems Ignition Switch

Brake Warning Light

Parking Brake Switch Fuse

Fluid Level Switch

Battery

Pressure Differential Switch

Proofing Circuit Ignition Switch

FIGURE 3-41  A brake warning light warns the driver of an issue in the braking system.

FIGURE 3-42  Brake-warning-light circuit.

Diagnosing the Brake Warning Lamp A mechanical brake warning lamp system (non-CANbus) circuit is a simple lightbulb in series, with as many as four switches that are connected in parallel with one another (­ FIGURE 3-43). Each switch can illuminate the warning lamp under the right conditions. The warning lamp should be off when the ignition key is in the Run position if the parking brake is released, and there are no faults in the hydraulic system. When the ignition is turned to the Crank position, the warning lamp should illuminate. B+

Fuse Block

Fuse

Ignition Switch

Brake Warning Light

Parking Brake Switch Brake Fluid Switch

Pressure Differential Switch

Proof Circuit Ignition Switch

FIGURE 3-43  Brake-warning-light circuit.



Brake-Warning-Light System

In the Run position, the warning lamp can be illuminated only when one of the switches is closed. One switch is on the parking brake assembly and illuminates the warning lamp when the parking brake is applied. This alerts the driver that the parking brake should be released before driving the vehicle. The second switch is located on the pressure-differential assembly and illuminates the warning lamp if there is a moderate-sized leak in the hydraulic system. The third switch, if the vehicle is so equipped, is located on the master cylinder reservoir and illuminates the warning lamp if the fluid level falls below a certain point. The last switch, if the vehicle is so equipped, is fed when the ignition switch is turned on, which illuminates the warning lamp when the ignition switch is in the Crank position. When diagnosing the brake warning lamp, it is good to start by verifying that the bulb is operational, using the circuit’s bulb check feature if the vehicle is so equipped. To do so, turn the ignition key to the Crank position. The brake warning lamp on the dash should illuminate on most vehicles. If the bulb illuminates, then that confirms that the bulb is good and has power and that the ground circuit through the ignition switch is operating as it should. If it does not illuminate, apply the parking brake, and then recheck to determine whether the warning light is on. If not, check the fuse with a test light (FIGURE 3-44). If the fuse is OK, remove the bulb, and verify that it is not burned out, by measuring its resistance with an ohmmeter and comparing it to specifications or the resistance of a known good bulb. If the brake warning lamp illuminated in the Crank position, turn the ignition key to the Run position. The brake warning lamp should be off. If it is, apply the parking brake. The warning lamp should illuminate. If it does not illuminate, refer to the vehicle’s wiring diagram to determine the best strategy to diagnose the system. If it is similar to the one in Figure 45-9, then locate the parking brake switch, disconnect the wire harness connector from the switch, and jump it to ground with a jumper wire. With the ignition switch on, the brake warning light should now be illuminated. If it is, then the switch needs either adjustment or replacement. If it is not, then there is a problem between the switch and the bulb. Use the same process for the low-brake-fluid switch, the pressure-differential switch, and the ignition switch. If the brake warning lamp always stays on in the Run position, each of the four switches need to be disconnected one at a time, checking to determine whether the warning lamp goes off. If it does, then each switch needs to be tested to determine whether it is adjusted properly or whether it is shorted. If the switches test OK, then there is likely a short circuit in one of the wires leading to the switches. This is the hardest part of testing the circuit; thankfully, it doesn’t happen very often. In a CANbus circuit, the parking brake sensor and the low brake-fluid sensors send signals over the CANbus network regarding their status (FIGURE 3-45). The appropriate module then signals the instrument panel module to illuminate the brake warning light. Diagnosis involves using a scan tool capable of interrogating the system, a wiring diagram, and a digital voltohmmeter (DVOM) to perform specific tests. Refer to Chapter 4, on the disc brake system, for more information on parking brakes.

FIGURE 3-44  Checking a brake fuse with a test light.

81

82

Chapter 3  Hydraulic Brake Systems

CAN-High IG+

+B

CAN-Low

25

11 A24 CAN-High

26

25 A24 CAN-Low

B 17 IG+

18

From Battery

ECU-B1

From Ignition Circuit

Meter Brake Warning Lamp (LED) IG

ECU-IG No 2

ET 15

E7 Combination Meter Assembly (instrument panel)

46 A24 IG1 28 A24

PKB

E53 Parking Brake Switch Assembly 1

2

A26 Brake Fluid Level Warning Switch

41 A24 LBL

1 A24 GND1 32 A24 GND2 4 A24 GND3 Master Cylinder Control Unit

FIGURE 3-45  CANbus circuit for the brake-warning-light circuit.

Checking the Brake-Warning-Light System To check a brake-warning-light system in a non-CANbus system, follow the steps in SKILL DRILL 3-16.

SKILL DRILL 3-16 Checking the Brake-Warning-Light System in a Non-CANbus System 1. Perform a bulb check and observe the brake warning light. If it is on, go to step 2. If it is off, check the fuse with a test light. If it is OK, check the warning lamp bulb with an ohmmeter.

Continued



Brake-Warning-Light System

83

2. Ensure that the parking brake is released and turn the ignition key to the Run position. The brake warning lamp should be off. If not, go to step 6.

3. If the warning lamp is off, apply the parking brake. The lamp should illuminate. If it does, check the operation of the other switches. If they work fine, there are no faults present in the system.

4. If the brake warning light is off when the parking brake is on, disconnect the wire from the parking brake switch, then ground the wire. The brake warning light should illuminate. If it does, test the parking brake switch with a DVOM.

5. If the warning lamp does not illuminate with the parking brake wire grounded, this is cause to suspect an open circuit between the parking brake wire and the warning lamp. Use a DVOM and wiring diagram to diagnose the fault.

Continued

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Chapter 3  Hydraulic Brake Systems

6. If the light stayed on in step 2, apply and release the parking brake several times. If this turns it off, clean or adjust the parking brake mechanism or switch. If it is still on, then disconnect each of the switches that activate it: parking brake, low fluid level, pressure differential, and ignition. If that turns the light off, then test that switch for misadjustment or a shorted condition. If the warning lamp is still on, suspect a short to ground in the wiring harness between the switches and the warning lamp bulb. Use a wiring diagram and DVOM to identify the location of the fault.

▶▶ Stop-Light 3-9 Diagnose stop-light operation.

Operation

Stop lights are designed to warn others that the vehicle is braking. This information is critical to helping avoid accidents. The regular stop lights are mounted on the rear of the vehicle and must conform to federal laws for brightness and location. They are activated by a normally open stop-light switch located on the brake pedal assembly. When the driver applies the brake, the brake pedal moves away from the stop-light switch. A spring in the switch closes the contacts, allowing current to flow and illuminate the stop lights (FIGURE 3-46). Releasing the brakes causes the stop-light switch to open, turning the stop lights off. In 1986, North America mandated that all new passenger vehicles be equipped with a center high mount stop lamp (CHMSL) (FIGURE 3-47). Light trucks and vans were added in 1994. This lamp is located higher than the regular brake lights, near the centerline of the vehicle. It is designed to be more in the driver’s line of sight while he or she is looking down the road to anticipate traffic hazards. This higher-mounted lamp helps reduce rear-end collisions by being more visible to drivers, giving them more warning time for stopping.

Diagnosing a Stop Light The mechanical system (non-CANbus) circuit is simply a normally closed switch in series with two to six brake lightbulbs connected in parallel with each other (FIGURE 3-48). This From Ignition

Locknut Pedal Released

Adjustment Thread Switch Contacts

From Ignition

Pedal Applied

Spring FIGURE 3-46  Brake-light-switch operation.

FIGURE 3-47  A CHMSL mounted on a vehicle.



Stop-Light Operation

85

B+

Fuse

Brake Switch Brake Pedal Stop/Tail Light Bulbs FIGURE 3-48  Typical nonCANbus stop-light circuit.

circuit is also protected by a fuse. Some vehicles use separate fuses for the CHMSL and the side stop lights. The switch is turned on and off by the movement of the brake pedal. Diagnosis starts with operating the stop lights, observing their reaction and comparing that to a wiring diagram to see how the manufacturer has wired the circuit. If they do not illuminate at all, check components common to all of the stop lights, such as the fuse, stop-light switch, and stop-light ground circuit. If individual lights do not illuminate, check those bulbs to determine whether they are burned out or whether they have bad connections, terminals, or wires.

Checking Operation of a Stop-Light System The stop lights are capable of operating at all times on most vehicles. This means the stop lights should come on whenever the brake pedal is depressed, regardless of whether the key is on or off. Checking the operation of the stop lights involves a visual inspection of each bulb. If the bulbs are on even when the brake pedal is released, suspect a misadjusted brake pedal, a defective brake switch, or a short to power in the circuit. If none of the bulbs light up when the brake pedal is depressed, suspect a fault that is common to all of the bulbs, such as the fuse, brake switch, feed wire, or ground wire. If there is a problem with only some of the bulbs, suspect a fault that is common to only the bulbs that do not operate, such as the bulbs or the individual wires. In any case, the DVOM will need to be used to locate the open circuit, high resistance, or short circuit. To check the operation of the brake stop-light system and determine any necessary action, follow the steps in SKILL DRILL 3-17.

SKILL DRILL 3-17 Checking the Operation of the Brake Stop-Light System 1. Verify that the stop lights are off when the brakes are released. If the lights are on, test the operation of the brake pedal and brake switch. Observe the stop lights when the brake pedal is applied. All lights, including the CHMSL, should be illuminated.

Continued

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Chapter 3  Hydraulic Brake Systems

2. If some or all of the stop lamps do not illuminate, test their resistance with a DVOM.

3. If the bulbs are OK, consult the wiring diagram to determine potential causes of the fault in the circuit. Use the wiring diagram and a DVOM to identify the cause of the fault.

▶▶Wrap-Up Ready for Review ▶▶

▶▶

▶▶ ▶▶

▶▶ ▶▶ ▶▶

The principle behind hydraulic brakes is Pascal’s law, which states that pressure applied to a fluid in one part of a closed system will be transmitted without loss to all other areas of the system. A substantial leak in the hydraulic braking system will prevent enough pressure from building to exert the ­necessary braking force. Engineers design brakes that have precise (but unequal from front to back) amounts of braking force at each wheel. The three variables related to pressure and force in hydraulic systems are input force, working pressure, and output force. Main components of the hydraulic braking system are brake pedal, brake fluid, and master cylinder. The brake pedal multiplies force from the driver’s foot to the master cylinder. Brake fluid has a high boiling point and a low freezing point and is hygroscopic (absorbs moisture).

▶▶

▶▶ ▶▶ ▶▶

▶▶ ▶▶

▶▶ ▶▶

The US Department of Transportation (DOT) grades brake fluids on pH value, viscosity, resistance to oxidation, stability, and boiling point. Master cylinders convert force exerted from the brake pedal into hydraulic pressure to activate wheel brake units. Types of master cylinders are single piston and tandem (required on modern cars). Single-piston master cylinders use a primary cup to seal pressure in the cylinder and a secondary cup to prevent fluid loss. A single-piston master cylinder traps brake fluid and ­forces it into the brake lines. Residual-pressure valves are used on drum brake systems to maintain brake-fluid pressure and prevent air entry when the brakes are off. Modern vehicles have tandem master cylinders to ensure braking ability in at least one circuit despite a leak. Differential pressure switches monitor loss of pressure between the hydraulic circuits.

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▶▶

▶▶ ▶▶ ▶▶

▶▶ ▶▶

▶▶

▶▶ ▶▶ ▶▶

Wrap-Up

Braking units can be split front-to-rear, diagonally, or in an L shape. Diagonal and L-shaped braking splits retain 50% braking capability even if half the system fails. Quick take-up master cylinders work to compensate for the large running clearance maintained by low-drag brake calipers. Hydraulic braking systems use proportioning valves, metering valves, pressure-differential valves, or antilock hydraulic control units to modify hydraulic pressure. Proportioning valves reduce brake pressure to the rear wheels and are pressure-sensitive or load-sensitive. Load-sensitive proportioning valves adjust rear brake pressure according to the weight of the vehicle’s load. Pressure-sensitive proportioning valves use a poppet piston to limit the rate of braking pressure increase to the rear brakes. Metering valves work to ensure rear brake pressure is applied before front brake pressure. The combination valve combines individually operating proportioning valves, a metering valve, and a pressure-­ differential valve in one unit and cannot be repaired. Brake warning lights alert drivers to engagement of the parking brake, low brake-fluid intake, and unequal ­pressure in the hydraulic brake system. Stop lights are mounted on the rear of a vehicle and alert other drivers that the vehicle is being braked. As of 1986, all vehicles must have a center high mount stop lamp (CHMSL) to reduce incidents of rear-end ­collisions. Power brake units are either vacuum assist (most ­common) or hydraulic assist.

Key Terms aerate  The tendency to create air bubbles in a fluid. bleeding  The process of removing air from a hydraulic braking system. brake fluid  Hydraulic fluid that transfers forces under pressure through the hydraulic lines to the wheel braking units. brake hose  A flexible section of the brake lines between the body and suspension that allows for steering and suspension movement. brake lines  Lines made of seamless, double-walled steel that are able to transmit over 1,000 psi (6,895 kPa) of hydraulic pressure through the hydraulic brake system. CANbus circuit  A two-wire communication network that transmits status and command signals between control modules in a vehicle. compensating port  A port that connects the brake-fluid reservoir to the master cylinder bore when the piston is fully retracted, allowing for expansion and contraction of the brake fluid. electrohydraulic braking (EHB)  A hydraulic braking system that uses an electrically driven hydraulic pump to pressurize fluid for use in the master cylinder. hygroscopic  A substance that attracts and absorbs moisture (e.g., brake fluid).

87

inlet port  A port that connects the reservoir with the space around the piston and between the piston cups in a brake master cylinder. input force  The force applied to the input piston, measured in either pounds or kilograms. International Standards Organization (ISO) flare, also called a bubble flare  A method for joining brake lines. It is created by flaring the line slightly out and then back in, leaving the line bubbled near the end. inverted double flare  A method for joining brake lines that forms a secure, leak-proof connection. load transfer  Weight transfer from one set of wheels to the other set of wheels during braking, acceleration, or cornering. metering valve  A valve used on vehicles equipped with older rear drum/front disc brakes to delay application of the front disc brakes until the rear drum brakes are applied. It is located in line with the front disc brakes. output force  The force that equals the working pressure multiplied by the surface area of the output piston, expressed as pounds, newtons, or kilograms. outlet port  A port that links the cylinder to the brake lines. Pascal’s law  The law of physics that states that pressure applied to a fluid in one part of a closed system will be transmitted equally to all other areas of the system. poppet valve  A valve that controls the flow of brake fluid at usually preset pressures. pressure-differential valve  A valve that monitors any pressure difference between the two separate hydraulic brake circuits. It usually contains a switch to turn on the brake warning light when there is a pressure difference. primary cup  A seal that holds pressure in the master cylinder when force is applied to the piston. primary piston  A brake piston in the master cylinder moved directly by the pushrod or the power booster. It generates hydraulic pressure to move the secondary piston. proportioning valves  Valves mostly on older vehicles equipped with rear drum brakes, used to reduce rear-wheel hydraulic brake pressure under hard braking or light loads. They are located in line with the rear brakes. quick take-up master cylinders  Cylinders used on disc brake systems that are equipped with low-drag brake calipers to quickly move the brake pads into contact with the brake rotors. quick take-up valve  A valve used to release excess pressure from the larger piston in a quick take-up master cylinder once the brake pads have contacted the brake rotors. recuperation  A process by which brake fluid moves from the reservoir past the edges of the seal into the chamber in front of the piston. This prevents air from being drawn into the hydraulic system caused by low pressure when the brake pedal is released quickly. residual-pressure valve (residual-check valve)  In drum brake systems, a valve that maintains pressure in the wheel cylinders slightly above atmospheric pressure so that air does not enter the system through the seals in the wheel cylinders.

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Chapter 3  Hydraulic Brake Systems

secondary cup  A seal that prevents loss of fluid from the rear of each piston in the master cylinder. secondary piston  A piston that is moved by hydraulic pressure generated by the primary piston in the master cylinder. single-piston master cylinder  A master cylinder with a single piston that creates hydraulic pressure for all wheel units. If there is a leak in the system, there is a loss of pressure for all wheel units. tandem master cylinder  A master cylinder that has two pistons that operate separate braking circuits so that if a leak develops in one circuit, the other circuit can still operate. working pressure  The pressure within a hydraulic system while the system is being operated.

Review Questions 1. All of the statements are true except: a. Pascal’s law is the principle behind hydraulic brakes. b. Varying amounts of mechanical force can be extracted from a single amount of hydraulic pressure. c. The smaller brake pistons on the front wheels give greater mechanical force and braking power to the front wheels. d. A power booster is fitted to the brake system to increase the driver’s brake pedal force to the master cylinder. 2. If 100 psi (689.475 kPa) of working pressure is applied to a 2 square inch output piston, how much force will be created? a. 50 lb (22.68 kg) b. 100 lb (45.36 kg) c. 200 lb (90.72 kg) d. 400 lb (181.43 kg) 3. Which of the following brake fluids is suitable for ABSequipped vehicles due to its high boiling point? a. DOT 2 b. DOT 5.1 c. DOT 3 d. DOT 5 4. The port that connects the reservoir to the cylinder, just barely ahead of the primary cup is the __________. a. compensating port b. inlet port c. outlet port d. primary port 5. Disc brake systems that are equipped with low-drag brake calipers use ___________. a. tandem master cylinders b. quick take-up master cylinders c. single-piston master cylinders d. modern master cylinders 6. Different lever designs can be engineered to alter the brake pedal effort required of the driver by using different levels of __________. a. input force b. output force c. working pressure d. mechanical advantage

7. A wheel’s braking ability depends on ____________. a. engine speed b. the type of brake fluid used c. the load it is carrying d. the type of transmission used 8. Which of the following flares is created by first flaring the end of the tube outward in a Y shape? a. Straight flare. b. Bubble flare. c. Inverted double flare. d. ISO flare. 9. Which of the following is used to hold off the application of the front brakes on vehicles with disc brakes on the front wheels and drum brakes on the rear wheels? a. Load-sensitive proportioning valve. b. Metering valve. c. Pressure-differential valve. d. Combination valve. 10. The valve that reduces rear brake pressure when the vehicle is lightly loaded and allows higher pressure when it is heavily loaded is the ________. a. load-sensitive proportioning valve b. metering valve c. pressure-differential valve d. combination valve

ASE Technician A/Technician B Style Questions 1. Technician A says that hydraulic pressure is applied equally in all directions throughout a closed system. Technician B says that air in the hydraulic system will cause the brake pedal to be spongy. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that a brake pedal that does not return all the way causes the brake warning light on the instrument panel to stay on. Technician B says that a brake pedal that does not return causes the brake lights to stay on. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that brake fluid should periodically be checked for excessive moisture content. Technician B says that brake fluid should be replaced every 12,000 miles. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that DOT 5 brake fluid should be used in all vehicles today because it is silicone based and will not



absorb water. Technician B says that DOT 4 and DOT 3 are not recommended for use in ABS systems. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that the secondary piston in a master cylinder is operated by hydraulic force. Technician B says that the brake pedal return spring returns the master cylinder pistons to their original position. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that the low-level brake-fluid switch on a master cylinder turns on the brake warning light when the system is low on fluid. Technician B says that the lowlevel switch also monitors the condition of the fluid and activates the warning light when the brake fluid needs to be replaced. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that the metering valve controls pressure to the rear brakes. Technician B says that the proportioning valve controls pressure to the front brakes. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

Wrap-Up

89

8. Technician A states that air in a braking hydraulic system may cause poor performance of caliper application. Technician B says that it is possible on some vehicles to use a scan tool to verify pressure in the hydraulic system. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says that input force divided by the area of the input gives working pressure. Technician B says that working pressure multiplied by the surface area of the output piston gives output force. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that quick take-up master cylinders use two additional pistons to move brake fluid faster. Technician B says that banjo fittings are commonly used to ­connect brake hoses to wheel brake units. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

CHAPTER 4

Disc Brake Systems Learning Objectives ■■ ■■ ■■ ■■ ■■

4-1 Explain the application of a disc brake system component. 4-2 Explain the operation of a disk brake caliper. 4-3 Classify disc brake pad and friction material. 4-4 Summarize the use of wear indicators on brake pads. 4-5 Explain the difference between the various brake disc rotors.

■■ ■■ ■■ ■■

4-6 Illustrate disc brake service. 4-7 Inspect and replace calipers. 4-8 Inspect and replace disc brake rotors. 4-9 Inspect and replace wheel studs.

You Are the Automotive Technician A customer comes into the service department. She has been experiencing a pulsating brake pedal and high-pitched squealing during braking on her 2013 Town and Country minivan, which has 41,000 miles (69,202 km) on the odometer. After reviewing the vehicle’s service history, you notice the vehicle has never had any brake service.You advise the customer that you will need to complete a more thorough inspection to determine what is causing the issues. After completing a visual inspection of the brakes and measuring the rotor’s thickness and parallelism, you see that the front brake linings have worn down to the wear indicators, and the rotors are warped beyond specifications. You inform the customer of your findings and recommend that the vehicle needs new front brake pads installed and the rotors refinished. You also ­recommend that the brake fluid be flushed, as the fluid is beyond the end of its two-year life.

1. What conditions can cause a rotor to become warped? 2. What conditions would require replacement of the rotors rather than just refinishing them? 3. What are the relatively common maximum specifications for rotor runout and thickness variation? 4. What are the benefits of using an on-car brake lathe to refinish the rotors?





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Chapter 4  Disc Brake Systems

▶▶ Introduction Disc brakes are so named because they create braking power by forcing flat friction pads against the sides of a rotating disc. This disc is also called a disc brake rotor, or rotor, and is bolted to the wheel (FIGURE 4-1). Disc brake calipers straddle the pads. Hydraulic pressure from the master cylinder causes the caliper to create a mechanical clamping action, forcing the brake pads onto the surface of the rotor, creating friction. By forcing friction materials against moving surfaces, the vehicle’s kinetic energy is transformed into heat energy. As the vehicle’s kinetic energy is transformed into heat energy, the vehicle’s speed decreases. Higher applied forces can be used in disc brakes than in drum brakes because the rotor has to withstand only compressive forces, whereas the drum has to withstand tension forces (FIGURE 4-2). Because heat is generated on the outside s­ urfaces of the rotor, it can be easily transferred to the atmosphere. Because of these and other design features, disc brakes are effective at creating substantial braking power by using a fairly simple design that is relatively easy for technicians to service and repair.

FIGURE 4-1  Disc brake operation.

Strong– No Deflection Heavy Pressure

Not as Strong– Deflection FIGURE 4-2  Disc versus drum brakes.

▶▶TECHNICIAN TIP The purpose of the disc brake system is to provide an effective means to slow the vehicle down under a variety of conditions in an acceptable distance and manner. The better a braking system can do this, the more likely the vehicle will avoid a collision.

Disc Brake Safety When dealing with a disc brake system, the technician must be able to identify what components are under pressure at any given moment. Operating with hydraulic pressures in the 1,000+ psi areas, technicians must know what to disassemble and what not to. Along with the potential high-pressure issues, the components of the braking system get very hot during regular use, so the technician must be aware of whether the vehicle had been operated prior to disassembly, because they may get burned if they try to take it apart too quickly. Modifying, substituting, or other usage of the brake components should be avoided as this could become a liability for the technician if the operator is injured after they have worked on the vehicle. Moderate Pressure

Hybrid or Other Vehicles That Cycle Their Antilock Braking System On and Off When repairing a hybrid vehicle, the technician must be aware of where the keys for the vehicle are. If the keys or fob are inside the vehicle’s cabin, the vehicle may turn on to recharge the battery. If this happens, injury could possibly result, so best practice would be to remove the key and keep it far enough away from the vehicle so that the vehicle cannot be turned on by it. Another issue with a hybrid vehicle is the high-voltage wiring that runs throughout the vehicle. The technician must be aware of the high-voltage wiring (usually brightly colored orange or yellow), so that they can use proper personal protection ­equipment (PPE) before touching it.

▶▶ Disc 4-1 Explain the application of a disc brake system component.

Brake System Component Application

Modern passenger vehicles are almost always equipped with disc brakes on at least the front two wheels, and many manufacturers are using them on all four wheels. The primary components of the disc brakes are as follows: ■■ ■■ ■■

rotor caliper brake pads (FIGURE 4-3).

The rotors are the main rotating part of this brake system. They are durable and resistant to damage by the high temperatures that occur during braking. In high-performance



Disc Brake System Component Application

vehicles, the rotors are made from composite materials, ceramics, or carbon fiber; otherwise, they are usually made of cast iron. The caliper straddles the rotor and houses the disc brake pads and one or more activating pistons. The calipers use hydraulic pressure from the master cylinder to apply the brake pads. They are usually bolted to the steering knuckle or, in the case of a non-steering axle, to a suspension component. Calipers should be inspected at the same time as the brake pads. The disc brake pads are located inside the caliper or caliper mounting bracket. The pads are forced against the rotor to slow down or stop the vehicle. The disc brake pad consists of a friction material bonded or riveted to a steel backing plate. With this design, the pads will wear out over time and must be replaced periodically.

Disc Brake Operation

FIGURE 4-3  The disc brake system.

Disc brakes can be used on all four wheels of a vehicle, or on the front wheels, with drum brakes on the rear. When the brake pedal is depressed, a pushrod transfers the force through a brake booster to a hydraulic master cylinder. The master cylinder converts the pedal force into hydraulic pressure, which is then transmitted via brake lines and hoses to one or more pistons at each brake caliper (FIGURE 4-4). The pistons operate on friction pads to provide a clamping force on a rotor that is attached to the wheel hub. This clamping action is designed to stop the rotation of the rotor and the wheel. The rotors are free to rotate with the wheels, because of wheel bearings and the hubs that contain them. The hub can be part of the brake rotor, called a hub-style rotor. Or it could be separate from the rotor, called a hubless-style rotor. In this case, the rotor slips over the hub and is bolted to it by the wheel and lug nuts (FIGURE 4-5). The brake caliper assembly is normally bolted to the steering knuckle or vehicle axle housing (FIGURE 4-6). In most cases, the brake is positioned as close as possible to the wheel, but there are exceptions. Some high-performance cars with independent rear ­suspension (IRS) use inboard disc brakes (Figure 4-26) on the rear wheels. The calipers are mounted

Master Cylinder

Pedal

Secondary Piston

Primary Piston

FIGURE 4-4  The master

cylinder converts the pedal force into hydraulic pressure, which is then transmitted via brake lines and hoses to one or more pistons at each brake caliper.

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Chapter 4  Disc Brake Systems

FIGURE 4-5  Hub and hubless rotors.

FIGURE 4-6  Floating or sliding caliper mounting methods.

on or next to the differential, which is directly mounted to the vehicle body. This configuration increases vehicle handling because it reduces the vehicle’s unsprung weight by taking the differential assembly and brakes from the suspension system and mounting them to the body. Because the wheels and axles are now lighter, the vehicle’s springs can do a better job of keeping the wheels on the ground, especially on uneven road surfaces. Disc brake pads require much higher application force to operate than drum brake shoes because they are not self-energizing. This additional clamping pressure is created by increasing the diameter of the caliper pistons. Unfortunately, this means the brake pedal travel is lengthened to move the additional fluid being displaced by the larger caliper pistons. Building in more pushrod travel would require more room under the dash for the brake pedal. Manufacturers have overcome this problem by equipping most disc brake systems with a power booster. Because of the high forces needed to apply a disc brake, using it as a parking brake is more challenging. Some manufacturers have chosen to design more complicated calipers, whereas others have built an auxiliary drum brake assembly into the center of the rear disc brake rotors to provide for parking brake operation. This is referred to as a top-hat design.

Advantages and Disadvantages

▶▶TECHNICIAN TIP Although there may appear to be more disadvantages to disc brakes than drum brakes, the advantages are generally considered much more critical than the disadvantages. Thus, disc brakes are preferred over drum brakes in most applications.

Disc brakes have a number of advantages over drum brakes. They also have some disadvantages. In most cases, the advantages outweigh the disadvantages. One of the biggest advantages is that disc brakes can generate and transfer greater amounts of heat to the atmosphere; because most of the friction area of a rotor is exposed to air, cooling is far more rapid in disc brakes than in drum brakes. This faster cooling makes them better suited for high-performance driving or heavy-duty vehicles and reduces the likelihood of brake fade. Also, because of their shape, rotors tend to scrape off water more effectively. After being driven through water, disc brakes operate at peak performance almost immediately. Further, due to their design, disc brakes are self-adjusting and neither need periodic maintenance nor rely on a self-adjusting mechanism that is prone to sticking. Lastly, disc brakes are in most cases also easier to service than drum brakes are. Although disc brakes have a number of benefits over drum brakes, there are some disadvantages. Probably the most apparent disadvantage is that disc brakes are much more prone to making noise. Their design tends to create squeals and squeaks, which can be very annoying. Many a technician has spent time replacing perfectly functional disc brakes because of excessive noise complaints. Another issue is that the rotors warp more easily than those in drum brake systems. Because the brake pads are pressing on each side of the rotor, thickness variations as small as 0.0003" (0.0076 mm) can cause brake pedal pulsations, requiring resurfacing or replacement. The last disadvantage is that because disc brakes are not self-energizing, they need higher clamping forces, which requires a power booster. This also makes it harder to use disc brakes as an effective parking brake.



▶▶ Disc

Disc Brake Caliper Operation

Brake Caliper Operation

4-2 Explain the operation of a disk In most applications, the disc brake caliper assembly is bolted to the vehicle axle housing, brake caliper. or steering knuckle, and it clamps the brake pads onto the rotors to slow the vehicle. There are two main types of calipers: fixed calipers and sliding or floating calipers (FIGURE 4-7). Sliding or floating calipers are the most common type used in passenger vehicles because they are easier to build and more compact. All calipers are fitted with a bleeder screw on the top of the piston bore, to allow for the removal of air within the disc brake system, as well as to help in performing routine brake fluid changes. Fixed calipers are rigidly bolted in place and cannot move or slide. This makes their application of braking forces more precise and more powerful than floating calipers. They commonly have one to four pistons on each side of the rotor (FIGURE 4-8). When the brakes are applied, hydraulic pressure forces the pistons on both sides of the caliper inward, causing the brake pads to come into contact with the rotor (FIGURE 4-9). Once the pad-to-rotor clearance is taken up, the hydraulic pressure rises equally on each side of the caliper, applying both brake pads equally. Pad replacement is done without removing the caliper; the technician must take caution to ensure that they don’t disassemble the caliper FIGURE 4-7  Fixed and sliding or floating calipers. when they remove the brake pads. The floating or sliding caliper has brake pads on each side of the rotor, but all of the pistons are only on one side, usually the inboard side of the rotor. Thus, the hydraulic force is generated on one side of the rotor, but the unique design applies equal braking force to both sides of the rotor. This distribution of force is possible because the caliper is mounted on pins, or slides, that allow it to move (float or slide) from side to side, as necessary. This movement allows pistons on one side of the rotor to generate equal force on both sides of the rotor at the same time. When the brakes are applied, hydraulic pressure forces the piston toward the rotor. This takes up any clearance between the inboard brake pad and the rotor and starts to push the pad into the rotor. Because the caliper is free to move on the pins or slides, it gets pushed away from the rotor, pulling the outboard brake pad into contact with the outboard side of the rotor. Once all FIGURE 4-8  Fixed calipers with multiple pistons. clearance is taken up on the outboard brake pad, the clamping force increases equally on both brake pads, applying the brakes (FIGURE 4-10). Floating calipers are mounted in place by guide pins and bushings (FIGURE 4-11). The pins allow the caliper to move in and out as the brakes are operated and as the brake pads wear. Because the calipers move on the pins, the bushings must be lubricated with high-temperature, waterproof disc brake caliper grease when they are serviced. This helps prevent them from binding or sticking. Inspecting, cleaning, and lubricating the pins and pin bores, including any bushings and dust boots, are important steps in disc brake repair, and if any of these components show wear they should be replaced. Sliding calipers have matching machined surfaces on the caliper and caliper mount that allow the caliper to slide on the mount (FIGURE 4-12). The sliding mount holds the caliper in position and prevents it from rotating when the brakes are applied. The machined mounts allow the caliper to move side to side, as necessary, to operate the brakes or adjust for brake pad wear. When the calipers are serviced, the surfaces must be cleaned and lubricated FIGURE 4-9  Fixed caliper being applied.

95

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Chapter 4  Disc Brake Systems

FIGURE 4-11  Floating caliper and guide pins.

FIGURE 4-10  Sliding or floating caliper application.

FIGURE 4-12  Sliding caliper.

with the same high-temperature, waterproof grease as the floating calipers were. There are several methods to holding sliding calipers in place. Because sliding calipers do not slide as easily as floating calipers, most light-duty vehicles use the floating caliper design.

O-Rings ▶▶TECHNICIAN TIP Because the force generated by the O-ring to retract the piston is fairly small, any corrosion or buildup on the piston or bore will cause the piston to stick and not retract. This holds the brakes in the applied position, causing brake drag, overheated brakes, and poor fuel economy. Technicians can also identify this situation by using an infrared temperature gun to measure the temperature of each brake rotor after test-driving the vehicle. The temperatures should be approximately the same on each side. If they are not, suspect a stuck or binding caliper.

In disc brake calipers, the piston is sealed by a stationary square-section sealing ring, also called a square-cut O-ring (FIGURE 4-13). This O-ring has a square cross section and is fitted into a machined groove in the caliper. The O-ring is compressed between the piston and caliper housing, creating a positive seal to keep the high-pressure brake fluid from leaking out. It also prevents air from being drawn into the system when the brake pedal is released quickly, creating a low-pressure situation in the hydraulic system. When the brakes are applied, the piston moves outward, slightly deforming the O-ring seal (FIGURE 4-14). When the brakes are released, the elasticity or flexibility of the seal causes it to return to its original shape. This action of the sealing ring retracts the piston to provide a small running clearance between the rotor and pads. As the brake pads wear, the piston must move outward a bit farther than the sealing ring can stretch or flex. The sealing ring is designed to allow the piston to slide through it in this situation, taking up the extra clearance and making the disc brakes self-adjusting. Some calipers, sometimes called low-drag calipers, are designed to maintain a larger brake pad–to-rotor clearance by retracting the pistons a little bit farther. This is accomplished by modifying the sealing groove in the caliper so that the outside of the groove is slightly angled toward the rotor (FIGURE 4-15). This position allows the seal



Disc Brake Caliper Operation

A: Square-Cut O-Ring

B: Square-Cut O-Ring Groove

B

A

Caliper Fluid Seal

Caliper Body

O-Ring Groove in Caliper

FIGURE 4-13  O-rings. A. Square-cut O-ring and O-ring cut to show the square section. B. Square-cut O-ring groove in caliper.

A

B

FIGURE 4-14  Square-cut O-ring. A. Square-cut O-ring during brake application. B. Square-cut O-ring during brake release.

Seal Travel Low Drag Caliper Seal Travel Standard Caliper

FIGURE 4-15  Low-drag caliper.

97

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Chapter 4  Disc Brake Systems

FIGURE 4-16  Corroded caliper piston bore.

A

B

FIGURE 4-17  Heat transfer. A. Phenolic piston (slow heat transfer).

B. Steel piston (fast heat transfer).

▶▶ Disc 4-3 Classify disc brake pad and friction material.

to flex a bit farther upon brake application and then retract the piston a greater distance. These ­systems use a “quick take-up” or “fast-fill” master cylinder to maintain adequate brake pedal reserve height. The primary sealing surface is the outside diameter of the piston. It is critical that this surface be smooth and free of pitting or rust; therefore, steel pistons are chrome plated. This gives the surface a hard, wear-resistant, and corrosion-resistant finish. Chrome can still rust, but it is much more corrosion-resistant than steel. Another way manufacturers have dealt with the corrosion issue is by making pistons out of a phenolic resin. Billiard balls also are made from phenolic resin, which is very dense when it hardens and which does not corrode or rust, making for a good sealing surface in brake systems. Although the phenolic pistons themselves do not corrode, the cast iron bore of the caliper does corrode and rust and can therefore cause a phenolic piston to seize in the bore (FIGURE 4-16). Phenolic pistons transfer heat more slowly than steel p ­ istons (FIGURE 4-17), which is a good thing because excessive heat transferred through the piston to the brake fluid may cause it to boil. Calipers with phenolic pistons are therefore less susceptible to brake failure from boiling brake fluid. However, care must be taken when servicing brakes that use phenolic pistons because they are more easily damaged from prying with screwdrivers or pliers. There is also a dust boot that seals the surfaces of the piston and caliper bore from outside dirt and moisture. This boot connects to both the piston and the caliper and must be expandable to allow the piston to move outward as the brake pads wear. It also must be free from cuts and holes; otherwise, the piston and bore could corrode and cause the piston to bind in the bore, causing brake drag.

Brake Pads and Friction Material

Disc brake pads consist of friction material bonded or riveted onto a steel backing plate (FIGURE 4-18). Bonded linings are more common on light-duty vehicles because they are less expensive to build and because the bonding agent can fail under the very high temperatures of heavy-duty use. Riveted linings are used on heavier-duty or high-performance vehicles. Metal rivets provide a mechanical connection to hold the lining to the backing plate, which is less susceptible to failure under high temperatures. At the same time, the rivet heads would contact the rotor and wear a groove in the face of the rotor. The backing plate has lugs that correctly position the pad in the caliper assembly and help the backing plate maintain the proper position in relation to the rotor (FIGURE 4-19). Disc brakes are usually designed so that the thickness of the pads can be checked easily once the wheel has been removed. Most disc brakes also are designed to allow the pads to be replaced with a minimum of disassembly. The composition of the friction material affects brake operation. Materials that provide good braking with low pedal pressures tend to lose efficiency when they get hot, thus increasing the stopping distance. They also tend to wear out sooner. Materials that maintain a stable friction coefficient over a wide temperature range generally require higher pedal pressures to provide efficient braking. They also have a tendency to put added wear on the disc brake rotor, reducing how long it’ll be useful for (FIGURE 4-20).

Brake Friction Materials Friction is the force that acts to prevent two surfaces in contact from sliding against each other. The amount of friction between two surfaces is expressed as a ratio and is called the



Disc Brake Pads and Friction Material

99

FIGURE 4-19  Brake pad locating lugs.

FIGURE 4-18  Bonded and riveted brake pads.

coefficient of friction. When friction occurs, the kinetic energy (motion) of the sliding surfaces is converted into thermal energy (heat). Some combinations of materials, such as a hockey puck on ice, have a very low coefficient of friction. There is very little friction between them and therefore almost no sliding resistance. Rubber tires against a dry, hard road surface have a high coefficient of friction, which means they tend to grip and resist sliding against each other. Disc brake pads and drum brake linings are made from materials that have a moderate coefficient of friction (TABLE 4-1). When replacing pads, the new pads must be put through a drive cycle to break in the pads to the rotors. Brake pads must be able to absorb and disperse large amounts of heat without braking performance being adversely affected. As the heat in brake pads and linings builds up, the coefficient of friction capability of the material—and consequently its stopping power—is reduced. This is called brake FIGURE 4-20  Brake rotor wear. fade. Minimizing or overcoming fade is a major factor in the design of brakes and the development of brake friction materials. Brake friction materials are made from asbestos compounds because of the excellent heat ▶▶SAFETY TIP resistance of that material. Now that asbestos has been proven to be hazardous, it is generally Don’t assume that a brake pad doesn’t banned and is not normally used. Today, brakes are manufactured from a variety of different contain asbestos; it is still found in some materials, including the following: ■■ ■■ ■■ ■■ ■■

non-asbestos organic (NAO) materials—organic materials such as Kevlar and carbon low-metallic NAO materials—small amounts of copper or steel and NAO materials semi-metallic materials—a higher quantity of steel, copper, and/or brass ceramic materials—ceramic fiber materials and possibly a small amount of copper carbon fiber materials—carbon fiber material of organic polymers.

TABLE 4-1  Brake Lining Coefficient of Friction (Sliding) Materials Involved

Coefficient of Friction-Dry Sliding

Rubber and concrete

0.6–0.85

Steel and cast iron

0.23

Copper and cast iron

0.29

Brass and cast iron

0.3

Leather and oak

0.52

Brake lining (FF rating)

0.35–0.45

applications.

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Chapter 4  Disc Brake Systems

Applied Math AM-1: Charts/Tables/Graphs: The technician can interpret charts, tables, and graphs to determine the manufacturer’s specifications for a given system; draw reasonable conclusions about a situation being modeled; and select, apply, and translate among mathematical representations to solve problems. Technicians regularly apply math concepts to disc brake diagnosis and repair. For example, a technician will measure the amount of rotor thickness variation and runout if there is a brake pedal pulsation problem. The technician may perform these measurements and come up with a thickness variation of 0.0025" (0.064 mm) and a runout of 0.0015"

(0.038 mm) on the left front rotor. Next, the technician will look up the manufacturer’s specification chart and find that the maximum allowable thickness variation is 0.0005" (0.013 mm) and the maximum allowable runout is 0.003" (0.076 mm) for the vehicle being worked on. Using the information from the chart, the technician will determine that the thickness variation is excessive by 0.002" (0.051 mm) (0.0025" [0.064 mm] – 0.0005" [0.013 mm] = 0.002" [0.051 mm]), and the runout is OK since it is under the maximum allowable specification. But even so, because the thickness variation is out of specifications, the rotor has to be refinished or replaced to bring it back within specification.

The choice of brake lining compound depends on the application. Lighter passenger vehicles generate less heat in the brakes than heavy or high-performance vehicles do. Living in a very hilly region of the country or where there is a lot of stop-and-go traffic puts added demands on the brake pads. The optimum brake composition for any given vehicle or use is a combination of weighted qualities, including the following: ■■ ■■ ■■ ■■ ■■ ■■ ■■ ■■ ■■

stopping power heat absorption and dispersion resistance to fade recovery speed from fade wear rate less dust on wheels performance when wet operating noise price.

For instance, owners of small economy vehicles tend to value a longer pad life and minimal operating noise rather than resistance to fade in extreme conditions. Owners of high-performance cars, however, may consider fade resistance and stopping power at high speeds more important than noise levels or wear rate. The Society of Automotive Engineers (SAE) has adopted letter codes to rate the coefficient of friction of brake lining materials. The rating is written on the edge of the friction linings and is called the edge code (FIGURE 4-21). The closer the letter is to Z, the less friction the material has and the harder the brake pedal must be applied to achieve a given amount of stopping power. These code letters represent the following coefficients of friction: ■■ ■■ ■■ ■■ ■■ ■■ ■■

FIGURE 4-21  Brake lining edge code (FF).

C: ≤0.15 D: 0.15–0.25 E: 0.25–0.35 F: 0.35–0.45 G: 0.045–0.55 H: >0.55 Z: Unclassified.

The lining is tested both cool and hot. The rating is a two-­ letter designation, such as “FF.” The first letter is for the cool performance, and the second letter is for the hot performance. For example, FF has a cool coefficient of friction of 0.35–0.45 and the same coefficient of friction at the hot temperature. It also is very possible that the hot and cold ratings will differ from each other. For example, if the rating is FE, the coefficient of friction reduces as the temperature of the lining heats up. Notice that the coefficient of friction range is quite wide for each letter designation. Linings with the same letter ratings may not have the same



Disc Brake Pads and Friction Material

101

braking performance as one another. This means that an EE-rated lining from one manufacturer is likely to have different braking characteristics than an EE-rated lining from another manufacturer. Always use high-quality brake lining from reliable companies to help avoid brake issues.

Antinoise Measures Disc brakes are more prone to annoying brake squealing than are drum brakes. Brake squealing is caused by vibrations set up between the brake pad and rotor. Manufacturers have addressed this problem in a number of ways: 1. Using softer linings with a higher coefficient of friction, which are less prone to noise than harder linings with a lower coefficient of friction. 2. Adding brake pad shims and guides to the brake pads, which help cushion the brake pad and absorb some of the vibration (FIGURE 4-22). 3. Using springs to tightly hold the pads in place to minimize vibration (FIGURE 4-23). 4. Contouring and grooving the lining material in a way that minimizes vibration. Grooving helps ventilate gases that build up at the surface of the friction material under heavy brake application. It also can help modify the harmonic vibration quality of the friction material to reduce brake squeal. On some pads, the depth of this groove is set so that as the pad wears thinner, the remaining groove gets smaller; when it can no longer be seen, the pad should be replaced (FIGURE 4-24). 5. Incorporating bendable tangs on the brake pad backing plate that allow technicians to crimp the tangs so that they are more firmly mounted in the caliper (FIGURE 4-25).

▶▶TECHNICIAN TIP Some manufacturers claim that refinishing their rotors removes enough material from the rotor that it can cause the brakes to squeal due to less mass, which changes the harmonic vibration qualities of the rotor. They recommend replacing the rotors any time the rotors are worn enough to need resurfacing.

FIGURE 4-22  Brake pad shims and guides.

FIGURE 4-23  Example of spring-loaded brake pad retainers.

FIGURE 4-24  Brake lining grooves and contouring.

FIGURE 4-25  Brake pad bendable tangs.

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Chapter 4  Disc Brake Systems

Technicians also can apply noise-reducing compounds to the brake pads. One is a type of high-temperature liquid rubber compound that is applied to the back of the brake pad. When it cures, it stays flexible, absorbs brake pad vibrations, and helps reduce brake noise. Another compound is a specially designed liquid that is applied directly to the face of the lining material. This compound helps to slightly modify the lining’s coefficient of friction, making it less likely that the lining will squeal. Be sure to apply the correct compound to the correct side of the brake pad.

Applied Science AS-1: Sound: The technician can demonstrate an understanding of the role that sound plays in identifying various problems in the vehicle. Sound is a series of waves that travel through a gas, liquid, or solid and can often be heard by the human ear. Sound waves are created by vibrating objects, such as a guitar string. Moving objects that are in sliding contact with each other are highly likely to create sound. One example of this is fingernails dragging on a chalkboard. The fingernails vibrate on the surface of the chalkboard and create sound waves that are then heard by the ear.

▶▶ Wear 4-4 Summarize the use of wear indicators on brake pads.

Technicians commonly use differences in sound to assist in diagnosing disc brake problems. For example, brake pads that are worn down to the metal backing plate make a deep grinding noise when the brakes are applied. Listening to hear which wheel or wheels the noise is coming from helps identify the source of the problem. In the same way, if the disc brakes are equipped with a scratcher style of brake warning system, the brakes will make a high-pitched screeching noise.This noise happens when the vehicle is being driven and the brakes are not being applied. One way to help determine which side the noise is coming from is by driving the vehicle next to a concrete wall or building. If the noise is on that side, it will get much louder than when not near the wall.

Indicators on Brake Pads

Some manufacturers provide a means of notifying the driver that the brake pad linings are worn to their minimum limit. This helps ensure that the brake linings do not wear down to the point that they cannot properly perform their job anymore. Excessively thin brake linings tend to heat up more quickly than thicker linings, which can lead to premature brake fade. Not all manufacturers use a brake lining wear indicator. It is especially important to inspect the brakes at regular intervals, usually during tire rotations or oil changes.

Types

FIGURE 4-26  The scratcher brake wear

indicator.

▶▶TECHNICIAN TIP In many cases, the scratchers start to make noise when the brakes are not applied and stop making noise when the brakes are applied, since applying the brakes tends to dampen the vibrations.

Some manufacturers use a mechanically operated wear indicator to notify the driver that the brake pads are worn to their minimum limit. This is achieved by a spring-steel scratcher mounted to the brake pad (FIGURE 4-26). Part of the scratcher extends below the brake pad backing plate at the lining’s minimum wear thickness. When the friction material wears down far enough, the scratcher contacts the surface of the rotor and makes a squealing noise similar to fingernails on a chalkboard. This distinctive noise means the brakes need service right away. When the brake pads are replaced, ensure they come equipped with new scratchers set to the correct depth so that they can function the next time the pads wear down. Some manufacturers use a warning lamp or warning message on the dash to alert the driver that the lining is worn to its minimum thickness. These systems have an electrical contact installed on the brake pad at the point of the lining’s minimum wear thickness. When the pad wears to this minimum thickness, the contact touches the rotor as the brakes are applied, prompting a warning light or warning message that tells the driver the disc brake pads are due for replacement. These types of sensors either complete the circuit to the electronic control module (ECM) or provide a ground to allow the computer to determine that the pads need to be replaced (FIGURE 4-27). These contacts can be manufactured into the pad or clipped onto the pad. The contacts are normally replaced when the pads are replaced. Make sure that the contacts either come with the new pads or are ordered along with the pads. To check the operation of the brake pad wear indicator system and determine any necessary actions, follow the steps in SKILL DRILL 4-1.



Wear Indicators on Brake Pads

103

Ignition Electrical Contact Low Brake Warning Lamp

Disc Pad

FIGURE 4-27  A brake pad wear indicator system illuminates a warning lamp on the dash when one or more brake pads wear down to a predetermined level.

SKILL DRILL 4-1 Inspecting Brake Pads and Wear Indicators 1. Determine the type of wear indicator system used. Check that it is not contacting or is only nearly contacting the rotor.

2. If the system is a sensor style, test the system to verify that the system is operational. Determine any necessary actions.

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Chapter 4  Disc Brake Systems

Checking Brake Pads During a routine maintenance inspection where there are no customer concerns regarding the brakes, a simple visual inspection and a measurement of the lining thickness is usually adequate. But if there are customer concerns with the brakes, it may be necessary to remove the pads for a more detailed inspection. Removing the brake pads is the most thorough way to inspect them because not only the thickness of the lining but also the condition of the friction surface can be inspected. This could show cracks in the lining or other defects that wouldn’t otherwise be seen from the outside. It also allows for a more thorough inspection of the retaining hardware. When removing pads, pay attention to the way the brake pads come off, because some pads have slight differences (such as locating nubs) that make it easy to install them incorrectly. To remove, inspect, and replace pads and retaining hardware, and to determine any necessary actions, follow the steps in SKILL DRILL 4-2.

SKILL DRILL 4-2 Checking Brake Pads 1. Remove the pads and retaining hardware.

2. Inspect all pads, retaining hardware, and antinoise shims for wear or damage.

3. Measure the remaining brake pad thickness and compare it to specifications. Determine any necessary action(s).



▶▶ Various

Various Disc Brake Rotors

105

Disc Brake Rotors

The brake disc or rotor is the main rotating component of the disc brake unit. The wheel and rotor rotate together, leading some manufacturers to integrate the antilock brake system (ABS) tone wheel into the rotor. Because friction between the rotor and brake pads generates a high level of heat, rotors must be able to withstand high temperatures. The pads are also forced onto the surface of the rotor with potentially thousands of pounds of force, so the rotor must be strong and have a durable surface. The rotors are usually made of cast iron. To reduce weight, some manufacturers use a two-part rotor with a cast iron disc and a stamped steel center hat (FIGURE 4-28). This style of rotor is called a composite rotor. Some heavy-duty and/or high-performance vehicles have rotors made of reinforced carbon, carbon ceramic, or composite ceramic substances to reduce weight and withstand much higher temperatures. For proper operation, rotors must maintain their shape and resist warpage under highheat and high-pressure conditions. Because of these requirements, they are usually made of cast iron. On motorcycles, rotors are often made of stainless steel for cosmetic reasons. Disc brakes also are equipped with a dust shield to help protect the rotor. Dust shields help keep dust, water, and other road debris away from the inside surface of the rotor (FIGURE 4-29). They also help direct air flow to the rotor to assist with heat transfer to the atmosphere. Dust shields are commonly made of plastic, but they can also be made of stamped sheet metal. Dust shields can become damaged during brake repair, so always inspect them for proper clearance before installing the wheel assembly.

4-5 Explain the difference between the various brake disc rotors.

▶▶TECHNICIAN TIP Rotors can be warped by improperly torquing the lug nuts. Always use a properly calibrated torque wrench (or the proper torque stick with an air impact wrench if the shop policy allows) to torque the lug nuts to the manufacturer’s specified torque.

Solid and Ventilated Rotors Rotors can be solid or ventilated (FIGURE 4-30). Solid rotors are less expensive and usually found on smaller vehicles. Ventilated rotors are used to improve heat transfer to the atmosphere. These passageways are designed to use centrifugal force to cause air to flow through the center of the rotor when it is rotating. Ventilated rotors are used on heavier vehicles or high-performance vehicles. Some ventilated rotors are directional, meaning they are designed to force air through the rotor in one direction only (FIGURE 4-31). If the rotor is rotated in the wrong direction, it will not pump air properly and will overheat more easily.

Cast Iron Disc

Stamped Steel Center

Composite Rotors Composite rotors use a steel center section and a cast iron wear surface that comes into contact with the brake pads (FIGURE 4-32). When replacing ­composite rotors a similar type of rotor must be used; otherwise, the brake components will not fit correctly, which will then cause issues when driving the

FIGURE 4-28  A composite rotor.

A

FIGURE 4-29  A typical dust shield.

B

FIGURE 4-30  Rotors. A. Standard solid rotors. B. Ventilated rotors.

Chapter 4  Disc Brake Systems

Direction of R ota tio n

106

FIGURE 4-31  Directional ventilated rotor.

FIGURE 4-32   Composite rotor.

vehicle. These types of rotors can be remachined, but that must be done properly; ­otherwise, failure of the rotor could happen. See the refinishing chapter for more information.

Ceramic Composite Rotors

▶▶TECHNICIAN TIP If an unusual scraping or grinding noise can be heard when test-driving a vehicle after servicing the brakes, check to see whether the dust shield is contacting one of the rotors. If so, it can make a lot of loud scraping or grinding noises. Because it is thin plastic or sheet metal, chances are good that it is either caught on something or bent. It can usually be put back into place or bent back into shape easily.

FIGURE 4-33   Ceramic composite rotor.

Ceramic composite rotors are a carbon fiber mixture blended with a resin of carbon and silicon to create a carbon ceramic compound that is nearly as hard as a diamond. This material has a low thermal expansion rate, which prevents it from deforming under heavy braking, and is totally resistant to corrosion (FIGURE 4-33). The ceramic features of this rotor allow for more favorable noise damping ability and long life. The downside to these types of rotors is the cost, which averages between $6,000 and $12,000 per application, but their life expectancy is around 180,000 miles (289,682 km). Disc brake rotors with holes or slots machined into their surface dissipate heat faster (FIGURE 4-34). They also help to quickly remove water from the surface of the pad in wet driving conditions. Because the pads wipe across the holes or slots, the surface of the pad is prevented from becoming hard and glassy smooth from the friction and heat of use. However, this scraping action reduces the overall life of the brake pad, so these types of rotors are generally used only in high-performance or heavy-duty vehicles. Most disc brake rotors are stamped with the manufacturer’s minimum-thickness s­ pecification (FIGURE 4-35). This minimum thickness ensures an adequate amount of thermal mass for stopping power. When material is removed, there is not as much material to absorb heat, so the rotor heats up faster (FIGURE 4-36). The excess heat can lead to brake

FIGURE 4-34  Slotted and drilled rotor.



Disc Brake Service

400°F Rotor-Standard Thickness (at end of moderate hill) FIGURE 4-35  Rotor with minimum thickness stamped on it.

500°F Rotor-Below Minimum Thickness (at end of moderate hill)

FIGURE 4-36  Rotor thickness and heat capacity.

fade sooner. Also, when the brake pads wear, if the thickness of the rotor were below this minimum, the piston could be pushed out beyond the edge of the sealing ring, which would cause the brakes to lose hydraulic pressure and fail. Make sure the rotors are always above the manufacturer’s minimum thickness before putting them back in service.

▶▶ Disc

Brake Service

When completing disc brake service, the technician must inspect the entire braking system to make sure that the rest of the system is serviceable. Fixing one portion of the braking system will cause the technician and customer to place faith in half of the braking system. When servicing disc brakes, the technician must use the manufacture’s operation specifications when evaluating what is happening on the vehicle. This information can be obtained from the service information system prior to doing any work on the braking system.

Diagnosis Disc brake diagnosis starts with understanding the customer’s concern. Communicating directly with the customer is the best way to do that, but the customer is not always available. An experienced service advisor will gather the required information, so the service advisor’s notes on the repair order should be read carefully or this person should be spoken with directly. Once the customer’s concern is understood, a test drive is usually needed to verify the accuracy of the concern. Depending on the concern, it may be as simple as stepping on the brake pedal without moving the vehicle and feeling the pedal sink to the floor, or it could require a more detailed test drive to observe the fault the customer is describing. This is a good opportunity to test the brakes under a variety of conditions. Replicating the customer concern is important in order to address the situation that the customer is experiencing. During the test drive, find a safe place to operate the brakes at a variety of speeds with a variety of brake pedal pressures, especially trying to mimic the conditions that the customer described. It may be necessary to go on a test drive with the customer driving, allowing them to operate the vehicle in the way that makes the problem evident and to point out the particular situation they’re experiencing. Also, it is good to have the customer along in case the problem does not occur, to avoid their thinking that the technician doesn’t believe there is a problem or that the technician is ignoring the issue. If there is a concern related to the ABS that requires a test drive, extreme caution is required so that an accident doesn’t happen. Because ABS operates only during extreme braking or poor traction conditions, a driver runs the risk of being rear-ended by a vehicle behind them or of losing control of the vehicle if the brakes are applied hard enough to activate the ABS. So make sure the vehicle is being tested away from all other

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▶▶TECHNICIAN TIP One way to identify whether a problem is coming from the front or rear brakes is to test-drive the vehicle in a safe place at a relatively low speed and lightly apply the parking brake. If the condition is still present, the problem is with the rear brakes, because the parking brakes are usually on the rear wheels. If the condition is not present, the problem is likely with the front brakes.

traffic. Remember that if the yellow ABS warning lamp is illuminated, the ABS system is deactivated, and it will not activate the ABS on a test drive. If the yellow ABS lamp is off, the ABS system should be active and ready to activate. It is helpful when testing ABS to do so on a surface with limited traction, such as wet pavement or a dirt road. When the brakes are applied firmly, ABS activation can typically be felt as pulsations in the brake pedal and possibly the steering wheel. The vehicle should brake quickly while maintaining steering control. In some cases, a “poor traction” lamp may illuminate, indicating that the ABS system had to activate. This warning lamp will usually turn off after several seconds. If the yellow ABS warning lamp illuminates, it typically means that the vehicle’s ECM has observed a fault in the ABS system and should be checked for diagnostic trouble codes (DTCs). Once the customer concern has been verified, enough information will hopefully have been gathered to research the concern in the service information and technical service bulletins (TSBs). Armed with this information, be ready to create a plan to begin testing the brake system. This could be as simple as performing a visual inspection of the brake fluid level and condition, removing the wheels to disassemble and inspect the brake units or measuring the thickness variation of the rotors. It could also involve an electrical diagnosis, such as locating a short circuit in the brake pad warning indicator system. Suspension and steering system faults can appear to be brake system faults. An example of this is a pulling condition while braking. If the strut rod bushings on the suspension are worn, then braking the vehicle will cause the wheel to move rearward; at the same time, that will cause the steering angle to change, causing the wheel to point in a direction other than straight down the road, imitating a brake pull. So always inspect the suspension and steering systems when diagnosing concerns related to a vehicle “pull.” The braking system on a vehicle must be restored to its proper operation if one or more braking system faults are present. In diagnosing any problem, all issues that would prevent the brakes from operating normally must be identified. Lawyers and technicians have been known to say, “Whoever touched the brake system last, owns it!” What this means is that if there is a problem in the braking system, the person who inspected it or worked on it is very likely liable for anything that went wrong with it. Brake system failures are more likely to lead to vehicle accidents than failures of most systems. Any diagnosis and subsequent repairs need to be thorough and complete, so specifications must be obtained and the manufacturer’s procedures must be followed. Once the cause of the fault has been identified, determine the action that will correct the fault. This information can then be used for an estimate of repairs and given to the customer for authorization to perform the needed work. Once the repair is complete, retest the system to confirm that the concern has been fully addressed and that no further issues are present.

Tools The tools that are used to diagnose and repair brake systems include those shown in FIGURE 4-37: Brake lining thickness gauges—used to measure the thickness of the brake lining. Brake wash station—used to clean drum and disc brake dust. Caliper piston pliers—used to grip caliper pistons when removing them. Disc brake rotor micrometer—used to measure the thickness and parallelism of a rotor. Dial indicator—used to measure the lateral runout (side to side) of the rotor. Parking brake cable pliers—used to install parking brake cables. Caliper piston retracting tool—used to retract caliper pistons with integrated parking brakes. H. Off-car/bench brake lathe—used to machine drums and rotors that are off the vehicle. I. On-car brake lathe—used to machine rotors that are on the vehicle. J. Caliper dust boot seal driver set—consists of a driver and a variety of adapters used to install various sizes of dust boot seals. K. C-clamp—used to push pistons back into the caliper bore on non-integrated parking brakes. A. B. C. D. E. F. G.



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B A

C

D

E

G

F

H

Caption appears on next page.

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I

J

FIGURE 4-37  Disc brake tools. A. Brake lining thickness gauges. B. Brake wash station. C. Caliper piston pliers. D. Disc brake rotor micrometer.

E. Dial indicator. F. Parking brake cable tool. G. Caliper piston retracting tool. H. Off-car brake lathe. I. On-car brake lathe. J. Dust boot seal/ bushing driver set.

Common Disc Brake Concerns To diagnose poor stopping, noise, vibration, pulling, grabbing, dragging, or pulsation concerns and determine any necessary actions, follow the steps outlined in the remainder of this chapter. Although braking concerns are generally easier to diagnose than most of the other systems on the vehicle, the large variety of conditions listed makes it imperative to have a good understanding of disc brake and hydraulic theory. It also helps to use all senses to assist in identifying the location of the fault. As a reminder, TABLE 4-2 lists some of the common faults to consider for each concern. When determining the different types of disc brake concerns, the technician must drive the vehicle to replicate the issue that the customer is concerned with. Driving the vehicle to determine whether the issue is present while not applying the brake could indicate a dragging or component loose situation. When inspecting the braking components after determining whether this is the issue, look for components not installed correctly or calipers not releasing properly, causing a dragging condition. If the concern happens when

TABLE 4-2  Common Brake Issues Concern

Fault

Poor stopping

• Power booster not operating properly • Internal master cylinder leak; air in the hydraulic system • Metering valve or proportioning valve blocking fluid flow • Improperly adjusted drum brakes • Improper friction lining material • Contaminated linings

Noise

• Friction lining material too hard • Wear indicator touching rotor (worn pads) • Lining worn down to metal • Worn caliper slides/guide pins • Component-specific noises

Vibration

• Improper friction lining material • Rotor surface finish not correct • Foreign object (mud, rocks, etc.) in rotor • Warped rotor/thickness variation excessive • ABS operation



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TABLE 4-2  Common Brake Issues (Continued) Concern

Fault

Pulling

• Plugged or restricted brake hose • Stuck caliper piston • Seized caliper guide pins • Contaminated lining • Lining worn down to metal • Air in the hydraulic system

Grabbing

• Contaminated lining • Stuck caliper piston • Internal master cylinder leak • Misadjusted drum brakes

Dragging

• Stuck caliper piston • Seized caliper guide pins • Misadjusted master cylinder pushrod length • Binding brake pedal • Plugged restricted brake line or hose

Pulsation

• Warped rotors • Rotor parallelism • ABS operation

the brakes are applied, which leads to a pull to one side or a pulsation/vibration, look at the opposing caliper to the way the vehicle was pulling to make sure it is operating correctly. The pulsation/vibration is usually caused by an out-of-round, excessive runout or by a non-parallel rotor. Verifying the proper installation of the rotor and components will allow the technician to narrow down the area of potential fault. The first step in any repair of a vehicle is verifying that the problem is still occurring, so that the fix can be verified once what is at fault has been determined. Once the issue is verified, the next step is to work through the components in the system to determine which one has failed. Understanding how the system works helps in quickly and correctly fixing the fault. The table will list some common issues that happen within the braking system.

▶▶ Removing

and Inspecting Calipers

Removing the caliper is necessary to replace the brake pads on most vehicles. It also allows access to machine the rotors on the vehicle, remove the rotors for replacement, or machine the rotors off the vehicle. Further, it allows for a thorough inspection of the caliper, pads, and rotor to determine the cause of a brake concern. If the caliper is likely to be reinstalled on the vehicle, or rebuilt and reinstalled, it is good practice to slightly loosen the bleeder screws and then retighten them. Doing so ensures that they are not seized in place and that they can be loosened later, when bleeding the system. Failure to do this now could waste a lot of time later trying to repair a bleeder screw when it breaks off. It is also good practice to flush the old brake fluid from the system at this time so that old brake fluid will not have to be bled through the new or newly rebuilt caliper. Also, leave the master cylinder reservoir level low so that pushing the caliper pistons into their bores will not overflow the reservoir. To push the caliper pistons back into their bores slightly, using a small pry bar or C-clamp will usually force the piston back just far enough so that the caliper can be removed from the pads and rotor once the caliper is loose from its mount (FIGURE 4-38). To remove the caliper assembly, inspect for leaks and damage to the caliper housing, and to determine any necessary actions, follow the steps in SKILL DRILL 4-3.

4-7 Inspect and replace calipers.

▶▶TECHNICIAN TIP Some technicians pinch off the flexible brake hoses by using vise-grip pliers. This should be avoided because it crimps the hose, potentially damaging it internally and/or externally. It is a better idea to pull the brake fuse and use the brake pedal holding tool to block off the master cylinder compensating ports, to prevent brake fluid from leaking out of the hose.

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FIGURE 4-38  Caliper retraction tools.

SKILL DRILL 4-3 Removing and Inspecting Calipers 1. Research the procedure for removing the caliper, using the appropriate service information. Loosen the bleeder screws slightly, and then retighten them.

2. If the caliper is being rebuilt or a new caliper will be installed, it is good practice to flush the old brake fluid from the system at this time.

3. Use a brake pedal holding tool to slightly apply the brakes and block off the compensating ports in the master cylinder to avoid excess fluid leakage.

Continued



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4. Remove the brake line or hose from the caliper. Be careful not to lose any sealing rings.

5. Push the caliper pistons back into their bores slightly. Although many technicians use a screwdriver, as shown, a pry bar or C-clamp is a safer choice.

6. Remove the caliper assembly from its mountings.

7. Inspect the caliper, including the piston dust boot, for leaks or damage. Determine any necessary actions.

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Inspecting Caliper Mountings, Slides, and Pins The caliper mountings and slides/pins are placed under heavy loads and forces, and they also operate in harsh environments. Therefore, they commonly experience wear over time. They can also corrode and bind up. Clean caliper mountings and slides/pins thoroughly, and inspect them closely for excessive damage or wear. To clean and inspect caliper mountings and slides/pins for operation, wear, and damage, and to determine any necessary actions, follow the steps in SKILL DRILL 4-4.

SKILL DRILL 4-4 Inspecting Caliper Mountings, Slides, and Pins 1. Clean the caliper mountings and slides/pins, using equipment/ procedures for dealing with asbestos/hazardous dust.

2. Once the dust has been taken care of, a brake cleaning solvent may need to be used to clean the components further.

3. Inspect the caliper mountings and slides/pins for wear and damage. Determine any necessary actions.



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Disassembling Calipers Calipers are disassembled and cleaned for a couple of reasons. The first is to diagnose a brake system concern related to one or more calipers. For example, if the vehicle has a brake pull, it could be caused by a sticking caliper piston. Whether or not this is the case can be verified by disassembling the caliper. They also need to be disassembled and cleaned if they are going to be rebuilt. Some shops rebuild the calipers themselves, but most shops just replace them with rebuilt calipers if necessary. Disc brake calipers are usually side specific, meaning they are designed to be installed on a particular side of the vehicle. Failure to install them on the correct side usually results in the bleeder screws being on the bottom of the caliper cylinder. This prevents air from being bled from the caliper and results in a very spongy brake pedal. When disassembling a caliper, compressed air is typically used to remove the piston. Be very careful because the piston can shoot out with great force, enough to break finger bones or pinch fingers off. Always use an approved wood or heavy cardboard cushion between the caliper piston and caliper housing. Keep all fingers away from the area. Clean all of the caliper parts according to the service manual procedure. Make sure the sealing ring groove is completely clean. This can be performed by scraping the groove with a variety of pick tools, such as dental picks, and then wiping it out with a rag. To disassemble and clean the caliper assembly; inspect parts for wear, rust, scoring, and damage; and replace the seal, boot, and damaged or worn parts, follow the steps in SKILL DRILL 4-5.

▶▶TECHNICIAN TIP Because the rotor is exposed, skip ahead to the sections on inspecting and servicing rotors, and complete the tasks listed there. This will save time and effort. Return here after finishing with the rotor.

SKILL DRILL 4-5 Disassembling Calipers 1. Disassemble the caliper, following the service manual procedure.

2. Clean all of the caliper parts, including the seal grooves, following the service manual procedure.

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3. Inspect each of the parts for damage, rust, and wear. Also check the caliper pin bores or bushings for wear or damage. Replace if they cannot be cleaned up.

4. Measure the caliper’s bore-to-piston clearance with a feeler gauge, and compare to specifications. Determine any necessary actions.

Reassembling Caliper and Pad Components Reassembling the caliper requires patience and attention to detail. Ensure that the sealing ring groove is spotless and that the O-ring gets seated fully in the groove. Use clean brake fluid or approved caliper piston assembly lube on the piston and sealing ring prior to installing it. Be careful not to pinch, twist, or cut the sealing ring and dust boot. Also check the service manual to find out when the piston dust boot needs to be installed. Some calipers require the dust boot to be installed in the caliper before installing the piston. In this case, a special technique of using air pressure to “balloon” the dust boot is required so that the piston can slip inside it. This takes a lot of experience, and it is easy to cut and bruise fingers or a hand. Ask a supervisor to demonstrate this technique. On most calipers, the seal can be installed after the piston is installed. This is usually much easier to do. When installing shims, springs, and clips, make sure to position them properly. Most of these parts can be installed in a variety of ways, many of which will cause rubbing, wear, noise, or a spongy pedal. To avoid this issue, pay attention to how they came off. Many times the wear patterns will determine the position the parts were originally in. Once all brakes have been assembled, seat the pads by applying the brake pedal several times. It is a good idea for a technician to place their left foot under the brake pedal so that when applying the pedal with their right foot, the pedal does not push the master cylinder pistons farther than normal into the master cylinder bore, which could dislodge sludge or cut the lips of the master cylinder primary seals. Applying the brake forces the brake caliper pistons to adjust to the proper clearance for proper brake application. The vehicle may need to be started to enable the power booster to help fully apply the brakes, especially if the vehicle is equipped with integrated parking brake calipers. To reassemble, lubricate, and reinstall the caliper, pads, and related hardware and the seat pads, and to inspect for leaks, follow the steps in SKILL DRILL 4-6.



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SKILL DRILL 4-6 Reassembling Calipers 1. Make sure the rotor has been properly installed on the hub/ spindle and that the hub surface is free of rust or dirt.

2. Install the piston seal, using brake fluid or brake assembly fluid.

3. Install the piston by hand, being careful not to pinch, twist, or cut the sealing ring and dust boot.

Continued

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4. Assemble the pads, hardware, and caliper on the caliper mountings, using the specified lubricant. Lubricate all moving parts.

5. Reinstall the brake line fittings, using two new copper washers (if the fittings are so equipped).

6. Tighten the brake line fitting and caliper bolts to the proper torque. Bleed the brakes, following the manufacturer’s procedure.

7. Seat the pads by applying the brake pedal several times, not allowing the pedal to go all the way to the floor.

Continued



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8. If the brake pedal is spongy, the brakes will need to be bled of any remaining trapped air in the system.

9. Inspect the system for any brake fluid leaks, no matter how small.

Retracting and Readjusting Pistons on an Integrated Parking Brake Retracting the caliper piston on an integrated parking brake system is different than it is on a standard caliper. Because the integrated parking brake system uses a threaded shaft to force the piston outward from the caliper bore, it cannot just be retracted with a C-clamp. The piston has to be screwed back in on the threaded shaft to retract it into the bore. This is accomplished by using a tool that mates to slots, grooves, or holes in the outer face of the piston. The tool is then turned by hand or wrench to screw the piston back into the bore. Several types of tools work for this purpose. One is a multi-sided cube with a variety of projections of varying configurations on each side. This cube fits on the end of a ratchet. The ratchet and tool must be held tightly against the piston so that it does not slip. There are other more application-specific tools that use the caliper housing to keep the tool from slipping out of the holes in the piston. This style works best if there is access to one. Be careful not to tear the piston dust boot while twisting the piston in. To retract the caliper piston on an integrated parking brake system, follow the steps in SKILL DRILL 4-7.

▶▶ Inspecting

▶▶TECHNICIAN TIP In some cases, the piston cannot be removed with compressed air, because it is seized in the bore. If this happens, reinstall the calipers on the vehicle, without the pads; bleed the brakes; and use the brake pedal to force the stuck piston out of the caliper. Any non-seized pistons may need to be blocked so that they don’t pop out; that way, just the seized piston will be pushed out.

and Measuring Disc Brake Rotors

The rotor thickness, lateral runout, and thickness variation must be within specifications for the rotor to function properly. Rotors that are too thin cannot handle as much heat and experience brake fade sooner than thicker rotors do. They also may cause the piston to be pushed out of the caliper bore far enough that the sealing ring no longer seals the piston. This leads to a lack of braking action on at least half of the system.

4-8 Inspect and replace disc brake rotors.

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SKILL DRILL 4-7 Retracting and Readjusting Pistons on an Integrated Parking Brake 1. Research the procedure for retracting the caliper piston. Select the proper adapter or tool to match the caliper piston.

2. Install the tool and turn it in the direction that causes the piston to retract.

3. Continue turning until the piston is lightly seated at the bottom of its bore.

4. Make sure the dust boot is seated properly in its grooves.



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Excessive thickness variation causes brake pedal pulsation and the vehicle to have a surging feeling while coming to a stop. Excessive lateral runout tends to cause the steering wheel to shimmy. It also can cause an excessive thickness variation problem due to the high spot of the rotor continuously hitting the brake pad while driving down the road. This constant rubbing on the high spot of the rotor wears it slightly, leading to excessive thickness variation across the face of the rotor. When measuring the rotor for minimum thickness, measure at the deepest groove or thinnest part of the rotor, and compare the result to specifications. If it is under the minimum thickness, it will have to be replaced. If it needs to be machined and is above the minimum thickness, check to see how badly it is scored. Remember that removing 0.015" (0.38 mm) on each side of the rotor results in the thickness being reduced by 0.030" (0.76 mm). Many rotors only start with 0.060" (1.52 mm) of machinable material when they are new. When measuring thickness variation, measure the thickness of the rotor in five to eight places around the face of the rotor. Calculate the maximum thickness variation by subtracting the minimum thickness from the maximum thickness, and compare the result to specifications. To clean, inspect, and measure rotor thickness, lateral runout, and thickness variation, and to determine any necessary action(s), follow the steps in SKILL DRILL 4-8.

SKILL DRILL 4-8 Inspecting and Measuring Disc Brake Rotors 1. Research the procedure and specifications for inspecting the rotor. If the caliper assembly, brake pads, and any hardware have not already been removed, do so. Clean the rotor with approved asbestos removal equipment.

2. Inspect the rotor for hard spots or hot spots, scoring, cracks, and damage.

3. Measure the rotor thickness at the deepest groove or thinnest part of the rotor, and compare the result to specifications.

Continued

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4. Measure the thickness of the rotor in five to eight places around the face of the rotor. Calculate the maximum thickness variation, and compare result to specifications.

5. Set up a dial indicator to measure lateral runout. Rotate the rotor and find the lowest spot on the rotor; then zero the dial indicator.

6. Slowly rotate the rotor to find the highest spot on the rotor. Read the dial indicator showing maximum runout.

7. Keep turning the rotor to make sure the dial indicator does not read below zero. If it does, re-zero the dial caliper on the lowest spot. Keep turning the rotor to find the highest spot, and reread the dial indicator. Compare all readings to the specifications, and determine whether the rotor is fit for service, is machinable, or needs to be replaced.



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Removing and Reinstalling Rotors Removing the rotor is usually required when the rotor needs to be replaced because it is under the specified minimum thickness or would be after machining. It also would need to be removed to be refinished on an off-car brake lathe or to service the wheel bearings or axle shaft. Also, hubless rotors that are being machined with an on-car brake lathe need to be removed to clean off the rust and dirt that accumulated between the rotor and the hub. The hub style has the wheel bearing hub cast into the rotor. This style generally requires disassembly of the wheel bearing hub to remove the rotor from the vehicle. The hubless style uses a wheel bearing hub separate from the rotor, with the rotor held onto the hub by the wheel studs and lug nuts. Some hubs also use small screws to hold the rotor on the hub. Removing hubless rotors is generally easier than removing hub-style rotors, although some manufacturers design their rotors to unbolt from the rear side of the bearing hub. In these applications, the wheel bearing hub must be removed before the rotor can be removed from the hub. When removing hubless rotors, mark the rotor so that it can be reinstalled in the same position. A permanent marker, crayon, or center punch can be used to do this. When removing a hub-style rotor, the wheel bearings need to be removed. First, remove the wheel bearing locking mechanism (cotter pin, lock nut, or peened washer). Remove the wheel bearing adjusting nut, thrust washer, and outer bearing. Use the following procedure to remove the inner bearing and grease seal. 1. Reinstall the adjusting nut onto the spindle about five turns. 2. Grasp the rotor on the top and bottom, and push it toward the center of the vehicle. 3. Hold slight downward pressure while firmly pulling the rotor away from the vehicle. This should cause the adjusting nut to catch the wheel bearing and to pull it and the seal out of the rear of the hub. 4. The grease seal will have to be replaced with a new one. 5. Now would be a good time to perform any other rotor-related tasks, such as inspecting and measuring a rotor, refinishing a rotor, and servicing wheel bearings. Perform those tasks, then return here to reinstall the rotor. 6. If a new rotor is being installed, make sure to clean off any anticorrosion coating it was shipped with. Follow the rotor manufacturer’s procedure to remove this coating. To remove and reinstall the rotor, follow the steps in SKILL DRILL 4-9.

SKILL DRILL 4-9 Removing and Reinstalling Rotor 1. Research the procedure for removing and reinstalling the brake rotor. If the caliper assembly, brake pads, and any hardware have not already been removed, do so. If the caliper mount straddles the rotor, remove it.

Continued

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2. To remove the hubless-style rotor, mark the rotor for proper reinstallation.

3. If there are screws or speed nuts holding the rotor to the hub, remove them.

4. Remove the rotor from the hub.

5. To remove the hub-style rotor, remove the wheel bearing locking mechanism.

Continued



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6. Remove the wheel bearing adjusting nut, thrust washer, and outer bearing.

7. Follow the steps of the inner bearing and grease seal removal procedure if the rear bearing must be removed.

8. To reinstall a hubless-style rotor, clean all mounting surfaces on the hub and rotor and remove any rust.

9. Slip the rotor over the wheel studs.

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10. If the rotor uses small screws to hold the rotor to the hub, reinstall those and tighten to the proper torque.

11. Spin the rotor to ensure it spins true and does not contact any other components, such as the dust shield.

12. To reinstall a hub-style rotor, see the wheel bearing service Skill Drills in Chapter 9, which covers wheel bearings. Be sure to install the locking device. Spin the rotor and listen for contact.

▶▶ Wheel 4-9 Inspect and replace wheel studs.

Stud Evaluation and Installation

Wheel studs have to be replaced when they have been damaged or broken off due to improper installation or normal wear and tear. Wheel studs can be damaged by being over-tightened and stretched. Look for a necked-down or thinned-out section of the stud, which is most likely to happen within the threaded area of the stud. Stretched studs must be replaced. Studs can also have their threads damaged by cross-threading or seizing of the lug nut on the stud. It is best to replace the stud and nut if this occurs. Lastly, wheel studs may even break off if they are over-tightened beyond their stretch point. It is always a good idea to consider replacing all of the studs on a wheel (and maybe the ones on the other wheels also) if one stud has broken off, because all of the others have likely been weakened.



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Some vehicles are designed to allow for the removal and replacement of the wheel studs while the hub and wheel flange are still installed on the vehicle. The manufacturer may have provided a recessed spot in the steering knuckle where the studs have enough clearance to be removed; thus, the flange has to be positioned in that particular position. Other vehicles do not have enough clearance on the back side for the stud to be removed; in these cases, the hub and flange must be removed from the vehicle. There are two primary methods of replacing lug studs: the drawing-in method and the hydraulic press method. The drawing-in method uses the lug nut to draw the wheel stud into the flange. It is accomplished by inserting the new stud into the wheel stud hole in the flange and installing enough heavy-duty washers over the stud to allow the lug nut to draw the wheel stud into the flange when tightened. The lug nut is placed on the stud, flat side in, and tightened. Make sure that as the stud is pulled in, the lug nut does not run out of threads on the stud. If it does, remove the nut and add washers. Keep tightening the lug nut until the wheel stud bottoms out in the flange. Verify that the head of the stud is fully seated in the flange. The hydraulic press method uses a press to force the wheel stud into the flange until it bottoms out. This method requires the hub and flange to be removed from the vehicle. Make sure the flange is positioned and supported properly on the press table. Sometimes it is best to use a short piece of pipe (a bit longer than the wheel stud) to support the flange while the stud is being pressed in. Once the stud is installed, verify that it is fully seated in the flange. To inspect and replace wheel studs, follow the steps in SKILL DRILL 4-10.

SKILL DRILL 4-10 Inspecting and Replacing Wheel Studs 1. Inspect the wheel studs and lug nuts for damage. Look for signs of stretched studs, cross-threaded lug nuts, and broken-off studs. Determine whether the stud can be removed with the flange on or off the vehicle.

2. To perform the drawing-in method, position the flange so that the stud has clearance on the back side to be removed.

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3. Using a hammer, remove any damaged studs. Be careful not to damage any of the surfaces on the flange and hub, including the wheel speed sensor and tone ring.

4. Insert the stud in the hole in the flange, and rotate it so that all of the flutes on the stud line up with the notches in the flange.

5. Place enough heavy-duty washers over the stud to prevent the lug nut from bottoming out on the threads.

6. Place the lug nut onto the stud, flat side in. Tighten it until the stud bottoms out in the flange. Inspect the threads on the stud and lug nut to make sure they did not get damaged.

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7. To perform the hydraulic press method, remove the hub and wheel flange from the vehicle, following the manufacturer’s procedure.

8. Use the press to push out any damaged lug studs.

9. Insert the new lug stud into the wheel flange hole. Line up the flutes on the stud with the notches in the flange.

10. Support the hub and flange so that the press is pushing the stud straight into the flange.

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11. Push the stud in until it bottoms out in the flange.

12. To complete the drawing-in method and the hydraulic press method, verify that each stud is fully seated in the flange.

Installing Wheels,Torquing Lug Nuts, and Making Final Checks This step, although fairly simple, can result in problems if it is not done properly. Over-­ tightening the lug nuts can cause the wheel studs to break, either immediately or, worse, after the vehicle has been driven for a period of time. Over-tightening also can cause warpage of the rotors, which results in the need to refinish or replace them. Under-tightening can lead to loosening of the lug nuts and result in the wheel working its way off the ­vehicle. This can cause the driver to lose control of the vehicle and potentially result in a ­collision. Lug nuts should be tightened to the proper torque, in the specified sequence. All manufacturers specify the sequences for each of their vehicles (FIGURE 4-39). The torque pattern is usually in some form of a star or cross. 4 Be careful which way the lug nuts are installed. Many wheels use a tapered hole that matches the tapered end of the lug nut and centers the wheel on the wheel flange. 2 1 Other wheels use a flat surface that matches flat surfaces on the lug nuts. But no matter what, the lug nut surface must match the mating surface of the wheel. Always check that these surfaces match. 5 3 When installing lug nuts, the weight should be off the vehicle so that the wheel is off the ground or is barely touching the ground. The lug nuts should easily center the wheel on the hub. This is especially important on aluminum wheels that use a flat lug nut seating surface. The lug nuts can dig into the sides of the lug nut holes and cause the wheel to not center properly. Once the lug nuts are against the wheel, work the wheel onto the lug nut shafts. Then they can be torqued down properly. FIGURE 4-39  Common torque sequences for lug nuts. Once the lug nuts are torqued properly and the brake system checked, it is time



Wheel Stud Evaluation and Installation

for a test drive, to verify proper brake operation and to burnish the new brake pads and rotor surfaces. Burnishing, also called bedding in, is the process of evenly transferring pad material onto the rotor, as well as cooking off the resins that are used to bind the friction material in the pad. Burnishing results in long, quiet brake life, which results in satisfied customers. For burnishing to happen properly, the rotor and pad material should be heated slowly and evenly. This is done by making a specified number of stops from a specified speed, with the appropriate wait times between stops. In some cases, brake lining manufacturers specify a series of stops performed at light to moderate brake application. Other manufacturers specify a series of moderate to heavy stops. Always check the manufacturer’s procedure to burnish the brakes once the brake job is complete. This process will help avoid the dreaded disc brake squeal. To install the wheel, torque lug nuts, and make final checks and adjustments, follow the steps in SKILL DRILL 4-11.

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▶▶TECHNICIAN TIP There is some controversy regarding the use of a lubricant or antiseize on wheel studs and lug nuts. Because the purpose of a lubricant or antiseize is to prevent the components from sticking, the issue is the lug nuts coming loose when the vehicle is moving. Also, the torque given for the lug nuts is dry (no lubricant), so torqueing them to specifications would lead to over-tightening. Following the manufacture’s procedures and specifications should be done at all times to decrease the possibility of being liable for a failure.

SKILL DRILL 4-11 Installing Wheels, Torquing Lug Nuts, and Making Final Checks 1. Start the lug nuts on the wheel studs, being careful to match up the surfaces.

2. Carefully run all of the lug nuts down lightly so that they are seated in the wheel.

3. Lower the vehicle so that the tires are partially on the ground to keep them from turning while torqueing the lug nuts.

Continued

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Chapter 4  Disc Brake Systems

4. Use a torque wrench to tighten each lug nut to the proper torque in the proper sequence.

5. Once all of the lug nuts have been torqued, go around them again, this time in a circular pattern to ensure that none were missed in the previous pattern.

6. If the vehicle was equipped with hubcaps and valve stem caps, reinstall them.

7. Check the brake fluid level in the master cylinder reservoir. Start the vehicle, and check the brake pedal for proper feel and height. Check the parking brake for proper operation. Also inspect the system for any brake fluid leaks and loose or missing fasteners.



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Disc brakes create braking power by forcing flat friction pads against the outer faces of a rotor. The vehicle’s kinetic energy is transformed into heat ­energy by the disc brake components, which slow the vehicle when applied. Disc brake assemblies consist of a caliper, brake pads, and a rotor. Caliper pistons use hydraulic pressure to create a clamping force of the brake pads to the faces of the rotor. Disc brake pads require much higher application pressures to operate than drum brake shoes because brake pads are not self-energizing. There are some advantages to using disc brakes rather than drum brakes: They are more effective at transferring heat to the atmosphere, self-adjusting, resistant to water fade, and straightforward to service. There are also some disadvantages to using disc brakes rather than drum brakes: They are more prone to noise, more prone to pedal pulsations due to warpage, and more difficult to use as an emergency brake. Disc brake calipers come in two main styles: fixed and floating/sliding. In disc brake calipers, the piston is sealed by a square-cut O-ring. Floating/sliding calipers require clean and lubricated pins, bushings, or guides for proper operation. Brake pad lining is either riveted or bonded to the pad backing plate. Brake pad lining is available in a variety of materials with varying amounts of coefficient of friction. Brake pads may use shims, spacers, guides, and bendable tangs to help minimize squealing. Brake pad wear indicators, if used, can be of the mechanical or electronic type. Rotors rotate with the wheels and are usually made of durable cast iron with friction surfaces that run true and parallel. Brake rotors can be solid or ventilated. Disc brake parking brakes can be of the integrated caliper style, top-hat drum style, electric pull-cable style, or the integrated electric motor caliper style. Diagnosing brake faults requires good information from the customer, an adequate test drive when possible, and a good understanding of brake theory. Special precautions are needed when testing ABS during a test drive. Removing the caliper is necessary for replacing the brake pads on most vehicles. To push the caliper pistons back into their bores slightly, use a small pry bar or C-clamp.

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The caliper mountings and slides/pins experience wear and corrosion over time. Removing the brake pads is the most thorough way to inspect them, because they can be inspected not only for the thickness of the lining but also the condition of the friction surface. Some shops rebuild calipers themselves, but most shops just replace them with rebuilt calipers if necessary. Once all brakes have been assembled, seat the pads by applying the brake pedal several times. Retracting the caliper piston on an integrated parking brake system requires the piston to be screwed back in on the threaded shaft to retract it into the bore. Excessive rotor thickness variation causes brake pedal pulsation and the vehicle to have a surging feeling while coming to a stop. Removing 0.015" (0.38 mm) on each side of the rotor ­results in the thickness being reduced by 0.030" (0.76 mm). Many rotors only start with 0.060" (1.52 mm) of machinable material when they are new. On-vehicle refinishing is preferred by most manufacturers because it minimizes runout issues between the hub and rotor. Hub-style and hubless-style rotors each require their own way of being mounted on the brake lathe. There are two primary methods of replacing lug studs: the drawing-in method and the hydraulic press method. Lug nuts should be tightened to the proper torque, in the specified sequence. Burnishing, also called bedding in, is the process of evenly transferring pad material onto the rotor, as well as cooking off the resins that are used to bind the friction material in the pad.

Key Terms backing plate  A metal plate to which the brake lining is fixed. bearing races  Hardened metal surfaces that roller or ball bearings fit into when a bearing is properly assembled. bendable tangs  Small tabs on the brake pad backing plate that are crimped onto the caliper, creating a secure fit and reducing noise. bleeder screw  A hollow screw that allows air and brake fluid to be bled out of a hydraulic brake system when it is loosened and seals the brake fluid in when it has been tightened. bonded linings  More commonly found on light-duty vehicles, brake linings that are essentially glued to the brake pad backing plate. brake booster  A vacuum or hydraulically operated device that increases the driver’s braking effort. brake lining thickness gauge  A tool used to measure the thickness of the brake lining.

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Chapter 4  Disc Brake Systems

brake pad shims and guides  Small pieces of metal that cushion the brake pad and absorb some of the vibration, helping to cut down on unwanted noise. brake wash station  A piece of equipment designed to safely clean and contain brake dust from drum and disc brake components. caliper  A hydraulic device that uses pressure from the master cylinder to apply the brake pads against the rotor. caliper dust boot seal driver set  A set of drivers used to install metal-backed caliper dust boot seals. caliper piston pliers  A tool used to grip caliper pistons while removing them. caliper piston retracting tool  A tool used to retract caliper pistons on integrated parking brake systems. C-clamp  A tool used to push pistons back into the caliper bore on non-integrated parking brakes. dial indicator  A tool used to measure the lateral runout of the rotor. disc brake pads  Brake pads that consist of a friction material bonded or riveted to a steel backing plate, which are designed to wear out over time. disc brake rotor micrometer  A specially designed micrometer used to measure the thickness of a rotor. drawing-in method  A method for replacing wheel studs that uses the lug nut to draw the wheel stud into the hub or flange. edge code  A two-digit code printed on the edge of a friction lining, where the code describes its coefficient of friction. electronic control module (ECM)  A computer that receives signals from input sensors, compares that information with preloaded software, and sends an appropriate command signal to output devices. It is used to manage the antilock brake system (ABS). fixed caliper  A type of brake caliper bolted firmly to the steering knuckle or axle housing, having at least one piston on each side of the rotor. guide pins  Pins that allow the caliper to move in and out as the brakes operate and as the brake pads wear. hydraulic press method  A method for replacing wheel studs that uses a press to force the wheel stud into the flange until it bottoms out. independent rear suspension (IRS)  A type of suspension system where each rear wheel is capable of moving independently of the other. lateral runout, also called warpage  The side-to-side movement of the rotor surfaces as the rotor turns. low-drag caliper  A caliper designed to maintain a larger brake pad–to-rotor clearance by retracting the pistons farther than normal. lug  A flange that is shaped to assist with aligning objects on other objects. off-car brake lathe  A tool used to machine (refinish) drums and rotors after they have been removed from the vehicle. on-car brake lathe  A tool used to machine (refinish) rotors while they are still attached to the vehicle.

parallelism, also called thickness variation  A situation where both surfaces of the rotor are parallel to less than 0.001" of each other, so that brake pulsations do not occur. parking brake cable pliers  A tool used to install parking brake cables. phenolic resin  A very dense material, used to create some brake pistons, that is very resistant to corrosion and heat transfer. pushrod  (braking system) A mechanism used to transmit force from the brake pedal to the master cylinder. riveted linings  Brake linings riveted to the brake pad b ­ acking plate with metal rivets and used on heavier-duty or high-­ performance vehicles. rotor  The main rotating part of a disc brake system. scratcher  A thin, spring-steel wear indicator that is fixed to the backing plate of the brake pad. It emits a high-pitched squeal when the brakes linings have become too thin. sliding or floating caliper  A type of brake caliper that has piston(s) only on the inboard side of the rotor. The caliper is free to slide or float, thus pulling the outboard brake pad into the rotor when braking force is applied. solid rotor  A type of brake rotor made of solid metal, not ventilated. square-cut O-ring  An O-ring with a square cross section that is used to seal the pistons in disc brake calipers. ventilated rotor  A type of brake rotor with passages between the rotor surfaces that are used to improve heat transfer to the atmosphere. wheel studs  Threaded fasteners that are pressed into the wheel hub flange and used to bolt the wheel onto the vehicle.

Review Questions 1. All of the following statements with respect to the disc brake system are true except: a. The disc brake rotor is bolted to the wheel. b. The hydraulic pressure from the master cylinder causes the caliper to create a mechanical clamping action. c. Drum brakes withstand higher applied forces than disc brakes do. d. The heat generated on the outside surfaces of the rotor is easily transferred to the atmosphere. 2. Which of the following primary components of the disc brake system uses hydraulic pressure from the master cylinder to apply the brake pads? a. Rotor. b. Caliper. c. Brake liners. d. Brake pads. 3. When the brake pedal is depressed, a pushrod transfers the force to a hydraulic master cylinder through the _________. a. brake booster b. brake liner c. hoses d. pistons



4. Which one of these is an advantage of the disc brake system? a. Disc brakes transfer less heat to the atmosphere. b. Disc brakes reduce the likelihood of brake fade. c. Disc brakes are self-adjusting and do not need maintenance. d. Disc brakes do not create annoying squeals and squeaks. 5. All of the following statements referring to disc brake calipers are true except: a. A disc brake caliper assembly clamps the brake pads onto the rotors to slow the vehicle. b. Calipers are fitted with a bleeder screw on the top of the piston bore. c. An O-ring is compressed between the piston and caliper housing, creating a positive seal to keep the high-pressure brake fluid from leaking out. d. Fixed calipers are the most common type used in passenger vehicles. 6. Which of the following brake friction materials have a high coefficient of friction? a. Leather and oak. b. Brass and cast iron. c. Rubber and concrete. d. Copper and cast iron. 7. The following edge code letters correspond to four different materials. Which of these has the lowest coefficient of friction? a. E b. F c. G d. H 8. All of the following statements with respect to wear indicators are true except: a. A spring-steel scratcher is a mechanically operated wear indicator. b. When the scratcher makes a squealing noise, it means the brakes need immediate service. c. Electrical contact installed on the brake pad touches the rotor, prompting a warning light or warning message. d. Electrical contacts cannot be manufactured into the pad, so they have to clipped onto the pad. 9. Which of the following statements is true with respect to parallelism? a. It creates less pressure on the brake pads. b. It results when the rotor’s thickness varies by as little as 0.0003". c. It is usually noticeable at high breaking speeds. d. It results in the rotor tending to push the brake pads inward. 10. In which type of parking brake does the rotor have a deeper offset than normal? a. Foot-operated parking brake. b. Integrated parking brake. c. Top-hat-design parking brake. d. Electric parking brake.

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ASE Technician A/Technician B Style Questions 1. Technician A says that disc brakes operate on the principle of friction. Technician B says that disc brakes operate on the principle of regeneration. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that some vehicles use fixed calipers. Technician B says that some vehicles use sliding/floating calipers. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that disc brake pads require higher application force than drum brake shoes. Technician B says that disc brakes are self-energizing. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that fixed calipers use one or more pistons only on one side of the rotor. Technician B says that sliding/fixed calipers use one or more pistons on both sides of the rotor. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that calipers use steel piston rings to seal each piston. Technician B says that calipers use a square-section O-ring to seal each piston. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that chrome-plated pistons resist rust. Technician B says that pistons made of phenolic resin transmit less heat and do not corrode. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that some vehicles are equipped with a spring-steel brake pad wear indicator that drags on the rotor when the lining thickness is low. Technician B says that some vehicles are equipped with an electric brake pad wear sensor that activates a warning on the dash. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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8. Technician A says that rotors should be measured for thickness variation (parallelism). Technician B says that rotors should be measured for lateral runout. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says that rotors that are too thin cannot handle as much heat and will experience brake fade sooner. Technician B says that brake pedal pulsation is the result of air in the hydraulic system. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

10. Technician A says that a micrometer is used to measure rotor thickness variation. Technician B says that a micrometer is used to measure rotor lateral runout. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

CHAPTER 5

Drum Brake Systems Learning Objectives ■■ ■■ ■■ ■■ ■■

5-1 Classify the types of drum brake systems. 5-2 Break down drum brake components. 5-3 Explain the use of wheel cylinders. 5-4 Summarize brake shoes and lining construction. 5-5 Explain the purpose of springs and hardware inside a drum brake system.

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5-6 Explain drum brake and parking brake systems. 5-7 Diagnose drum brake operation. 5-8 Summarize how to evaluate a brake drum repair. 5-9 Demonstrate the inspection of brake shoes and hardware.

You Are the Automotive Technician You are working in San Francisco, California, on your company’s fleet vehicles, which are about six years old. Today you are performing a rear brake inspection on a light-duty truck.The driver has indicated that the parking brake isn’t able to completely hold the vehicle on some of the steepest hills he has to park on.You operate the parking brake and notice that it can be pushed all the way to the floor before it is completely tight. This means you will have to visually inspect the rear brake assemblies.

1. How is the parking brake similar to and different from the rear drum brakes? 2. What are the possible faults in the service brakes (drum brake style) that could cause the parking brake to be out of adjustment? 3. In what order should the drum brakes and parking brakes be adjusted?





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▶▶ Types

of Drum Brake Systems

Drum brakes get their name from the rotating drum-shaped component called a brake drum (or simply drum). The lug nuts clamp the drum between the wheel and the axle flange. This means the wheels and drums rotate together. Friction lining on brake shoes is forced against the inside of the drums by hydraulic wheel cylinders, causing friction and absorbing the vehicle’s kinetic energy. Because heat is created inside the drums, the heat has to transfer through the drum material before it can be transferred to the atmosphere. Situations such as heavy loads or long downhill grades, where the brakes are used heavily, can lead to overheating of the lining, drums, or brake fluid, resulting in a loss of braking, called brake fade. To combat brake fade, heavier-duty vehicles, like pickup trucks, use larger-diameter brake drums and shoes as well as wider brake shoes and drum surfaces. These factors allow the drum brakes to create, absorb, and transfer a greater amount of heat energy. The main components of the drum brake system are presented here (FIGURE 5-1):

5-1 Classify the types of drum brake systems.

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Brake drum: The brake drum fits over the brake linings and forms the braking surface for the brake linings. It is usually made from cast iron and machined so that the inside surface rotates true. Backing plate: The backing plate is made from stamped steel and is bolted to the steering or suspension components. It supports the wheel cylinder(s), brake shoes, and hardware. Wheel cylinder: The wheel cylinder is attached to the backing plate. The wheel cylinder pistons push the brake shoes into contact with the brake drum to slow down or stop the vehicle. Brake shoes: The brake shoe consists of the steel shoe and the brake lining friction ­material. The brake shoes are held against the backing plate by hold-down springs and clips. Springs and clips: Return springs retract the brake shoes when the brakes are released. Other springs work with the self-adjuster and with parking brake linkage operation. Automatic brake self-adjuster: The automatic self-adjuster automatically adjusts the brakes to maintain a specified amount of running clearance between the shoes and drum. It operates in one of two ways: either when using the brakes while backing up or as part of applying the parking brake. It also makes periodic brake adjustment unnecessary. Parking brake mechanism: The parking brake linkage mechanically operates the brake shoes (service brakes) to hold the vehicle stationary when the driver applies the parking brake.

Drum Brake Operation In drum brake systems, when the brake pedal is depressed, a pushrod transfers the force to a hydraulic master cylinder. The master cylinder converts the brake pedal force into Wheel Cylinder Hole Cover Loading Shoe Backing Plate

Shoe Hold Spring

Hold-Down Pin or Rod

Star Wheel Adjusting Screw

Anchor Parking Lever

Lining Trailing Shoe

Adjusting Lever

FIGURE 5-1  The main components

of a drum brake system.



Types of Drum Brake Systems

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Direction of Rotation Anchor Pin Wheel Cylinder Piston Push Rod

Brake Shoes

FIGURE 5-2  The pistons within each

Brake Drum Floating Link

wheel cylinder are forced outward by the hydraulic pressure and apply force to the brake shoes, forcing them into each rotating drum.

hydraulic pressure, which is then transmitted without loss via the brake lines and hoses to one or two wheel cylinders at each drum brake assembly. The pistons within each wheel cylinder are forced outward by the hydraulic pressure and apply force to both brake shoes, forcing them into each rotating drum (FIGURE 5-2). Because the brake shoes are anchored to the backing plate, preventing them from rotating freely with the drum, the friction generated between the moving surfaces slows down the rotation of the drum and the wheel. Each drum brake has two brake shoes with an attached friction material, called a lining. These shoes expand against the inside surface of a brake drum and slow the wheel down. The harder the linings are forced against the brake drum, the greater the braking force applied. They can be expanded mechanically or hydraulically. Drum brake systems have to be adjusted to allow for wear of the lining. As the lining wears, the brake shoes must be pushed farther outward to contact the drum. Over time, this wear causes only the top portion of the linings to contact the drum, reducing the surface area of the lining that can dissipate the created heat. Also, if the brakes are not adjusted, the brake pedal reserve height will be too low to be safe. Because of this, most drum brake systems incorporate an automatic adjuster to keep the brakes properly adjusted.

Self-Energizing and Servo Action Drum brakes are self-energizing. This means they can increase the force with which they are applied. When brake shoes come into contact with the moving drum, the friction tends to carry them in the direction the drum is rotating. Because the brake shoes are inside the drum and anchored at one end, this has a wedging effect on the brake shoe. This wedging effect assists the driver in applying the brakes, making it so that the driver does not have to push so hard on the brake pedal (FIGURE 5-3). The positioning of the brake shoes determines whether a brake shoe is self-energizing. Brake shoes are designed in a leading or trailing manner. Leading shoes are installed so that the direction they are applied in is the same as the forward rotation of the drum. Leading shoes are self-energizing. Trailing shoes are installed so that the direction they are applied in is opposite to the forward rotation of the drum. Trailing shoes are not self-energizing. In fact, they tend to have reduced energization, meaning they are not nearly as efficient at developing braking force as leading shoes are. Servo action as related to brakes means that one brake shoe, when activated, applies an increased activating force to the other brake shoe, in proportion to the initial activating force. This further enhances the self-energizing feature of some drum brakes. We cover this topic in greater depth later in this chapter.

Types of Drum Brake Systems There are three types of drum brake systems: twin leading shoe, leading/trailing shoe (also called single leading shoe), and duo servo. Each type uses similar drum brake components

▶▶TECHNICIAN TIP Servicing the wheel bearings is a common part of a brake job if the vehicle has serviceable wheel bearings. If the vehicle uses non-serviceable bearings, the bearings should be checked during a brake job and replaced if they are worn out. See Chapter 9 for more information on wheel bearings and service.

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Chapter 5  Drum Brake Systems Tangent brake force (T) is the sum of the apply force (A) and the self-energizing force (S). Direction of Rotation

A: Apply Force (e.g., 100 lb)

Leading Shoe T=A+S T = 100 lb + 20 lb T = 120 lb

Trailing Shoe T=A−S T = 100 lb − 20 lb T = 80 lb Center of Drag

Center of Drag

T: Tangent Brake Force

S: Self-Energizing Action (e.g., 20 lb) at Heel of Shoe (pivot point)

Actual Brake Force = Tangent Brake Force multiplied by the coefficient of friction FIGURE 5-3  Drum brake shoes are self-energizing.

but functions a bit differently. All three types are self-energizing in at least one direction. The first two types are non-servo brakes. The duo-servo drum brake system uses servo action in both directions. Each of the three types has pros and cons, and designers use the type that best fits the vehicle application.

Twin Leading Shoe Drum Brake Systems The twin leading shoe drum brake system is the least common type in modern automotive use. This system was once popular on front wheels because it is very efficient at braking in the forward direction. Because the vehicle travels much faster forward than it does in reverse, this matched the braking needs well. The large forward stopping power that the twin leading shoe drum brake system generated also allowed the system to operate without a power brake booster. Twin leading shoe drum brake systems use two single-piston wheel cylinders (also called single-acting wheel cylinders): one near the top of the backing plate and the other near the bottom (FIGURE 5-4). Each wheel cylinder activates one of the brake shoes. The brake shoes are anchored at the closed end of the opposite wheel cylinder. It is called a twin leading shoe drum brake system because both shoes are arranged in a leading shoe (self-energizing) configuration in the forward direction. This arrangement gives very good stopping power in the forward direction. When applied in the reverse direction, the braking force is much less, only about 30% as efficient. This type of drum brake system was usually accompanied by one of the other types of brakes on the rear wheels, to be used as a parking brake. The twin leading shoe drum brake system is very well suited for motorcycles because they are driven mostly in the forward direction and rarely in reverse.

Leading/Trailing Shoe Drum Brake Systems The leading/trailing shoe drum brake system is very common on the rear wheels of front-wheel drive vehicles because of their equal braking forces in both the forward and reverse direction. This system uses a single-wheel cylinder with two pistons (also called a double-acting wheel cylinder), usually mounted near the top of the backing plate (­ FIGURE 5-5). Each piston operates one of the brake shoes, and each shoe is anchored at the bottom of the backing plate. This arrangement makes one shoe a leading shoe and the other a trailing shoe. In the forward direction, the front piston forces the front brake shoe into the drum, and it acts like a leading shoe. The rear piston pushes the rear brake shoe into contact with the drum, but it acts as a trailing shoe, so it does not create as much braking power.



Types of Drum Brake Systems Direction of Rotation Shoe Retaining Clip Return Spring Piston

141

Direction of Rotation Parking Brake Pushrod Wheel Cylinder and Brake Clearance Adjuster Return Spring Self-Adjuster

Wheel Cylinder Brake Adjuster

Leading Shoe

Trailing Shoe Backing Plate

Brake Drum (sectional view not rotating) Parking Brake Cable FIGURE 5-4  A twin leading shoe drum brake works very well

in the forward direction.

Shoe Retaining Clip Parking Brake Lever Return Spring

Anchor Point

FIGURE 5-5  The leading/trailing shoe drum brake system.

When the car is in reverse or facing uphill and the brakes are applied, the rear shoe becomes a leading shoe and the front shoe becomes a trailing shoe. The leading/trailing shoe drum brake system works equally well in both directions. It is also important to note that it does not provide maximum braking in either direction; it tends to produce a lesser but equal amount of force in both directions. This makes it ideal for the rear wheels of front-wheel-drive vehicles because approximately 70–80% of the braking power needed under heavy braking occurs at the front wheels and only 20–30% at the rear wheels.

Duo-Servo Drum Brake Systems Duo-servo drum brake systems get their name from using the servo action in both the forward direction and the reverse direction. Like the leading/trailing system, the system uses a single-wheel cylinder with two pistons (also called a double-acting wheel cylinder), usually mounted near the top of the backing plate (FIGURE 5-6). The bottom of each brake shoe is not anchored to the backing plate, but is instead connected by an adjustable, floating link. This configuration allows the bottom of the brake shoes to move in the direction of the drum. There is an anchor pin at the top of the backing plate above the wheel cylinder that prevents each shoe from rotating past that point. The shoes can move away from the anchor pin, but they are stopped by it when they rotate toward it. When the brakes are applied, the front piston overcomes the weaker front return spring tension to move the forward shoe into contact with the drum. When the shoe contacts the drum, the friction causes it to start to rotate with the drum. This puts a small force through the bottom connecting link and applies (servo action) Direction of Rotation the bottom of the rear shoe, pushing it into contact with the rotating drum. Anchor Pin The top of that shoe is thus carried into the anchor pin at the top of the Wheel Cylinder backing plate, causing both shoes to stop rotating. The brakes generate a small amount of force at this point. As the driver applies more force to the brake pedal, the forward pisShoe Return Springs ton in the wheel cylinder pushes the front shoe harder into the rotating drum, which causes the front shoe to apply more force to the rear shoe. This forces the rear shoe harder into the drum as the anchor pin prevents it from rotating. Because hydraulic pressure is the same on both pistons in Secondary Shoe the wheel cylinder, the rear piston also tends to push the rear shoe outward into the drum, which helps apply it, although not in a completely compleBrake Drum mentary direction. Because the front shoe multiplies the force applying the rear shoe, Primary Shoe Adjustable Floating Link the rear shoe does more of the braking work. If the linings were the same length front to rear, then the rear one would wear out much faster. This is FIGURE 5-6  A duo-servo drum brake system.

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why manufacturers put more lining on the rear shoe than on the front shoe. They might also use linings with different coefficients of friction for each of the shoes to get the desired braking load between the two shoes. It is important, therefore, to install the correct shoe in the correct position. Failure to do so will cause the brake linings to wear unevenly as well as work improperly.

▶▶ Drums 5-2 Break down drum brake components.

and Related Components

Brake drums provide the rotating friction surface that the brake lining contacts. They are usually made from cast iron because of its ability to withstand high temperatures, absorb a lot of heat, and maintain its shape. To enhance the cooling of the brake drum, some manufacturers add cooling fins to the outside of the brake drum, and others make the brake drum out of aluminum (FIGURE 5-7).

Types of Brake Drums Like rotors, brake drums can have an integrated hub, called a hub-style drum, or a separate hub called a hubless-style drum (FIGURE 5-8). Hubless drums are the most common in passenger vehicles because of the high percentage of sealed wheel bearings being used. Because the wheel bearings in this case do not have to be periodically packed with grease, they do not have to be removed unless they wear out. Manufacturers design the brake drum to slip over the lug studs on the wheel flange and to be held on by the wheel and lug nuts. Also, hubless drums are less expensive to replace when they no longer meet the manufacturer’s specifications. Hub-style drums have a one-piece integrated hub/drum assembly. The wheel bearings are housed in the hub and are usually serviceable. If they are serviceable, then it is standard procedure to pack the wheel bearings with the specified grease and replace the grease seals during a brake job. More steps are required to remove hub-style drums from the vehicle than are required for hubless drums, and the hub-style drums are more expensive to replace.

Backing Plate All of the brake unit components, except the brake drum, are mounted on a backing plate bolted to the vehicle axle housing or suspension. The backing plate is usually pressed into a very specific configuration from heavy-gauge steel (FIGURE 5-9). It uses a labyrinth seal on its outer edge to help keep out dirt and water spray. The drum brake’s labyrinth seal is made up of a raised edge on the outer surface of the backing plate that fits into a groove or recess in the brake drum (FIGURE 5-10). This makes it so that there isn’t a direct path for water spray and dirt to enter the drum. The backing plate also has holes stamped in it for the wheel cylinder, hold-down pins, and the parking brake cable to pass through.

A

B

FIGURE 5-7  Brake drums. A. Without cooling fins. B. With cooling fins.

A

FIGURE 5-8  A. Hubless drum. B. Hub-style drum.

B



Wheel Cylinders

143

Backing Plate Brake Drum

Labyrinth Seal

FIGURE 5-9  The backing plate.

FIGURE 5-10  A labyrinth seal prevents a direct path for water spray and dirt to enter the brake drum.

Some vehicles have one or two openings in the backing plate to allow for manual adjustment of the brake shoes. These holes are plugged with rubber grommets that have to be reinstalled after adjusting the brakes. One of the most important components of the backing plate is the anchor pin (or anchor block). The anchor pin must be able to take all of the braking force when the brakes are applied, so it must be strong and firmly attached to the backing plate ­(FIGURE 5-11). The anchor pin may also hold the shoe guide, return springs, and self-adjuster cable on duo-servo-style brakes. The inside surface of the backing plate has flat or raised brake shoe contact pads stamped into it. The brake shoes are held against the brake shoe contact pads by spring pressure and are free to move side to side. Over time, the brake shoes can wear a groove in the surface of the contact pad. During brake shoe replacement, these contact pads have to be cleaned and inspected. FIGURE 5-11  Two styles of anchor pins that mount on backing plates. If there is no wear, then a very light coat of white lithium grease can be applied to the contact pads to lessen any wear during the life of the new brake shoes. If there is light wear, this can be cleaned up with a file or small handheld grinder. If the grooves are too deep, the backing plate will have to be replaced.

▶▶ Wheel

Cylinders

The wheel cylinder is located inside the brake drum and is either bolted or firmly clipped to the backing plate (FIGURE 5-12). It converts hydraulic pressure from the master cylinder into mechanical force that pushes the brake linings against the inside of the brake drum. Wheel cylinders usually contain one cylinder housing, one or two pistons, a lip seal for each piston, a spring and expander set, a dust boot for each open end of the cylinder, a pushrod for each piston, and a bleeder screw (FIGURE 5-13). Wheel cylinder housings are usually made of cast iron or aluminum alloy, and they operate under high pressures and temperatures. The cylinder bore, or inside diameter of the cylinder, is created by drawing a properly sized ball bearing through the bore. This technique gives the surface a hard, smooth

5-3 Explain the use of wheel cylinders.

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Chapter 5  Drum Brake Systems Double Acting (cross-section) Fluid Pipe

Boot FIGURE 5-12  The wheel cylinder mounted on the backing plate.

Piston

Seal

Bleeder Nipple

Cylinder Body

FIGURE 5-13  Wheel cylinder cutaway.

finish. It is also why most manufacturers recommend replacing rather than honing the wheel cylinder if it has pits or corrosion. Cylinder bores on aluminum wheel cylinders are usually anodized to help resist corrosion. They also should not be honed, because this would remove the protective finish. Some cylinder bores are sleeved with stainless steel to be longer wearing and more resistant to corrosion. No matter the type of material used, the cylinder and pistons are manufactured to a precise diameter. This provides the proper amount of clearance between them. It also maintains the proper tension of the piston seal to the cylinder bore, which helps hold pressure in and keep the air out. These are other reasons why honing the cylinder bore is frowned upon. Contamination, particularly from water, causes pitting and rusting of the inner surface of the wheel cylinder. Such damage can result in leakage of brake fluid from the cylinder or stuck pistons. The sealing surface in a wheel cylinder is the inside surface of the cylinder bore. This surface must be clean, smooth, and free of pitting. The wheel cylinder cups the seal against the surface of the cylinder bore and prevents the brake fluid from leaking out of the cylinder. Hydraulic pressure forces the lip of the seal into the surface of the cylinder bore even harder when the brakes are applied. The sealing cups are made of materials that are compatible with the type of brake fluid specified for the particular vehicle. Using the wrong seal compound in a brake system or mixing the wrong brake fluid in the brake system can cause seal damage and brake failure. Always verify that the brake fluid and components are compatible with each other. Wheel cylinder pistons are usually made of anodized aluminum. They have a built-in mating surface for the pushrods or brake shoes to engage. Most pistons use a flat surface that supports the piston seal, but some pistons use a seal that fits within a machined groove at its inner end. Piston-to-cylinder clearance is critical for proper operation, so verify that the cylinder and ­piston are not worn beyond the specified clearance. Wheel cylinders may be fitted with a spreader and a light expansion spring to keep the lips of the seal in contact with the cylinder bore during times of low pressure, such as during retraction and while at rest (FIGURE 5-14). This helps keep air from being drawn into the cylinder. A flexible dust boot fits over the open ends of the cylinder and allows for piston movement. At the same time, it helps keep brake dust and moisture away from the inside of the cylinder and piston. Wheel cylinders are fitted with a bleeder screw to allow for bleeding of air and old brake fluid from the hydraulic brake FIGURE 5-14  Wheel cylinder components.



Wheel Cylinders Bleeder Screw Open

145

Bleeder Screw Closed

Dust Cap Installed Air Bleeding

Fluid Pipe Dust Cap

Bleed Passage

Cylinder Body Bleeder Screw

Tapered Seat

FIGURE 5-15  Bleeder screw allows air and fluid to be bled from the system.

system (FIGURE 5-15). The bleeder screw is a hollow screw. It has a taper on the end that mates with a matching tapered seat in the wheel cylinder. These tapered seats seal when the bleeder screw is closed. The bleeder screw has been cross-drilled into the center hole just above the taper. This way, when the bleeder screw is loosened slightly, the tapered seat opens, and then brake fluid enters the cross-drilled passage into the center hole and flows out the end of the bleeder screw. Tightening the bleeder screw closes off the tapered seat and holds pressure in the wheel cylinder. Bleeder screws have a small rubber dust cap that fits over the exposed end of the screw to keep water, dirt, and debris from entering the center hole and plugging it up or rusting it in place. Always remember to replace these caps when bleeding the drum brakes has finished.

Types of Wheel Cylinders Wheel cylinders come in different configurations. They are either single acting or dual acting. Single-acting cylinders use a single piston, meaning that the force is generated in one direction only. Dual-acting cylinders use two pistons opposite of each other, meaning that the force acts in two different directions (FIGURE 5-16). Most modern vehicles use double-acting wheel cylinders because they are simpler to design, install, and bleed. Dual-acting wheel cylinders use a common cylinder with a piston and lip seal in each end. There is usually a coil spring with expanders on each end, positioned between the lip seals. The expander helps to hold the seal lips against the cylinder bore when there is little or no hydraulic pressure. Single Acting (with brake adjuster)

Double Acting (cross-section) Fluid Pipe

Boot

Piston

FIGURE 5-16  Types of wheel cylinders.

Bleeder Nipple

Seal

Cylinder Body

▶▶TECHNICIAN TIP During a brake inspection, it is good practice to carefully peel back the dust boot from the wheel cylinder to see whether there is brake fluid behind the dust boot. If there is, the wheel cylinder is starting to leak and should be replaced.

Chapter 5  Drum Brake Systems

146

Single-acting wheel cylinders are used on some non-servo drum brakes. Each wheel cylinder has only one piston, so the cylinder bore is closed off at the opposite end. There are two of these cylinders on each wheel assembly, one for each brake shoe. The wheel cylinder is very similar to the double-acting cylinder in that it is made of the same materials and has an aluminum piston, a lip seal, a spring and expander, a dust boot, a pushrod, and a bleeder screw.

▶▶ Brake 5-4 Summarize brake shoes and lining construction.

▶▶SAFETY TIP Brake friction materials are made from asbestos compounds because of the excellent heat resistance of that material. Asbestos compounds are now known to be hazardous and are generally banned. Do not assume that a brake shoe does not contain asbestos, because it is still found in some applications. Brakes are now manufactured from a variety of different materials that may be non-asbestos organic (NAO), low-metallic NAO, semi-metallic, and ceramic.

Shoes and Lining Construction

The drum brake system uses metal brake shoes that have holes, slots, and tabs for springs and hardware to attach to. The metal brake shoes have friction material, called linings, attached to them. Linings can be riveted, but they are more often bonded to the brake shoes. Even though the brake shoe is technically just the metal portion, technicians generally use the term “brake shoe” to means the metal shoe and lining assembly. The composition of the lining material affects brake operation. Materials that provide good braking with low pedal pressures tend to lose efficiency when they get hot (FIGURE 5-17). This means the stopping distance will be increased. They also tend to wear out more quickly. Materials that maintain a stable coefficient of friction over a wide temperature range generally require higher pedal pressures to provide efficient braking. They also tend to put added wear on the brake drum friction surface, reducing its useful life. The Society of Automotive Engineers (SAE) has adopted codes to rate the coefficient of friction of brake lining materials. The rating is written on the edge of the friction linings, which is called the edge code. Chapter 4, on disc brake systems, discusses these ratings in detail. Drum brakes are usually designed so that the condition of the lining can be checked only once the drum has been removed.

Primary and Secondary Brake Shoes ▶▶TECHNICIAN TIP As the heat in brake pads and linings builds up, the coefficient of friction capability of the material—and consequently its stopping power—is reduced. This reduction is called brake fade. Minimizing or overcoming brake fade is a major factor in the design of brakes and the development of brake friction materials.

High COF Material

0.45

Friction Coefficient

The terms “primary” and “secondary” refer to the brake shoes in a duo-servo brake system. The primary shoe goes toward the front of the vehicle, and the secondary shoe goes toward the rear of the vehicle (FIGURE 5-18). In most cases, the primary and secondary metal shoes are the same, but the linings installed on the brake shoes have different lengths. Because the primary shoe applies the secondary shoe, the secondary shoe is responsible for doing most of the braking work. Therefore, the primary shoe lining is shorter in length, and the secondary shoe lining is longer. The primary lining may also have a different coefficient of friction than the secondary lining. New shoes are normally labeled with “pri” (for primary) or “sec” (for secondary) on the edge of the lining. Verify that the lining is being installed in the proper position.

0.4 0.3

Secondary Shoe

Primary Shoe

0.2

Low COF Material

0.1 0

0

100

300

500

700

900

Surface Temperature °F FIGURE 5-17  The brake lining coefficient of friction is affected by temperature.

FWD FIGURE 5-18  Primary and secondary brake shoes.



Brake Shoes and Lining Construction

147

▶▶TECHNICIAN TIP New technicians tend to make a couple of rookie mistakes when it comes to duo-servo brake installation. The first relates to the way the new brake shoes are packaged in their box. The manufacturer generally puts the brake shoes in the box in like pairs. This means both primary shoes are in the bottom of the box and both secondary shoes are in the top of the box (or vice versa). When students remove the top two shoes to install them, and compare them to each other, they find that the shoes match (they shouldn’t if they are for a duo-servo system). The students assume the matching shoes belong on the same side of the vehicle and install them accordingly (FIGURE 5-19). They then do the same with the other two brake shoes (which also match each other) on the other side of the vehicle. So both primary shoes are now installed on one side of the vehicle, and both secondary shoes are installed on the other side. The second mistake involves installing the shoes in the wrong position on the backing plate. When students install the brake shoes on the first assembly, they may look carefully at the assembly and determine (correctly) that the primary shoe goes on the forward side of the backing plate, which on the driver’s side is the left side of the backing plate. They then determine that the secondary shoe goes on the right side of the backing plate. When they move to the passenger side of the vehicle, they believe they have already determined that the primary shoe goes on the left side of the backing plate just like the other side. Unfortunately, that is the rear side of the backing plate, and the secondary shoe should be installed in that position. So, the vehicle has the brake shoes installed correctly on one side, but incorrectly on the other side (FIGURE 5-20). Watch for these mistakes when performing brake inspections and service.

FIGURE 5-19  A common installation error: The primary shoes are on one side of the vehicle, and the secondary shoes are on the other.

FIGURE 5-20  Another common installation error: The primary and secondary shoes are swapped from side to side.

Riveted and Bonded Friction Materials Lining on brake shoes is much thinner than on disc brake pads due to the much greater surface area of the brake shoe lining. Drum brake shoes consist of friction material or lining bonded or riveted onto a steel shoe (FIGURE 5-21). Bonded brake linings are more common on light-duty vehicles because they are less expensive to build. Also, lighter vehicles do not subject the brake linings to as much heat, so the bonding agent is not as likely to fail on them as it would on heavier vehicles. Bonded linings are glued to the metal brake shoe under high pressure and temperature to ensure that the bonding is as strong as possible. Riveted linings are used on heavier-duty or high-performance vehicles. Metal rivets, usually made of copper or aluminum, provide a mechanical connection to hold the brake

FIGURE 5-21  A. Bonded brake shoe lining. B. Riveted brake shoe lining.

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Chapter 5  Drum Brake Systems

lining to the shoe. This means rivets are less susceptible to failure under high temperatures. However, because the rivets actually pinch some of the brake lining between the rivet head and the metal shoe, the brake linings cannot wear down as much before the rivets contact the drum. Riveted linings must be changed sooner than bonded linings to prevent the rivet heads from contacting the drum and wearing a groove in the friction surface.

▶▶ Springs

and Hardware Inside a Drum Brake System

5-5 Explain the purpose of springs and hardware inside a drum brake system.

Drum brakes use a variety of springs and hardware to control the action of the brake shoes (FIGURE 5-22). Each type of brake uses its own arrangement of these components. Before removing them, pay close attention to how they are installed. It is good practice to disassemble and reassemble the brake components on only one side of the vehicle at a time. That way the other side can be referred to if it’s hard to remember how something came apart. Generally speaking, there are several categories of springs used in drum brakes: return, hold down, and specialty.

Return Springs Return springs retract the brake shoes when the driver releases the brake pedal. Each return spring either connects to one brake shoe and the backing plate anchor pin or connects directly to the other brake shoe. In both cases, they pull the brake shoes back into their rest position (FIGURE 5-23). Return springs can integrate one or more coil springs in the length of the spring, or it can be a large U-shaped spring. Return springs are generally quite stiff, making them a challenge to install. Brake pliers can make installation easier. Even though return springs may look the same front to rear inside a particular brake assembly, be aware that they can have different amounts of strength. On duo-servo brakes, the primary return spring is weaker than the secondary return spring, so the primary shoe can be applied before the secondary shoe. The coils on return springs should be tightly wound together, meaning that the coils should be touching each other, with no space between them. Any space indicates that the return spring has been stretched and must be replaced. Return springs are normally replaced in sets. If one spring has to be replaced, they all should be replaced to avoid mismatched components. Also, because of the high temperatures and the large number of apply-andreturn cycles that drum brake springs must endure, some manufacturers and shops recommend replacing the return springs and hardware during every brake job.

Hold-Down Springs Hold-down springs do just what their name suggests: They hold the brake shoes against the backing plate (FIGURE 5-24). They can be coil springs in combination with a spring

FIGURE 5-22  Examples of brake springs and hardware.

FIGURE 5-23  Return springs pull the shoes back to their rest position.



Self-Adjusters

A

B

C

FIGURE 5-24  Hold-down springs. A. Coil spring style. B. Sideways coil style. C. U-shaped clip style.

retainer and a pin, sometimes referred to as a brake nail, that extends through the backing plate, brake shoe, and center of the spring; or they can be coil springs lying on their side and connected to the pin beside them. They can also use U-shaped spring-steel clips in combination with a pin that extends through the backing plate and brake shoe.

Specialty Springs Specialty springs are used to return links and levers on the parking brake system or the self-adjuster mechanism. Specialty springs can be of different shapes and sizes and can be used to push or pull components into proper position. Be sure to install each specialty spring in the correct position so that each of the brake systems works as intended. If in doubt, check the other wheel brake unit or the service information to see how it is assembled.

▶▶ Self-Adjusters Brake shoe lining wears over time and increases the clearance between the brake lining and drum. This increased clearance causes the wheel cylinder pistons to travel farther in order to move the brake shoes out to contact the surface of the drums. It also causes the brake pedal to travel farther before activating the brakes, reducing the reserve pedal height, which leaves the driver vulnerable in the event that a hydraulic failure were to occur. To safeguard against these dangers, all manufacturers have been required since 1968 to incorporate a self-adjusting mechanism into their drum brake systems that is capable of maintaining proper shoe-to-drum clearance.

149

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Chapter 5  Drum Brake Systems

Types of Self-Adjusters

▶▶TECHNICIAN TIP

The first type of self-adjuster is used on most duo-servo brakes. It has an adjustable threaded star wheel assembly holding the bottoms of the brake shoes apart. The link has three main parts: the threaded barrel, the non-threaded barrel (and possible thrust washer), and the star wheel, which is threaded on one end and smooth on the other. Both barrels have slots that fit to tabs on the brake shoes, preventing them from ­turning. The star wheel’s threaded end screws into the threaded barrel. The smooth end of the star wheel fits into the n ­ on-threaded barrel (FIGURE 5-25). As the star wheel is turned, the thread causes the self-adjuster to lengthen. This takes up excess brake shoe ­clearance, as needed. A movable self-adjuster link is held against the star wheel (FIGURE 5-26). The link is moved up and down by the action of applying the brakes while the vehicle is backing up. If the link moves far enough, it will catch another tooth of the star wheel, and the spring action will thus turn the star wheel one tooth, adjusting the brakes shoes outward just a bit. This process continues until the brake shoe clearance is taken up enough so that the self-­ adjuster link cannot move far enough to catch another tooth on the star wheel. As the brake linings wear, the clearance increases, and the movement of the link increases to the point ▶▶TECHNICIAN TIP that it catches another tooth. This continues for the life of the brake lining. Be careful to not switch the self-­adjusters The self-adjuster link is moved by a cable or rod, which is connected to the anchor from one side of the vehicle to the other. pin at the top of the backing plate. The movement of the secondary brake shoe, when the If this happens, the self-adjuster will brakes are applied while backing up, causes the self-adjuster link to be pulled by the cable or retract the adjustment, causing the brake rod. The position of the link and cable or rod is critical to the operation of the self-adjuster shoe clearance to increase as the brakes assembly. The link should be level with the star wheel and close to its centerline. Also, the adjust. The customer will report that the star wheel assembly is specific to a given side, meaning that the threads on each star wheel brake pedal keeps getting lower and are opposite each other, so they turn in opposite directions. lower but was fine when they picked up A similar style of self-adjuster works off movement between one brake shoe and the vehicle. the parking brake strut. It is usually used on non-servo brakes. The star wheel linkage is threaded and is usually mounted between the brake shoes just below the wheel cylinder (FIGURE 5-27). It acts as the parking brake strut. There is a pivoting self-adjuster lever attached to one brake shoe, and a spring pulls the lever down when the service brake pedal is applied. The pivoting lever is pushed up by Adjuster Screw Washer Socket the end of the parking brake strut when the brake pedal is released. If the lever travels far enough when the service brake is applied, it will catch the next tooth on the star wheel. As the brake pedal is released, the end of the strut pivots the self-adjuster link upward Button Star Wheel against spring pressure. This turns the star wheel linkage slightly, FIGURE 5-25  A star wheel assembly. lengthens the parking brake strut slightly, and reduces the brake Some drivers never use their brakes when backing up. Instead, they back up slowly, put the transmission in first gear, and use their clutch or automatic transmission to stop the vehicle and start going forward at the same time. Backing without braking prevents the self-­adjusters from adjusting the drum brakes on vehicles equipped with this type of self-adjuster. Educating customers on how the self-adjuster works will help them keep their brakes adjusted as well as get longer life out of the linings.

Brakes Applied in Reverse Direction

Self-Adjustment Action on Brake Release

Anchor Pin

Cable Pivot Control Cable Self-Adjuster Arm (pulled up by control cable)

Adjustable Floating Link FIGURE 5-26  A self-adjuster on a duo-servo-style brake.

Self-Adjuster Arm Adjuster Star Wheel

(turns adjuster star wheel as brakes are released)



Self-Adjusters Brakes Released

Brakes Applying and Self-Adjusting Self-Adjuster Arm

Adjuster Star Wheel

Self-Adjuster Spring

FIGURE 5-27  Alternate style of automatic star wheel brake adjuster.

shoe clearance slightly. This process continues as the brake pedal is applied and released until the excess clearance of the brake shoe to the drum is taken up. The self-adjuster lever will then be unable to catch another tooth on the star wheel until the clearance opens a bit more as the brake linings continue to wear. Some manufacturers use a ratcheting-style self-adjuster, sometimes called a cam-style adjuster (FIGURE 5-28). This type uses two toothed pieces held in contact with each other by spring pressure. They can slide over each other in one direction but hold in the other direction. The two pieces allow excessive shoe-to-drum clearance to be taken up when the brake pedal is applied and then prevent the shoes from fully returning to their rest position when the brakes are released. As the service brakes are applied, the shoes move apart. If they travel far enough, the toothed pieces slide over each other until the shoes contact the drum. When the brakes are released, the components allow a small amount of inward brake shoe movement to occur, which creates the proper shoe-to-drum clearance. This style of self-adjuster can generally be adjusted in one brake pedal application, reducing the amount of time spent preadjusting the brake shoes when they are being replaced.

Brake Shoes Brake Drum

Adjuster Link Park Brake Lever

Floating Pivot: - Fixed to Cam - Floats in the Link Serrated Teeth

FIGURE 5-28  A ratchet-style adjuster adjusts if excess brake clearance is present when the service brake is applied.

151

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Chapter 5  Drum Brake Systems

▶▶ Drum 5-6 Explain drum brake and parking brake systems.

▶▶ Drum 5-7 Diagnose drum brake operation.

Parking Brake Systems

Drum parking brake systems mechanically apply the regular service brake shoes. Because drum brakes are self-energizing, it is easier to generate the force needed to apply them mechanically than it is to apply disc brakes. The parking brake cable attaches to the bottom of the parking brake actuating lever in the drum brake assembly. The other end of the lever is attached to the top end of one of the brake shoes with a pin or tang. A strut rod runs from near the top of the actuating lever to the other brake shoe (FIGURE 5-29). When the cable is pulled, the lever pivots on the strut rod, pushing the top of the brake shoe rearward and the strut rod forward. Because the strut rod is connected to the other shoe, pulling the lever forces the brake shoes apart and into firm contact with the drum, holding the vehicle stationary. Releasing the cable allows the brake retracting springs to pull the brake shoes away from the drums.

Brakes Operation

Drum brake diagnosis starts with understanding the customer’s concern. Communicating directly with the customer is the best way to do that, but the customer is not always available. An experienced service advisor will gather the required information, so carefully read the service advisor’s notes on the repair order or speak with them directly. Once the customer’s concern have been understood, a test drive is usually needed to verify the accuracy of the concern. Depending on the concern, it may be as simple as stepping on the brake pedal without moving the vehicle and feeling the pedal sink to the floor, or it could require a more detailed test drive to observe the fault that the customer is describing. This is a good opportunity to test the brakes under a variety of conditions. Replicating the customer ­concern is important in order to address the situation that the customer is experiencing. During the test drive, find a safe place to operate the brakes at a variety of speeds with a variety of brake pedal pressures, especially trying to mimic the conditions that the customer described. It may be necessary to go on a test drive with the customer driving, allowing them to operate the vehicle in the way that makes the problem evident and to point out the particular situation they are experiencing. Also, it is good to have the customer along in case the problem does not occur, to avoid their thinking that the technician doesn’t believe there is a problem or that the technician is ignoring the issue. If there is a concern related to the antilock brake system (ABS) that requires a test drive, extreme caution is required so that an accident doesn’t happen. Because ABS operates only during extreme braking or poor traction conditions, there is a risk of being rear-ended or losing control of the vehicle if the brakes are applied hard enough to activate the ABS. So make sure the vehicle is being tested away from all other traffic. Remember that if the ­yellow ABS warning lamp is illuminated, the ABS system is deactivated, and in this case, do not try to activate ABS on a test drive. If the yellow ABS lamp is off, the ABS system should

Parking Brake Pushrod Parking Brake Handle

Parking Brake Cable Adjustable Floating Link FIGURE 5-29  A parking brake assembly for a drum brake.

Pivot Point Parking Brake Lever



be active and ready to activate. It is helpful when testing ABS to do so on a surface with limited traction, such as wet pavement or a dirt road. When the brakes are applied firmly, ABS activation can typically be felt as pulsations in the brake pedal and possibly the steering wheel. The vehicle should brake quickly while maintaining steering control. In some cases, a “poor traction” lamp may illuminate, indicating that the ABS system had to activate. This warning lamp usually turns off after several seconds. If the yellow ABS warning lamp illuminates, it typically means that the vehicle’s electronic control module (ECM) has observed a fault in the ABS system and will have to be checked for diagnostic trouble codes (DTCs). Once the customer’s concern has been verified, there will hopefully be enough information to research the concern in the service information and technical service bulletins (TSBs). Armed with this information be ready to create a plan to begin testing the brake system. This could be as simple as performing a visual inspection of the brake fluid level and condition, removing the wheels to disassemble and inspect the brake units, or measuring the runout of the drums. It could also involve electrical diagnosis, such as locating a short circuit in the parking brake warning indicator system. Suspension and steering system faults can appear to be brake system faults. An example of this is a pulling condition while braking. If the control arm bushings on the suspension are worn, then braking the vehicle will cause the wheel to pivot outward, making the wheel point in a direction other than straight down the road, imitating a brake pull. So always inspect the suspension and steering systems when diagnosing concerns related to a vehicle “pull.” The braking system on a vehicle must be restored to its proper operation if one or more braking system faults are present. Diagnosis of any problem must identify all issues that would prevent the brakes from operating normally. Brake system failures are more likely to lead to vehicle collisions than failures of most systems. Any diagnosis and subsequent repairs have to be thorough and complete. Once the cause of the fault has been identified, determine the action that will correct the fault. This information can then be used to prepare an estimated cost of repairs and given to the customer for authorization to perform the repairs. Once the repair is complete, retest the system to confirm that the concern has been fully addressed and that no further issues are present.

Drum Brake Noises Drum brakes are not prone to squealing like disc brakes are, but that does not mean they never make noise. One of the most common sounds that drum brakes make is a groaning noise. It can be caused by excessive brake dust in the drum that causes the brakes shoes to skip and catch, which sounds like a groan. Removing the drum and cleaning the excess brake dust or replacing the brake shoes (due to wear) usually resolves this issue (FIGURE 5-30). Another noise that drum brakes can make is a grinding noise. If the friction lining wears all the way down to the metal shoe, the shoe and drum can make a metal-on-metal grinding noise. This noise can be resolved only by replacing the brake shoes and refinishing or replacing the drum. Keeping drum brakes from making unwanted noises is pretty simple: keep them clean and inspect the brake lining thickness periodically, replacing it when it gets to the specified minimum thickness. The last common noise that drum brakes can make is a clicking noise. This can be caused by two situations. One, it could be that the brake shoes have worn grooves in the contact pads on the backing plate and are causing the clicking noise when the shoe moves into and out of the groove. Two, it could be that the brake drum’s surface finish cut is too rough and acts like the threads on a screw. When the lining contacts the surface of the drum, the lining follows the threaded finish of the drum, and the linings thread themselves away from the backing plate. When the brakes are released, the hold-down springs snap the brake shoes back FIGURE 5-30  Brake dust. against the backing plate.

Drum Brakes Operation

153

▶▶TECHNICIAN TIP It is common to have more than one fault present in a brake system when it is diagnosed, such as worn brake shoes, a leaky wheel cylinder, and a leaky seal in the master cylinder. It is good practice to perform a thorough inspection of the brake system whenever one condition is present, to determine if any others are present at the same time. This prevents having to go back to the customer to get permission to perform the additional work after they have agreed to the initial repair, giving the customer cause to doubt the technician’s competence or integrity.

▶▶TECHNICIAN TIP One way to help identify whether a problem is coming from the front or rear brakes is to test-drive the vehicle in a safe place at a relatively low speed and lightly apply the parking brake. If the condition is still present, the problem is with the rear brakes, because the parking brakes are usually on the rear wheels. If the condition is not present, the problem is likely with the front brakes.

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Chapter 5  Drum Brake Systems

▶▶ Brake 5-8 Summarize how to evaluate a brake drum repair.

Drum Repair

Drum brakes require periodic maintenance and repair as the vehicle ages. As stated previously, visually inspecting the braking system is part of the diagnostic procedure. It is also a critical part of maintenance and repair. A good visual inspection can turn up issues such as lining that isn’t wearing evenly, contaminated linings from a leaking wheel cylinder, a warped drum, or even broken springs and hardware. Given the critical role that the braking system plays, a good periodic inspection can address issues before they become life-­ threatening. The following topics and Skill Drills will address the common maintenance and repair procedures of drum brake systems.

Tools The tools used to diagnose and repair drum brake systems include the following (FIGURE 5-31): Brake wash station—used to clean drum and disc brake dust. Brake spring pliers—used to remove and install return springs. Hold-down spring tool—used to remove and install hold-down springs. Drum brake micrometer—used to measure inside brake drum diameter. Brake shoe adjustment gauge—used to preadjust brake shoes before installing the drum. F. Brake spoon—used to adjust brake shoes when the drum is installed. G. Wheel cylinder piston clamp—used to hold the pistons in the wheel cylinder while the brake shoes are removed. H. Off-car brake lathe—used to machine drums and rotors that are off the vehicle. I. Parking brake cable pliers—used to install parking brake cables on the parking brake lever. J. Parking brake cable removal tool—used to remove the parking brake cable from the backing plate. A. B. C. D. E.

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B

C

D

Caption appears on next page.



Brake Drum Repair

E

F

G

H

I

J

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FIGURE 5-31  Drum brake tools. A. Brake wash station. B. Brake spring pliers. C. Hold-down spring tool. D. Drum brake micrometer. E. Brake shoe adjustment gauge. F. Brake spoon. G. Wheel cylinder piston clamp. H. Off-car brake lathe. I. Parking brake cable pliers. J. Parking brake cable removal tool.

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Inspecting and Measuring Brake Drums

▶▶TECHNICIAN TIP When backing off the self-adjuster, remember that it is necessary to hold the self-adjuster link away from the star wheel so that the star wheel can be retracted.

Brake drums need to be removed for a variety of reasons, such as inspecting the thickness of the brake linings, checking for wheel cylinder leaks, and packing wheel bearings. While removed, the brake drums should be inspected visually for any damage. They should also be measured with a drum micrometer to verify that they are smaller than the specified maximum diameter. If they are not smaller, they should be replaced. Before removing the brake drum, determine whether it is a hub style or hubless (slip-off) style. Most light-duty vehicles now use hubless drums. Looking at the holes for the lug studs will usually reveal which style of brake drum is being worked on. If there is clearance between the stud and drum, it is likely a hubless brake drum. Also look at the clearance between the center hole of the brake drum and the hub. A hubless brake drum has a parting line at the centering hole. A hub-style brake drum appears to be one solid piece. Several steps can be taken to ease the drum removal process. First, back off the brake shoe adjustment before removing the drum. The shoes and drum are in close proximity to each other, and if the adjustment is backed off, they can bind during the removal process, especially if there is a ridge on the edge of the drum or if the brake shoes are heavily grooved. Second, because of the tight clearances between the hub and brake drum, the surfaces can rust together. A medium or large ball-peen hammer may need to be used to hammer on the drum between the lug studs. Make sure not to hit the lug studs, because hitting them will damage them. Some drums can be rusted on very solidly. Sometimes a torch may be needed to heat up the drum to break the rust bond. Some drum brakes have two threaded holes in the drum that allow bolts to be installed. By tightening the bolts, the drum is forced off of the hub. In case the drum still can’t be removed, cut the heads off the hold-down pins with a diagonal cutter at the backing plate. This releases the brake shoes from the backing plate and allows the shoes to come off with the drum. If breaking the drum loose causes difficulty, seek the assistance of a supervisor. To remove, clean, inspect, and measure brake drums, and to determine any further actions, follow the steps in SKILL DRILL 5-1.

SKILL DRILL 5-1 Removing, Cleaning, Inspecting, and Measuring Brake Drums 1. To perform this procedure on a hubless-style drum, first make matching marks on the drum and hub for reinstallation in the correct position. A permanent marker, crayon, or center punch can be used.

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2. If there are screws or speed nuts holding the drum to the hub, remove them in accordance with the specified procedure. The speed nuts can be discarded and are not needed upon reassembly. The screws will be reused.

3. Remove the drum from the hub.

4. To perform this procedure on a hub-style drum, remove the wheel bearing locking mechanism (cotter pin, lock nut, or peened washer).

5. Remove the wheel bearing adjusting nut, thrust washer, and outer bearing.

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6. If the rear bearing needs to be removed for service or replacement of the drum, use the following procedure to remove the inner bearing and grease seal: i. Reinstall the adjusting nut onto the spindle about five turns. ii. Grasp the drum/hub assembly on the top and bottom. Push it toward the center of the vehicle. Hold slight downward pressure as the drum/hub assembly is firmly pulled away from the backing plate. This should cause the adjusting nut to catch the wheel bearing and pull it and the seal out of the rear of the hub.

7. Clean the drum with approved asbestos removal equipment. Inspect the drum for hard spots/hot spots, scoring, cracks, and damage.

8. Measure the drum diameter at the deepest groove or most worn part of the drum, and compare the result to specifications. If it is over the size limit, it will need to be replaced. If it needs to be machined and is below the maximum diameter, check to see how badly it is scored.

9. Find the smallest diameter of the drum and write down the reading. Find the largest diameter of the drum and write down that reading. Calculate the difference between the two readings (out-of-round), and compare the result to specifications. Determine any necessary actions.



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▶▶ Removal

and Inspection of Brake Shoes and Hardware

Drum brakes need to be disassembled for a variety of reasons, such as when the brake lining has worn beyond specifications; the wheel cylinders are faulty; the self-adjuster is stuck or damaged; or the axle seal has failed and leaked gear oil or wheel bearing grease onto the brake lining. Drum brakes are generally more complicated to change than disc brake pads are, so care needs to be taken when working on them. There is usually a combination of springs, links, levers, guides, and retainers. It is easy to install these items incorrectly. It is good practice to carefully examine each brake assembly before disassembling, to verify that it was assembled correctly previously. Brake shoes and springs are always replaced in axle sets. If one rear brake assembly has linings contaminated with grease from a failed axle seal, the lining on both wheel brake units will have to be replaced. The same goes with springs. The other parts are left to the discretion of the technician and any shop policies. Regarding star wheel assemblies, there is some controversy as to whether the threads should be lubricated. The argument on one side is that if the threads are not lubricated, then they will likely rust and freeze up. The other side argues that any lubricant on the threads will attract brake dust and gum up the threads. There is some truth to both arguments. General practice would suggest using a light coating of lubricant on the threads in areas where the star wheel is likely to come in contact with salt, mud, or water because rust and corrosion are a greater hazard than dust gumming up the threads. In areas where the assembly is unlikely to come in contact with those conditions, use no lubricant on the threads. But always follow the manufacturer’s recommended procedure, if specified. To remove, clean, inspect, and reassemble a duo-servo brake, follow the steps in SKILL DRILL 5-2.

5-9 Demonstrate the inspection of brake shoes and hardware.

▶▶TECHNICIAN TIP Because it can be hard to remember where all of the parts fit inside of a drum brake assembly, it is good practice to disassemble only one side at a time, leaving the other side as a reference for reassembling the first side. Or take a couple of reference pictures before disassembly.

SKILL DRILL 5-2 Removing, Cleaning, Inspecting, and Reassembling a Duo-Servo Brake Note: In the appropriate service information, research and follow the procedure for disassembling the brake assembly. Manufacturers use many drum brake configurations, so the following steps are general in nature and should not be substituted for the manufacturer’s procedure. 1. Clean the brake shoes, hardware, and backing plates by using equipment and procedures for dealing with asbestos and/or dust.

2. To disassemble a duo-servo brake, first remove the return springs, cable guide (if installed), and shoe guide. Set everything aside in the order it will go back on. Remove the parking brake strut and spring.

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3. Remove the primary shoe hold-down spring, retainer, and pin.

4. Remove the self-adjuster spring, star wheel assembly, and primary shoe.

5. Remove the secondary hold-down spring, retainer, pin, and secondary shoe.

6. Disassemble the parking brake lever from the brake shoe and hardware from the backing plate, being careful not to lose any parts, and remember how they go back together.

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7. Finally, clean and inspect all parts according to the manufacturer’s procedure.

8. To reassemble a duo-servo brake, first reassemble the parking brake lever on the brake shoe and parking cable.

9. Reassemble the star wheel assembly, lubricate the floating end, and set the assembly aside. Also lubricate the contact pads on the backing plate.

10. Install both shoes onto the backing plate with the hold-down spring assemblies.

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11. Install the shoe guide and self-adjuster cable over the anchor pin.

12. Install the cable guide and return spring in the secondary shoe. Also align the wheel cylinder pushrod in the shoe.

13. Position the secondary shoe in place and use brake spring pliers to install the return spring over the anchor pin.

14. Install the parking brake strut rod onto the secondary shoe, and pull the primary shoe engaged with the parking brake strut rod.

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Removal and Inspection of Brake Shoes and Hardware

15. Install the return spring in the primary shoe, and use brake spring pliers to stretch the return spring over the anchor pin.

16. Install the self-adjuster link, cable, and spring into position.

17. Install the star wheel between the bottoms of the two shoes.

18. Finally, check the fit of all springs, clips, and levers. Operate the self-adjuster cable or lever.

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To disassemble, clean, inspect, and reassemble a non-servo brake, follow the steps in

SKILL DRILL 5-3.

SKILL DRILL 5-3 Dissembling, Cleaning, Inspecting, and Reassembling a Non-servo Brake Note: In the appropriate service information, research and follow the procedure for disassembling the brake assembly. Manufacturers use many drum brake configurations, so the following steps are general in nature and should not be substituted for the manufacturer’s procedure. 1. Clean the brake shoes, hardware, and backing plates by using equipment and procedures for dealing with asbestos and/or dust.

2. To disassemble a non-servo brake, first remove the hold-down springs, retainers, and pins.

3. Spread the shoes apart and remove the parking brake strut and self-adjuster components.

4. Remove the return springs.

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5. Disassemble the parking brake lever from the brake shoe and hardware from the backing plate, being careful not to lose any parts, and remember how they go back together.

6. Clean and inspect all parts according to the manufacturer’s procedure.

7. To reassemble a non-servo brake, first assemble and lube the selfadjuster/parking brake strut assembly. Lube the backing plate pads.

8. Place one shoe on the backing plate and install the hold-down spring and pin.

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9. Place the self-adjuster/parking brake strut on the installed brake shoe.

10. Place the retracting springs on both shoes, and fit the loose shoe to the backing plate, being sure to line up the wheel cylinder pushrods, the self-adjuster, and the parking brake mechanism.

11. Install the hold-down spring and pin.

12. Finally, check the fit of all springs, clips, and levers.



Removal and Inspection of Brake Shoes and Hardware

Removing, Inspecting, and Installing Wheel Cylinders If a wheel cylinder is leaking or binding, there is a good chance that the cylinder bore is corroded and pitted, requiring removal and replacement of the wheel cylinder. Wheel cylinders also have to be removed if the backing plate is being replaced. Very few shops rebuild wheel cylinders anymore. If the wheel cylinder is to be reused, make sure the bleeder screw can be opened. Because the bleeder screw is hollow, it can easily break off, which would make bleeding nearly impossible. To remove, inspect, and install wheel cylinders, follow the steps in SKILL DRILL 5-4.

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▶▶TECHNICIAN TIP If one wheel cylinder is leaking, how long until the other wheel cylinder starts leaking? And if it does leak, will it contaminate the new brake lining? Take into consideration the age and condition of the components when the system is inspected, and give the customer an appropriate recommendation.

SKILL DRILL 5-4 Removing and Installing Wheel Cylinders 1. Peel back the dust boots and check for brake fluid behind them. Determine any necessary actions.

2. Use a flare nut or line wrench to unscrew the brake line from the wheel cylinder.

3. Remove wheel cylinder mounting bolts and the wheel cylinder from the backing plate.

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4. Acquire a new wheel cylinder and verify that it matches the one that was removed.

5. Install and tighten any mounting screws. After that, tighten the brake line with a flare nut or line wrench.

6. Bleed the braking system starting from the farthest point of the vehicle from the master cylinder and work toward the master cylinder.

Preadjusting Brakes and Installing Drums and Wheel Bearings Preadjusting the brake shoes and parking brake is a routine step in a drum brake job. It saves time because it is faster to adjust the brakes with the drum off than it is when the drum is installed. The preadjustment sequence is important. Start by making sure the parking brake is fully released. Then look at the shoes to make sure the parking brake adjustment is not



Removal and Inspection of Brake Shoes and Hardware

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holding the brake shoes in the applied position. This can be seen by verifying that the brake shoes are up against their stops on the top and bottom of each shoe. If they are not firmly against them, loosen the parking brake adjustment. Then adjust the service brakes. Once they have been adjusted, adjust the parking brake. To preadjust brake shoes and the parking brake and to install brake drums or drum/ hub assemblies and wheel bearings, follow the steps in SKILL DRILL 5-5.

SKILL DRILL 5-5 Preadjusting Brakes and Installing Drums and Wheel Bearings 1. Make sure the brake shoes are fully up against their stops and centered on the backing plate.

2. Set the preadjustment gauge to the drum diameter and lock it in place.

3. Place the preadjustment gauge over the center of the brake shoes.

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4. Adjust the star wheel until the centers of the brake shoes barely contact the preadjustment gauge.

5. Test install the brake drum to verify the drum fits. Adjust the shoes as necessary.

6. Adjust the parking brake according to the manufacturer’s procedure. If the drum has serviceable wheel bearings, repack, install, adjust, and secure them.

Installing Wheels and Torquing Lug Nuts This procedure, though fairly simple, can result in problems if it is not performed properly. Over-tightening the lug nuts can cause the wheel studs to break either immediately or, worse, after the vehicle has been driven for a period of time. Under-tightening can lead to loosening of the lug nuts and result in the wheel working its way off the vehicle. This can cause the driver to lose control of the vehicle and will potentially result in an accident. Lug nuts should be tightened to the proper torque, in the specified sequence. All manufacturers specify these details for each of their vehicles. The torque pattern is usually in some form of either a star or cross.



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Be careful about how the lug nuts are installed. Many wheels use a tapered hole that matches the tapered end of the lug nut and centers the wheel on the wheel flange. Other wheels use a flat surface that matches flat surfaces on the lug nuts. No matter what, the lug nut surface must match the mating surface of the wheel. Always check that these surfaces match. When installing lug nuts, the weight should be off the vehicle. The lug nuts should easily center the wheel on the hub. Proper centering is especially important on aluminum wheels that use a flat lug nut sealing surface. The lug nuts can dig into the sides of the lug nut holes and cause the wheel to not center correctly. Once the lug nuts are up against the wheel, work in a star pattern to evenly install the wheel on the hub. To install the wheel and torque lug nuts and to make final checks and adjustments, follow the steps in SKILL DRILL 5-6.

SKILL DRILL 5-6 Installing Wheels and Torquing Lug Nuts and Making Final Checks 1. Start the lug nuts on the wheel studs by hand. Carefully run all of the lug nuts down so that they are seated in the wheel. Lower the vehicle so that the tires are partially on the ground. Use a torque wrench to tighten each lug nut to the proper torque in the proper sequence. Once all of the lug nuts have been torqued, go around them again, this time in a circular pattern.

2. Reinstall hubcaps and valve stem caps.

3. Check the brake fluid level in the master cylinder reservoir. Start the vehicle and check the brake pedal for proper feel and height. Check the parking brake for proper operation. Inspect the system for any brake fluid leaks and loose or missing fasteners.

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▶▶Wrap-Up Ready for Review ▶▶

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The main components of the drum brake system are brake drum, backing plate, wheel cylinder, brake shoes, springs and clips, automatic brake self-adjuster, and parking brake mechanism. Drum brakes are usually located on the rear wheels of disc-drum applications. Hydraulic pressure forces pistons to move the brake shoes into contact with the inside of the brake drums. Leading brake shoes are applied in the same direction as the brake drum’s forward rotation, and they are self-­ energizing; trailing brake shoes are applied in the opposite direction of the brake drum’s forward rotation, and they are not self-energizing. Drum brake systems can be twin leading shoe (least ­common), leading/trailing shoe, and duo servo. Twin leading shoe brake systems are more efficient in forward braking than in reverse braking. Leading/trailing shoe brake systems provide equal, but not maximum, braking in both directions. Duo-servo drum brake systems use servo action for the front brake shoe to multiply the force to the rear shoe, causing the rear brake shoe to do more braking work. Brake drums are machined to the manufacturer’s specified standard diameter and should be replaced once worn or refinished beyond specifications. Brake drum styles are hub or hubless (most common in passenger vehicles). All brake unit components, except the brake drum, are mounted to the backing plate. Brake shoe contact pads provide a smooth surface for the brake shoes to ride on. Wheel cylinders contain a housing, pistons, piston lip seals, a spring and expander set, dust boots, push rods, and a bleeder screw. Wheel cylinders should be replaced rather than honed if corrosion or pitting occurs. Wheel cylinders are single acting (one piston, with force generated in one direction) or dual acting (two pistons, with force generated in two directions). Steel brake shoes have lining material bonded (or riveted) to them to create friction for braking. Duo-servo brake systems have primary brake shoes (front of vehicle, shorter shoe lining) and secondary brake shoes (rear of vehicle, longer shoe lining). Drum brakes may make the following noises: groaning (excess brake dust in the drum), grinding (worn friction lining), or clicking (grooves worn into backing plate ­contact pads or rough finish cut on the drum). Drum brakes use return, hold-down, and specialty springs. Drum brakes must have self-adjusters to maintain proper shoe-to-drum clearance.

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Proper diagnosis involves understanding the customer concern, test-driving the vehicle, and visually inspecting the brake system. Drum brakes require periodic maintenance and repair as the vehicle ages. Brake shoes and springs are always replaced in axle sets. It is good practice to disassemble only one side at a time, leaving the other side as a reference for reassembling the first side. If a wheel cylinder is leaking or binding, the wheel ­cylinder will have to be replaced. Preadjusting the brake shoes and parking brake saves time because it is faster to adjust the brakes with the drum off than it is when the drum is installed. Lug nuts should be tightened to the proper torque, in the specified sequence.

Key Terms anchor pin  A component of the backing plate that takes all of the braking force from the brake shoes. automatic brake self-adjuster  A system on drum brakes that automatically adjusts the brakes to maintain a specified amount of running clearance between the shoes and drum. backing plate  A plate bolted to the steering or suspension components that supports the wheel cylinder(s), brake shoes, and hardware. brake drum  A short, wide, hollow cylinder that is capped on one end and bolted to a vehicle’s wheel. It has an inner friction surface that the brake shoe is forced against. brake lathe  A tool used to refinish the drum surface by removing a small amount of metal and returning it to a concentric, nondirectional finish. brake shoe  A steel shoe and brake lining friction material that apply force to the brake drum during braking. brake shoe adjustment gauge  An adjustable tool used to ­preadjust the brake shoes to the diameter of the brake drum. brake spoon  A tool used to adjust the brake lining–to-drum clearance when the drum is installed on the vehicle. brake spring pliers  A tool used for removing and installing brake return springs. cylinder bore  The inside diameter of a cylinder. drum brake micrometer  A tool used for measuring the inside diameter of the brake drum. duo-servo drum brake system  A system that uses servo action in both the forward direction and the reverse direction. hold-down springs  Springs that hold the brake shoes against the backing plate. hold-down spring tool  A tool used for removing and installing hold-down springs. leading shoes  Brake shoes that are installed so that they are applied in the same direction as the forward rotation of the drum and thus are self-energizing.



leading/trailing shoe drum brake system  A type of brake shoe arrangement where one shoe is positioned in a leading manner and the other shoe in a trailing manner. parking brake cable removal tool  A tool used to compress the spring-steel fingers of the parking brake cable so that the cable can be removed from the backing plate. parking brake mechanism  A mechanism that operates the brake shoes or pads to hold the vehicle stationary when the parking brake is applied. return springs  Springs that retract the brake shoes to their released position. self-energizing  The property of drum brakes that assists the driver in applying the brakes. When brake shoes come into contact with the moving drum, the friction tends to wedge the shoes against the drum, thus increasing the braking force. servo action  A drum brake design where one brake shoe, when activated, applies an increased activating force to the other brake shoe, in proportion to the initial activating force. It further enhances the self-energizing feature of some drum brakes. specialty springs  Springs used to return links and levers on the parking brake system or the self-adjuster mechanism. springs and clips  Various devices that hold the brake shoes in place or return them to their proper place. trailing shoes  Brake shoes installed so that they are applied in the opposite direction of the forward rotation of the brake drum. They are not self-energizing and are less efficient at developing braking force. twin leading shoe drum brake system  Brake shoe arrangement in which both brake shoes are self-energizing in the forward direction. wheel cylinder  A hydraulic cylinder with one or two pistons, seals, dust boots, and a bleeder screw that pushes the brake shoes into contact with the brake drum to slow down or stop the vehicle. wheel cylinder piston clamp  A tool that prevents the pistons from being pushed out of the wheel cylinders while the brake shoes are being replaced.

Review Questions 1. The component of the drum brake system attached to the backing plate to slow down or stop the vehicle is the ___________. a. automatic self-adjuster b. brake shoes c. wheel cylinder d. backing plate 2. All of the following statements with respect to brake systems are true except: a. Drum brakes are self-energizing. b. Trailing shoes are self-energizing. c. Leading shoes are self-energizing. d. Wedging effect assists the driver in applying the brakes.

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3. Which one of these drum brake systems uses servo action in both directions? a. Leading shoe drum brake system. b. Trailing shoe drum brake system. c. Twin leading shoe drum brake system. d. Duo-servo drum brake systems. 4. Choose the correct statement: a. Brake drums are usually made from fiber. b. Hubless drums are more expensive than hub-style drums. c. The wheel bearings are housed in the hub and are usually non-serviceable. d. The backing plate has holes stamped in it. 5. The component that prevents a direct path for water spray and dirt to enter the brake drum is the ________. a. labyrinth seal b. wheel bearing c. anchor pin d. anchor block 6. All of the following statements with respect to a wheel cylinder piston are true except: a. Piston-to-cylinder clearance does not affect proper operation. b. It is usually made of anodized aluminum. c. It is located inside the brake drum and is firmly clipped to the backing plate. d. It is fitted with a bleeder screw to air to bleed. 7. Choose the correct statement: a. The composition of the lining material does not affect brake operation. b. The primary shoe is responsible for doing most of the braking work. c. Riveted linings are used on heavier-duty or high-­ performance vehicles. d. Drum brakes are not prone to squealing like disc brakes are and hence never make noise. 8. When the driver releases the brake pedal, the brake shoes are withdrawn by the __________. a. hold-down spring b. return spring c. specialty spring d. coil spring 9. Among the tools used to diagnose and repair drum brake systems, the wheel cylinder piston clamp is used to ____________________. a. install parking brake cables on the parking brake lever b. hold the pistons in the wheel cylinder while the brake shoes are removed c. preadjust brake shoes before installing the drum d. remove and install hold-down springs 10. Which of the following brake tools is used to adjust brake shoes when the drum is installed? a. Off-car brake lathe. b. Brake spring pliers. c. Brake spoon. d. Brake wash station.

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ASE Technician A/Technician B Style Questions 1. Technician A says that parking brakes on drum brake vehicles use separate brake shoes for backup in an emergency. Technician B says that parking brakes on drum brake vehicles mechanically operate the standard drum brake shoes. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that duo-servo brake shoes are anchored only on the top. Technician B says that duo-servo brakes typically use single-piston wheel cylinders. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that both secondary shoes should be installed on the passenger side of the vehicle. Technician B says that the lining on the primary shoe is typically shorter than the lining on the secondary shoe. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that a cleaned-up self-adjuster can be installed on either side of the vehicle. Technician B says that grease seals are meant to be reusable. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says to use an air hose to clean the backing plate of dust and contamination. Technician B says to use a water-based cleaning solution to clean the backing plate of dust and contamination. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that riveted lining is usually for heavyduty or high-performance vehicles. Technician B says that bonded lining is glued to the metal brake shoe. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

7. Technician A says that a metal-on-metal grinding noise in drum brakes generally requires replacing the brake shoes and resurfacing or replacing the drums. Technician B says that one way to help identify whether a problem is coming from the front or rear brakes is to test-drive the vehicle in a safe place at a relatively low speed and lightly apply the parking brake. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says that brake shoe linings saturated with brake fluid from a leaky wheel cylinder can be cleaned with brake cleaner and reused as long as they aren’t worn out. Technician B says that brake shoes should be replaced in axle sets. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says that when performing a brake job on the rear axle of an older vehicle, inspection finds brake fluid under the dust boot, so the wheel cylinder should be replaced. Technician B says that when a drum brake return spring has failed, springs on both rear wheels need to be replaced. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that adapting brake shoes from different vehicles will increase the performance of a stock drum brake setup? Technician B says that the technician must restore the fit and function of the original equipment to minimize liability. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

CHAPTER 6

Refinishing Brake Rotors and Drums Learning Objectives ■■ ■■

6-1 Explain the purpose of machining a rotor. 6-2 Interpret rotor deformation.

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6-3 Explain the purpose of machining a drum.

You Are the Automotive Technician The customer just had their brake pads replaced at another repair shop but still report a rough braking event where they explain that it feels “like the brakes are releasing and then regrabbing.” You as the technician must decipher this to possibly mean that the previous repair facility didn’t turn the brake rotors. What does turning the brake rotors do for the vehicle when a braking event happens?

1. It makes the rotors bigger. 2. It makes the rotors more parallel or truer to the rest of the rotor. 3. It doesn’t do anything to the rotor.





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Chapter 6  Refinishing Brake Rotors and Drums

▶▶ Introduction When completing a brake repair, the technician should always verify the flatness of the brake pad and shoe surfaces so that brake pulsation will not be an outcome of a brake s­ ervice. Standard practice for replacing brake pads or shoes is to machine or replace the rotor or drum. By machining the metal braking component, the brake pads or shoes have the best possible chance of wearing correctly and the customer will have the best outcome from the brake service. There are different ways to remachine the surface of the rotor or drum to achieve a truly flat surface.

▶▶ Rotors A brake rotor is used in a disc brake setup on a vehicle and is the main means of transferring the kinetic motion of the wheel assembly to the stationary brake pad, which then converts that energy into heat (FIGURE 6-1). By doing this type of energy transfer, the brake rotor gets very hot when the brakes are applied because it is trying to stop an object that is ­moving down the road at a high rate of speed. Because of this type of design, the rotor must be made of a high-strength material to absorb the repetitive thermal cycles that the braking system encounters. To absorb the heat being generated, rotors are created thicker than they need to be, which helps with heat dissipation. With the extra material added to the rotor, when the rotor becomes worn to the point that it is no long in a serviceable condition, it can be machined to be brought back to within specification. This is not the case for all rotors, however: some applications for certain vehicles require the rotor to be of a specific thickness so that performance metrics can be met. In these situations, replacement of the worn rotor is the only option. To determine whether this applies to the vehicle being worked on, verify it within the service literature. The material that the rotor is constructed from is important to the person designing the braking system since certain materials react differently when high heat is applied. The most common type of material is cast iron, which is relatively FIGURE 6-1  The carbon brake rotor is very similar in design to the cost-effective to produce, and it has good reliability. When cast iron version. The hub is attached differently and the material is different than iron. heat is applied to the cast iron rotor, it is able to withstand the constant thermal cycles to maintain performance of the rotor with the brake pad. The downside to using cast iron rotors on a vehicle is the increased weight that they brought to the braking system. Composite Heat Dissapation rotors were designed to combat this excess weight. This type of rotors has a stamped steel hub, which is then attached to the cast iron rotor surface. This allows for a 20% decrease in weight per rotor and maintains the reliability of the cast iron wear surface. Machining these types of rotors requires special adapters to support the rotor because the steel center is more flexible than its cast iron counterpart. When replacing these types of rotors, they must be replaced with the exact style of rotor; otherwise, a brake pull may result. Carbon ceramic brake rotors are becoming more ­common within the performance applications from original equipment manufacturers (OEMs) (FIGURE 6-2). This type of rotor is harder than its cast iron companion, which increases the life of the rotor. The carbon ceramic material that the rotor is made of has a higher working temperature than cast iron: FIGURE 6-2  The brake rotor is used to transfer the kinetic 700°F (371.11°C) for cast iron vs. 1,350°F (732.22°C) for ceramic. This energy of the vehicle to the brake pads, which changes the allows the rotor to last a long time without becoming out of round or energy into thermal energy, because of the dissipation of warped. The hindrance to using this type on vehicles is the initial cost kinetic energy, the vehicle slows down. This process creates a lot of heat inside the rotor/caliper assembly. of installation: carbon ceramic rotors cost thousands of dollars for a

6-1 Explain the purpose of machining a rotor.



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complete set. Along with the high initial cost, the lack of applications will limit their use to only those vehicles that have been designed to use them.

Looking Up Specifications on Rotors Most rotors have the minimum thickness specification stamped on their edge (FIGURE 6-3). Now sometimes those numbers are not legible for various reasons, at that point the technician must go into their information system to acquire the proper minimum machining specifications (FIGURE 6-4). Failure to keep the rotor thickness above the minimum specification could result in the rotor flying apart under a heavy breaking load. Along with possible failure, the rotor will become too thin and will warp more quickly than if it were still within specification. FIGURE 6-3  The edge of the brake rotor will show the minimum The technician must understand that a lot of vehicles (espe- machining specification. cially pickup trucks) have ­different regular production option (RPO) codes that allow them to be built with ­heavier-duty braking systems (FIGURE 6-5). This must be taken into account when looking up specifications for brake rotors because these heavier systems have thicker rotors, which have d ­ ifferent discard and minimum machining specifications than the lighter duty variants. Along with pickup trucks, performance vehicles have a lower-performance version that is also sold to the public. For example, the 2017 Chevy Camaro has six different options for braking c­ omponents. The RPOs are as follows: ■■

■■

■■ ■■

■■ ■■

J55—brakes, Brembo four-piston front, performance, four-wheel antilock, four-wheel disc J6G—brakes, Brembo four-piston front and rear, performance, four-wheel antilock, four-wheel disc J6H—brakes, Brembo six-piston front and four-piston rear, performance J6M—brakes, Brembo Red, six-piston front Monobloc calipers, four-piston rear calipers, two-piece rotors, performance 5LQ—LPO, six-piston front brake kit 5JL—LPO, four-piston front brake kit.

Determining what is to be worked on is the first step in determining whether the rotor has the available material to machine. To measure brake rotors, follow the steps in SKILL DRILL 6-1.

FIGURE 6-5  The RPO sticker identifies which options have been FIGURE 6-4  If the specifications on the rotor are not legible, they

must be looked up in the service information.

added to this vehicle so that technicians can order the correct pieces when replacing components.

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SKILL DRILL 6-1 Measuring Brake Rotors 1. Remove the brake rotor from the vehicle.

2. Clean the residue from inside of the brake rotor so that it is clean (using soap and water), and at the same time inspect the rotor for obvious damage.

3. Verify the minimum machining specification on the edge of the rotor. If it is illegible, use the service information to locate the minimum machining specification.

Continued



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4. Take the rotor micrometer and adjust it to the size that will fit the deepest groove on the rotor.

5. Take a reading in the spot that it is in and move the micrometer to two other points on the rotor to determine whether all the readings are the same.

6. Compare the readings with the service specifications, and machine or replace the rotor as necessary.

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▶▶ Brake 6-2 Interpret rotor deformation.

Rotor Deformation

Because the rotor surfaces are squeezed between two brake pads, any unevenness of the rotor surfaces causes the brakes to pulsate as the thicker and thinner portions pass between the brake pads. The usual complaint from the customer is that when they step on the brake pedal, the vehicle, pedal, or steering wheel begins shaking. The technician must be able to determine which axle’s rotors should be inspected for this situation. When the test drive of the vehicle is conducted, the technician must determine whether the vibration is felt in the steering wheel or in the seat. If the vibration is determined to be very apparent in the steering wheel, the technician should look at the front rotors for parallelism and runout. If the vibration is more concentrated in the seat area, the technician should look at the rear rotors for parallelism and runout. The rotor surfaces must be parallel to each other to avoid this situation. Rotors can fail in two ways: parallelism, which is also called thickness variation, and lateral runout (FIGURE 6-6).

Parallelism/Thickness Variation Parallelism is the measurement that compares multiple spots on the rotor to each other so that the technician can determine whether the rotor is the same thickness all the way around. A rotor that does not have the same thickness at any given point is the most common cause of pedal pulsation when the driver is applying the brakes at higher speeds. If the rotor’s thickness varies by as little as 0.0003'' (0.0076 mm), the rotor tends to push the brake pads outward at any high spots (FIGURE 6-7). Some common causes of a non-parallel rotor are soft spots in the rotor casting that wear at a different rate than the others, though the major cause of this issue is rust buildup from either using sub quality metal or not driving the vehicle enough to evaporate the moisture off of the rotor. Other issues affect the parallelism of a rotor, such as the quality of the wheel hub that the rotor is installed on. Having rust buildup, wheel bearing looseness, or any hub distortion will have a profound effect on the rotational trueness of the rotor. If the rotor is removed from the hub for any reason, the surface of the hub should be cleaned to remove any rust buildup, which is necessary to minimize the possibility of distortion once the rotor has been reinstalled. If the wheel bearing is loose or damaged or has change in some way, it must be corrected before replacing the brake rotor.

Lateral Runout Lateral runout, also called warpage, is the side-to-side movement of the rotor surfaces as the rotor turns. A warped rotor can be within specifications for parallelism but out of specification for lateral runout. Lateral runout tends to move the caliper pistons in the same Disc Thickness Variation (DTV)

0.897" (22.8mm)

Lateral Runout

0

10 20

10

0.905" (22.98mm) A

10

5 0

B

FIGURE 6-6  A. Example of thickness variation. B. Example of lateral runout.



Brake Rotor Deformation Disc thickness variation

Lack of parallelism or “lateral runout”

Thick Thin

Brake pads Brake pads FIGURE 6-7  A rotor that has a parallelism

issue will have an unevenness of the rotor surface that cannot be seen by the naked eye. Use measuring equipment to determine how far the rotor is out of specification in order to recommend service.

FIGURE 6-8  When checking runout on a vehicle, the technician must

spin the rotor that is attached to the hub so that they can see how much runout there is on the rotor.

direction as one another so that brake fluid is not pushed back to the master cylinder. However, the caliper tends to be moved side to side (FIGURE 6-8). This movement can cause the steering wheel to shimmy as the warped rotor follows the brake pads, if the lateral runout is greater than about 0.003'' (0.076 mm). The lateral runout issue could be the result of using a substandard rotor because of the cost of the rotor. The lower cost could mean the manufacturer simply took out some of the material, thus allowing the rotor to bend more easily where it is attached to the hub. This situation may be correctable if the rotor has enough material to allow the rotor to be machined into specification. Another issue that arises in a runout problem is the technician not torqueing the lug nuts to specification. Using an impact wrench without a torque stick can cause over-tightening of the lug nut, thus distorting the hub of the rotor. To prevent this type of situation, always use a torque wrench to install lug nuts.

Refinishing Rotors off the Vehicle Refinishing a rotor while it is off the vehicle is a bit different than on-vehicle refinishing. The major difference is in the setup. Hub-style and hubless-style rotors each require their own way of being mounted on the brake lathe. Most hub-style rotors use the bearing races to drive and center the rotor on the lathe spindle. Thus, bearing adapters of the proper size have to be selected and used. The spindle nut then clamps the rotor onto the spindle through these bearing adapters and races. Hubless rotors are centered by using a springloaded centering cone to align the rotor’s centering hole with the spindle. Clam shell clamps are then used on each side of the rotor to clamp it to the lathe spindle. Composite rotors use a special adapter that drives the rotor from the center hole while clamping it firmly between solid plates. To refinish a rotor while it is off the vehicle and measure final rotor thickness, follow the steps in SKILL DRILL 6-2.

Refinishing Rotors on the Vehicle Rotors need to be refinished when they have excessive runout, thickness variation, or grooving. A brake lathe refinishes the rotor surfaces by removing metal and truing the surfaces. If the grooving or surface defects are too great, the rotor may require the removal of too much metal to satisfactorily refinish the surfaces. The rotor thickness should always be remeasured once the refinishing is complete, to ensure it is above the manufacturer’s minimum thickness.

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SKILL DRILL 6-2 Refinishing Rotors Off Vehicle 1. Research the brake lathe manufacturer’s procedure for properly refinishing the rotor. Clean nicks, burrs, or debris from the mounting surfaces of the rotor, including the centering hole.

2. Mount the rotor. Check that the rotor is running true on the lathe.

3. Install the antichatter band or antichatter pucks onto the rotor.

4. Position the cutting head about one-quarter the way in from the outer diameter of the rotor. Turn on the lathe.

Continued



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183

5. Set the cutting bits to the proper cutting depth for machining the ridge.

6. By hand, move the cutting head outward toward the ridge. Slowly remove the ridge.

7. Once the ridge has been removed, run the cutting head all the way inside the inner face of the rotor.

8. Set the cutting bits to the proper cutting depth for machining the rotor face.

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9. Engage the automatic feed on “fast” cut, and watch for proper machining action. Some single-cut machines have only one cutting speed. If necessary, repeat this step until all damaged surface areas have been removed on both sides of the rotor.

10. If necessary, perform a finish cut on the rotor. This is usually done on “slow” speed.

11. Use sandpaper or a drill with a sanding pad to give the rotor faces a nondirectional finish.

12. Remeasure the rotor thickness to determine if the rotor is above minimum thickness specifications. Wash the machined rotor in a hot, soapy water solution or a parts washing cabinet to remove any metal particles, and then dry it.



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185

Rotors can be refinished while on the vehicle or off the vehicle. On-vehicle refinishing is preferred by most manufacturers (those who allow refinishing of their rotors) because it minimizes runout issues between the hub and rotor. Later-model, higher-performance, and luxury vehicles recommend replacing rotors instead of machining them. By doing this, the vehicle will always have the proper thickness of rotor, and they will thus perform as they were intended to by the OEM. Because the rotor is being machined as it is mounted on the vehicle, it is being refinished true to the hub and other brake components. This minimizes any lateral runout issues. Because the rotor is still on the vehicle, the brake lathe drives the hub that the rotor is mounted on, during the refinishing process. This requires using the proper adapter and mounting it to the lug studs. Following the directions for the brake lathe, install the adapter and use a torque wrench to torque the lug nuts to the proper ­specification indicated inside the machine’s service manual. Before refinishing can begin, verify that the mating surfaces of the hubless rotor and the hub are free from any dirt, rust, and debris. On hub-style rotors, the wheel bearings must be adjusted so that their end play is within specification. Just remember to readjust them after refinishing is complete. When making a cut, set the cutting bits to the proper cutting depth for machining. This is usually between 0.004'' (0.101 mm) and 0.015'' (0.381 mm). Too little removal tends to overheat the edge of the cutting bit. Too much can overload the lathe, as well as make the rotors much thinner. Many newer machines use an elliptical motion to give a nondirectional finish, so a ­finish cut or separate nondirectional finish is not needed on these machines. If using an older machine, the rotor may need a nondirectional finish by using sandpaper or a special tool. The nondirectional finish allows the pads to correctly burnish into the pad while minimizing the possibility of lines forming in the pad or the rotor. Lines or grooves in the rotor lead to the pads making noise when the brakes are applied. A nondirectional finish will lead to better pad wear, longer life, and a better customer experience. To refinish a hubless-style rotor while it is on the vehicle and to measure final rotor thickness, follow the steps in SKILL DRILL 6-3.

SKILL DRILL 6-3 Refinishing a Hubless-Style Rotor on Vehicle 1. Research the brake lathe manufacturer’s procedure for properly refinishing the rotor. This includes determining whether the rotor is machinable, whether run out is within specification, and whether the rotor needs to be replaced. Mount the on-car brake lathe to the rotor after cleaning the rust and dirt from between the rotor and hub or adjusting the wheel bearing so that there is no end play.

2. Perform the runout calibration on the brake lathe. Some brake lathes require manual compensation, whereas other machines can perform this automatically.

Continued

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3. Adjust the cutting bits, and cut off any lip at the edge of the rotor.

4. Make sure the cutting bits will not contact the rotor face, and move the cutting head toward the inner diameter of the rotor face. Set the cutting bits to the proper cutting depth for machining.

5. Install the antichatter device, if specified.

6. Engage the automatic feed and watch for proper machining action. If necessary, repeat this step until all damaged surface areas have been removed on both sides of the rotor.

Continued



Drums

187

7. If necessary, perform a finish cut on the rotor.

8. Remeasure the rotor thickness to determine whether the rotor is above minimum thickness specifications. Readjust the wheel bearings if necessary.

Cut Depths from Machine to Machine or Multipass Versus Single Pass This topic will depend on a lot of different variables to answer whether single pass or multipass is most appropriate when machining rotors. If too much of the material is taken off at one time, bit chatter may occur, causing the finish on the rotor to be poor. If too little of material is taken off each time, the technician will be turning the rotor for longer than they are being paid for. Determining the right amount of material to take off at once depends on a lot of variables, which are different for each application. The following are some of the variables that the technician must consider: ■■

■■

■■

The type of machine they are using: some are adjustable enough to make the single pass possible; some are not that flexible. The size and type of rotor: some rotors, such as composite ones, do not respond well to single pass machining; also, the bigger the rotor, the harder it is to successfully turn in one pass. The amount of material left to machine off of the rotor: the slimmer the margin to getting below specification, the more attention must be paid to the machining process.

▶▶ Drums Brake drums are machined to a specific diameter by the manufacturer, which is called its standard diameter. Manufacturers specify the maximum allowable inside diameter that a brake drum can be worn or machined to and usually stamp or cast that

6-3 Explain the purpose of machining a drum.

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Chapter 6  Refinishing Brake Rotors and Drums

FIGURE 6-9  The minimum machining specification is cast into the

outside of the drum and should be referred to before and after machining the drum, to make sure it is still in specification.

specification on the outside of the brake drum (FIGURE 6-9). This specification is commonly 0.060'' (1.524 mm) over the standard diameter on many brake drums, but it can be as low as 0.030'' (0.762 mm) over standard or as high as 0.090'' (2.286 mm) over standard on some passenger vehicles. Always check the manufacturer’s specifications. Remember that taking 0.015'' (0.381 mm) off of the brake drum surface also removes 0.015'' (0.381 mm) from the other side, for a total of 0.030'' (0.762 mm). Brake drums that are heavily grooved or out of round probably need to be replaced. Even if they are not grooved, they have to be measured because they may have been refinished one or more times before, making them oversized. Never put an oversized brake drum back in service, because it does not have as much material to absorb brake heat, and it also is not as strong as a drum that is within specifications. To measure brake drums, follow the steps in SKILL DRILL 6-4.

SKILL DRILL 6-4 Measuring Brake Drums 1. Remove the brake drum from the vehicle.

2. Clean the residue from inside of the brake drum so that it is clean, and at the same time inspect the drum for obvious damage, such as cracks, discoloration, or trueness to the center line.

Continued



Drums

189

3. Verify the minimum machining specification on the outside of the drum. If it is illegible, use the service information to locate the minimum machining specification.

4. Take the drum gauge or micrometer and adjust it to the size that will fit the drum.

5. Take a reading in the spot that it is in, and move the micrometer to two other points within the drum to determine whether the readings are the same.

6. Compare the readings with the service specifications, and machine or replace the drum as necessary.

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Chapter 6  Refinishing Brake Rotors and Drums

Refinishing Brake Drums

▶▶TECHNICIAN TIP Resurfacing or machining the brake drum is an important task that requires precise measurements and the use of a brake lathe. Care must be taken while machining a drum to avoid an inadvertent adjustment that renders it scrap metal with only a quarter turn of the hand wheel.

Brake drums must be refinished with a brake lathe when they have excessive grooving or are out of round. A brake lathe refinishes the drum friction surface by removing metal and making it perfectly round with the proper finish. If the grooving or surface defects are too great, the drum may require the removal of too much metal to satisfactorily refinish the surface. The drum diameter should always be remeasured once the refinishing is complete, to ensure that it is under the manufacturer’s maximum diameter. Never put a brake drum that is over the maximum size back in service, because it does not have the ability to absorb as much heat and will experience brake fade much more quickly. Hub-style and hubless-style drums are mounted on the brake lathe differently. Most hub-style drums use the bearing races to drive and center the drum on the lathe spindle. Bearing adapters of the proper size need to be selected and used. The spindle nut then clamps the drum onto the spindle through these bearing adapters and races. Hubless drums can be mounted in two ways: using a composite rotor or clamshell or using centering cone. The composite rotor adapter mounts in a similar manner as on a disc brake rotor. The clamshell system uses a spring-loaded centering cone to align the drum’s centering hole with the spindle and clamshell clamps to clamp it to the lathe spindle. The following Skill Drill describes how to follow the composite rotor adapter method. To refinish a brake drum and measure final drum diameter, follow SKILL DRILL 6-5.

SKILL DRILL 6-5 Refinishing a Brake Drum 1. Clean any nicks, burrs, or debris from the mounting surfaces of the drum—including the centering hole, if used.

2. Mount the drum on the brake lathe.

Continued



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191

3. Visually check that the drum is running true on the lathe and is not wobbling. If it wobbles, turn off the lathe and recheck for any condition that could cause it to wobble.

4. Install the antichatter band on the drum. It will prevent the drum from vibrating during the machining process, because vibration would create a rough finish and dull the cutting bit.

5. Set the position of the cutting tool such that the brake drum is close to the brake lathe when the cutting bit is in the far corner of the drum.

6. Make sure the cutting bits will not contact the face of the drum, and move the cutting head about 0.5'' (1.27 mm) in from the outside of the drum.

Continued

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Chapter 6  Refinishing Brake Rotors and Drums

7. Turn on the brake lathe and set the depth of the cutting tool so that it just touches the surface of the drum. Rotate the brake lathe’s handwheel so that the drum moves outward and the cutting bit contacts the ridge. Keep turning slowly to remove the ridge.

8. Once the ridge has been removed, run the drum all the way in so that the cutting bit is in the inner corner of the drum. Set the cutting bit to the proper depth for machining the surface of the drum and lock it in place.

9. Engage the automatic spindle feed, and then set it to the proper speed, lock it in place, and watch for proper machining action. Some single-cut machines have only one speed.

10. If necessary, repeat this step until the worn surface areas have been removed all the way around the surface of the drum. If the brake lathe is not a single-cut machine, perform a finish cut on the drum. This cut is usually done on a slower spindle feed speed.

Continued



Wrap-Up

193

11. Move the drum well away from the cutting bit, and use sandpaper to give the drum surface a nondirectional finish.

12. Remeasure the drum diameter to determine whether the drum is above the maximum-diameter specifications; if so, discard the drum.

▶▶TECHNICIAN TIP Vehicle Brake Lathes and Matching the Rotor to the Hub On the car, brake lathes may be the best way to get a near-perfect finish on a rotor. These can include getting a rotor cut in a single cutting pass, applying nondirectional finishes, and also appropriately matching the rotor to the hub as it relates to runout. The two common name brands that are on the market are Pro-Cut and Hunter Engineering. Most vehicle manufacturers, especially high-end ones, require on-car lathes in their dealerships due to the accuracy and ability to match the rotors. The runout tolerances on performance vehicles are so precise now that it is difficult to meet them by using a traditional bench lathe.

▶▶Wrap-Up Ready for Review ▶▶ ▶▶

▶▶

Brake rotors must be within the specification before and after they are machined. Verifying that the rotor is machinable before it is removed will allow the technician to make an informed decision on whether they can reuse or replace the rotor. Brake rotors need to be refinished if they have excessive grooving or are heavily rusted.

▶▶ ▶▶

▶▶

The lateral runout on the rotor must be verified once it has been installed on the vehicle. Brake drums should be inspected visually for any damage and measured with a drum micrometer to verify that they are smaller than the specified maximum diameter and not warped. Brake drums need to be refinished when they have excessive grooving or are out of round. They can be refinished by using a brake lathe.

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Chapter 6  Refinishing Brake Rotors and Drums

Key Terms bearing races  Races that the wheel bearings ride in and that are used to mount the hub-style rotor onto the brake lathe for turning. lateral runout, also called warpage  The side-to-side movement of the rotor surfaces as the rotor turns. machining  A process in which the brake rotor or drum is cut by a steel cutter to remove the upper layers of metal to restore the trueness of the surface that wears on the shoe or pad. parallelism  The measurement that compares multiple spots on the rotor to each other so that the technician can determine whether the rotor is the same thickness all the way around.

Review Questions 1. Choose the correct statement: a. Rotors can have metal added to them so that they can be balanced. b. Drums must be within minimum machining specification before starting the machining process. c. It’s OK for drums to have grooves in them. d. Modifying the braking system is the proper way to repair a brake problem. 2. If the lateral runout is too large, the driver might complain of a steering wheel vibration when the brakes are applied. True or false? a. True. b. False. 3. Which of the following must not be present on a rotor if it is going to be machined to bring it back to service specifications? a. Cracks. b. Heat spots. c. Rust. d. Grooves. 4. The on-vehicle brake lathe will help with improving which measurement? a. Rotor thickness. b. Lateral runout. c. Drum thickness. d. None of the above. 5. Thick rust ridges on the brake rotors can ____________. a. cause a vibration in the steering wheel b. cause the rotor to become balanced and true c. extend the life of the brake pads d. create a soft pedal feeling 6. What is the proper solution to clean brake rotors before measuring and machining? a. Caustic cleaner. b. Brake fluid. c. Soap and water. d. Penetrating oil. 7. Brake drums can have a rust ridge buildup on their lip. How should this ridge be removed? a. With brake cleaner. b. On the brake lathe. c. It does not need to be removed. d. None of the above.

8. If a rotor is under specification after machining it, __________________. a. allow the rotor to be used in the brake repair b. replace the rotor before it is installed c. it’s not a problem, because the owner of the vehicle will never know d. add metal to the rotor to make it in specification 9. The drum on the pickup truck has been hit with something, and most of the cooling fins have been broken off. What should be done? a. Nothing, because the inside is still good. b. Replace it, because the cooling function of the drum is no longer good. c. Add fins to the drum by using a welder. d. Grind the outside smooth, to enhance cooling. 10. A hubless front-vented rotor can be replaced by which of the following? a. A hubless vented rotor. b. A solid rotor. c. A drum. d. A vented rotor with a hub integrated into it.

ASE Technician A/Technician B Style Questions 1. Technician A says that most brake drums are designed to be machined if minor surface issues are present. Technician B says that brake drums can be reused if they are machined larger than specifications, as long as the surface is smooth. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that most brake drums are designed to encase the brake shoes and components. Technician B says that brake drums cannot be reused, because they are a onetime-use-only part. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that brake rotors have the minimum machining specification cast into the edge of the rotor. Technician B says that brake rotors can be machined on the vehicle. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that wheel bearings will have to be repacked on rotors that have hubs integrated into the rotor. Technician B says that brake rotors will have to be replaced if they are machined thinner than specifications. Who is correct? a. Technician A b. Technician B



c. Both A and B d. Neither A nor B 5. Technician A says that brake drums are used in the rear of most vehicles because the rear brakes do the least amount of braking. Technician B says parallelism is a minor specification and shouldn’t be worried about. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that hard spots in the rotor should be inspected, and if they cannot be machined out, then the rotor must be replaced. Technician B says the drums can cause the rotors to become overheated and fail. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says using an on-car brake lathe can help with correcting the lateral runout of the wheel bearing. Technician B says a worn wheel bearing could cause a steering wheel vibration like a warped rotor. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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8. Technician A says leaving deep grooves in the rotor will be OK because the brake pad will not touch them. Technician B says making sure that the finish on the machined rotor has a nondirectional surface will ensure proper brake pad break in. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says drums can be turned on the vehicle just like rotors. Technician B says that using the wrong rotor for the application could cause problems with the braking system. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says measuring the brake rotor should be done with an outside rotor micrometer. Technician B says using a tape measure to measure the rotor will get the technician a close enough measurement to start the machining process and micrometers are needed only for the final measurement. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

CHAPTER 7

Parking Brakes Learning Objectives ■■

7-1 Explain how parking brake systems operate.

■■

7-2 Develop diagnostic procedures to repair parking brake failures.

You Are the Automotive Technician A customer brings in a 2015 Chevy Impala with an electronic parking brake system. The customer reports that the parking brake system is inoperative and that the service parking brake message is coming across the driver’s information center. What should be the first step you as the technician take?

1. Check the brake fluid. 2. Check the parking brake shoes/pads. 3. Scan the parking brake module for faults.





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Chapter 7  Parking Brakes

▶▶ Introduction The parking brake is a secondary brake used for parking and in an emergency situation. If the main hydraulic brakes fail, the parking brake system will help the operator safely slow the vehicle down. While this is not the main feature of the parking brake system, it can be used in an emergency to help control the vehicle. There are many kinds of parking brakes that are different from the ones integrated with the rear calipers: top-hat style brakes, drum-mounted parking brakes, electric-over-hydraulic brakes, driveline brakes, and electric brakes. The application will depend on the vehicle that it is installed on since some vehicles have more space than others do.

▶▶ Parking 7-1 Explain how parking brake systems operate.

Brakes

Parking brakes are designed to hold the vehicle stationary when parked. Manufacturers are required to design the vehicle so that the parking brake will hold the vehicle for a given amount of time on a specified grade in both forward and reverse directions. The parking brake must be separately activated from the service brakes, and the driver must be able to latch it into the applied position. Parking brakes can be foot operated or hand operated. Because disc brakes require higher applied forces to operate, they are a bit more difficult to use as parking brakes. However, manufacturers have overcome this challenge in a couple of ways, which are discussed in this chapter.

Types of Parking Brakes Currently, most parking brakes are mechanically applied by using a cable and ratcheting lever assembly (FIGURE 7-1). Parking brakes on disc brake units come primarily in two types: an integrated parking brake caliper and the top-hat drum style. Alternatively, electric parking brakes are being used on some vehicles. The electric motor can either pull on a conventional parking brake cable or be mounted on the caliper and directly drive the caliper piston to apply force to the brake pads.

Conventional Drum Brake Parking Brake Feature The conventional drum brakes that are used for parking brakes have a lever inside the drum that moves the front shoe into the drum when the parking brake is applied (­ FIGURE 7-2). As this happens, when the wheel starts to move, the rear shoe is drawn into the drum, which then increases the friction area that is acting on the drum to keep it from moving. This function is the simplest version of a parking brake that is found on most conventional vehicles today that have drum brakes in the rear. This type of system uses cables from the parking brake lever to both rear drums. These cables may become corroded due to lack of use, and then once they are used, they become stuck, not allowing the Drum Brakes brake to retract. When a customer comes in to Brake report a problem with a parking brake on their Booster vehicle and that vehicle is equipped with this Master type of parking brake system, the technician Cylinder must start with the basics of the system, verifying that the cables are loose and movable. Once that is verified, move into the drum to verify Wheel components inside the drum are in operating Cylinder order. Working through the basics of the sysDisc Brakes tem is how the technician is able to diagnosis issues with the system. To get the drum brake parking brake feature to operate on a vehicle, a lot of components Brake Shoes must be working correctly. Inspecting the condition of the return springs, parking brake strut, FIGURE 7-1  A basic cable-actuated parking brake system.



Parking Brakes

Primary Shoe Return Spring

Backing Plate Secondary Shoe Return Spring Wheel Cylinder Assembly

Primary Shoe

Cable Guide

Parking Brake Strut Shoe Hold-Down Parts Parking Brake Cable Adjuster Lever Spring

Primary Shoe Return Spring

Parking Brake Lever Adjusting Cable Secondary Shoe Adjuster Lever Adjuster Assembly

Backing Plate Secondary Shoe Return Spring Wheel Cylinder Assembly

Primary Shoe

Cable Guide

Parking Brake Strut Shoe Hold-Down Parts Parking Brake Cable Adjuster Lever Spring

Parking Brake Lever Adjusting Cable Secondary Shoe Adjuster Lever Adjuster Assembly

FIGURE 7-2  The brake shoes on the top are not applied, the bottom are applied when the parking brake is actuated.

See how the shoes move out into the drums to stop the rotation of the wheel.

parking brake lever, adjusting screw, and drum internal size. If any one of these components is not operating properly, the parking brake will not operate correctly. Understanding how these pieces work together will help with diagnosing what is happening with the parking brake system.

Integrated Mechanical Parking Brake Calipers The integrated parking brake mechanically forces the disc brake piston outward, forcing the brake pads to clamp the rotor when the parking brake is applied (FIGURE 7-3). A lever on the back side of the caliper is pulled by the parking brake cable. The lever converts that motion to rotary motion on a shaft that enters the rear of the caliper cylinder. The shaft uses a seal to prevent fluid leakage from the bore. The shaft has a coarse thread machined into

199

200

Chapter 7  Parking Brakes Locating notch (prevents piston rotating as the hand brake is applied)

Disc Rotor Brake Pads

Piston with oneway clutch

Hand Brake Apply Screw

Hand Brake Cable

Hand brake lever

FIGURE 7-3  Integrated parking brake operation.

FIGURE 7-4  A top-hat rotor and drum parking brake assembly.

Caliper Body

it, which threads into a nut assembly inside the caliper ­piston. As the shaft is turned by the parking brake lever, the nut causes the piston to be forced outward, applying the brakes. Releasing the parking brake cable allows a spring to unwind the shaft and release the ­pressure on the brake pads. The use of the integrated parking brake within the rear c­ aliper is one of the first ways of integrating the parking brake when a vehicle has a rear caliper/rotor setup. This complicates the rear caliper and allows potential corrosion to intrude inside the caliper, causing more issues as time goes on. The drum brake parking brake ­system and the integrated mechanical parking brake calipers have the same issues with corrosion inside the cables.

Top-Hat-Design Parking Brake The top-hat design gets its name from the shape of the rotor. The rotor has a deeper offset than normal, giving the appearance of a top hat. The offset portion allows room for a drum surface within the center of the rotor (FIGURE 7-4). Drum brake shoes are mechanically forced outward into contact with the inside of the brake drum, which locks the wheel. Releasing the parking brake allows the springs to retract the brake shoes from contact with the drums. This design of parking brake is like a drum brake and rotor combined so that the s­ implicity of the drum brake as a parking brake can be used on a rotor type of braking system. This type of parking brake can have the problem of the drum trapping all of the rust and corrosion, and it will not allow it to dissipate so that the brake shoes can operate properly. With the nonuse of the parking brake, the buildup of material in the drum can cause sticking when used. Along with corrosion buildup, the shoes can disintegrate over time with nonuse, which will allow the friction material to become lodged between the drum and the shoe backing. This can cause the wheel to lock up and the customer to report that the wheel is getting overheated.

Electric Parking Brake

The electric parking brake uses an electric motor to apply the disc brake assemblies. The cable style uses an electric motor to pull stanBrake Pads dard parking brake cables, which apply standard integrated mechanical parking brake calipers. The electrically integrated caliper style uses an electric motor mounted on the caliper to directly apply the Wedge brakes (FIGURE 7-5). Pushing a parking brake button on the dash Self Adjuster causes the motor to either tension the cable or directly apply the Disc parking brake. Electric parking brakes can also be integrated with the Park Motor controller area network bus (CANbus) system to provide additional features beyond just holding the vehicle when it is parked. It can be FIGURE 7-5  An electrically integrated parking brake caliper has used to automatically hold the vehicle while it is stopped on a hill, a built-in motor that mechanically applies the brake pads. to prevent it from rolling backward or forward. It also can be automatically released by the vehicle’s electronic control module (ECM) when the throttle is applied to start moving again. It also may work with the vehicle’s proximity detector when backing up. If the system detects the vehicle getting too close to an object, the ECM can apply the electric parking brake to stop the vehicle and prevent it from striking the object. Starting in the early 2000s the appearance of electronic parking brakes appeared on some European vehicles, which began to simplify the parking brake systems so that there



Diagnosis and Service

201

were fewer parts that could fail. Along with fewer parts, the control over when the parking brake would be employed would now be controlled by a control module networked with the other modules in the vehicle. By networking the parking brake module, a technician with a scan tool can diagnosis various electrical problems that can come up within the system. The technician must think ahead to determine whether their organization has the capability to repair these types of vehicles. As the technology advances, the need for understanding of how these advanced parking brake systems operate is becoming necessary for servicing and repairing these advanced systems. Referring to the manufacture’s service information should be the first step in starting to diagnosis a fault in the electric parking braking system.

Driveline Parking Brakes The driveline parking brake is usually situated at the front of the rear drive shaft attached to the rear of the transmission or transfer case. The brake is a simple cable-actuated drum brake that has the drum attached to the drive shaft and the backing plate attached to the transmission/transfer case (FIGURE 7-6). This type of brake will limit the movement of the mechanical parts within the driveline so that the vehicle will not move and will be held by some of the most robust components on the vehicle. The application for this brake is one where the load that is being carried by the truck would overcome a normal parking pawl within the transmission. The components that are designed to move the load are better suited to holding the load when the vehicle is parked. This component is usually not used on vehicles that are under 2 tons, which means that most technicians will not see this type of system. Awareness of these types of systems will allow the technician to apply this knowledge to other aspects of vehicle repair.

FIGURE 7-6  The driveline brake attaches to the drive shaft and the

pads or shoes are attached to the transmission. Stopping the vehicle movement this way puts less load on the transmission parking pawl.

Electric Over Hydraulic

The electrohydraulic parking brake system uses an electric motor FIGURE 7-7  The hydraulic pressure unit can control the electrohydraulic parking brake through the same lines that control the to create pressure to apply the brake caliper to the rotor. These foundation brakes. types of systems are usually used as part of a larger ­electronic stability program (ESP) on the vehicle (FIGURE 7-7). Using the hydraulic portion of the brake system as the parking brake, it eliminates the possible problems inherent in cable-driven parking brake systems. Integrating the parking brake with an electronic stability system allows for precise control on when to apply the brake. When the electronic stability program decides there is a call for the parking brake, it uses the hydraulic pressure unit to create pressure and apply the calipers. Once the brake is applied, the unit holds the pressure until the call for release of the caliper and the solenoid within the unit releases the pressure. This is sometimes integrated inside the electrohydraulic unit of the antilock brake system (ABS).

▶▶ Diagnosis

and Service

When diagnosing a parking brake issue, the technician must look at the complete system so that they get a clear understanding of what part of the system failed. As with most parking brake systems, the major issue is the lack of use, and when they do get used, the cables get stuck within their sheaths because of the rust buildup (FIGURE 7-8).

7-2 Develop diagnostic procedures to repair parking brake failures.

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Chapter 7  Parking Brakes

Looking for obvious signs of damage from materials on the road, improper installation or repair, and age should allow the technician to make an educated decision. Stretching of the parking brake cables is something that is not easily diagnosed. There is no specification for the amount of stretch of the cables, but a good way to diagnosis this condition is to verify that the rest of the system is operating correctly and then determine whether the tension on the cables can support the actuation of the brakes far enough to stop the vehicle. The customer is the main source of knowing what is failing inside the vehicle’s parking brake system, because they can point to what it is not doing. Once what the vehicle is not doing has been determined, the technician can start to determine which part of the system is not working correctly, so that it can be corrected. For example, the stretching of parking brake cables would probably lead to a complaint if the vehicle ended up moving slowly or creeping once the parking brake is FIGURE 7-8  When the parking brake cable sticks, the cable will hang engaged. This would lead the technician to determine that the lower than where it should be and will not move freely in and out of brake is not being applied enough to push the pads or shoes the sheath. into the drum or rotor to effectively stop the vehicle. From here, the technician would determine whether the parking brake cables appear to be damaged or stretched, in which case replacement is the only option. The driveline parking brakes are similar to the wheel parking brakes, except the brake assembly is on the drive shaft. This poses its own set of problems, like a U-joint failure causing issues with the brake assembly. Or a major transmission/transfer case leak in the rear will soak the brake in transmission fluid, causing it to fail. These types of brakes can be either manually actuated or electronic, depending on the application. Checking the same basic functions as other parking brake applications will yield the failure and ultimately what needs to be repaired. The electronic parking brake does get a little more complicated than the older traditional parking brakes. The inclusion of a control module into the operation of the brake can cause some electrical problems that must be addressed if there is a failure in the system. Using a scan tool to diagnosis the parking brake system is common in the electronic systems (FIGURE 7-9). When a component in the system fails, a code is generated, which will lead the technician in the right direction before they even pick up a tool. A common issue on the cabled and cable less systems, like the electronic system, is a lack of use causing the components to corrode and eventually freeze up within the housings they are in. A careful inspection of the components should yield any obvious issues that could be resolved by simply cleaning up the corrosion.

FIGURE 7-9  Using a scan

tool to diagnosis a parking brake failure is the first step in the procedure for repairing an electric parking brake system.



Wrap-Up

203

Integrating ECMs inside the parking brake system may also cause issues with the programming that is used to control how, when, and the duration of the parking event in which the brake is applied. Errors inside the program can cause unwanted engagement, no engagement, or delayed engagement. Updates for the software are uploaded the same way that the powertrain control module (PCM) is updated through the data link connector (DLC) and manufacturer-specific online software program. Checking the technical service bulletins (TSBs) first should be at the top of the diagnostic procedures for these types of systems.

Inspecting and Maintaining Parking Brakes Basic inspection of the parking brake system includes checking for any broken pieces, corroded cabling, and leaking brake components as well as inspecting the general brake ­system. Actuate the parking brake should only if the customer wants to get it working again. If it’s actuated and there is a problem, the customer may have to pay for something that they did not want to be fixed. Looking for TSBs on the system will help the technician expedite the repair process since they are more homed in on the particular issues with that vehicle. Maintaining the parking brake requires using it from time to time over its life span so that the cables and components do not become corroded in their housing, because such corrosion would cause it to fail. This must be done by the customer since the technician does not see the vehicle often enough to follow through on this type of policy. Customer awareness is vital to ensuring that the customer takes full advantage of as much longevity as possible from their parking brake components.

▶▶Wrap-Up Ready for Review ▶▶ ▶▶ ▶▶ ▶▶ ▶▶ ▶▶ ▶▶ ▶▶

Parking brakes are used to help keep the vehicle stationary when parked. Rear brakes are normally used as the parking brake and are a separate system from the main braking system. Some parking brakes use integrated mechanical parking brake calipers. To save space, rear disc brake rotors operate as an internal parking brake and external disc brake. Electric parking brakes could either pull a cable or directly actuate the caliper. Electric-over-hydraulic parking brakes generate their own hydraulic pressure when the parking brake is activated. In this chapter, technicians learned about the basic inspection of broken, loose, or frozen components. To properly maintain a parking brake, customers must use it on a regular basis.

Key Terms driveline parking brake  A driveline parking brake system is one that has a drum or disc brake assembly on the drive shaft or the pinion yolk on the rear end, where the driver can operate the brake to hold the vehicle stationary. electric parking brake  An electric parking brake is controlled by a switch, a module, and actuators that apply and release the brake based on control from the module. electronic control module (ECM)  A small computer that performs a multitude of operations to control its system. These operations include turning components on and off, making

calculations on the fly, adjusting the components in the vehicle, responding to the needs of the operator. electronic stability program (ESP)  A program run in an ECM type of component, which has been developed to operate under specific conditions. This type of program helps the operator to maintain control over the vehicle.

Review Questions 1. The customer states that the parking brake works only some of the times when they pull the level in their console. What could be the cause? a. The parking brake cables are stretched. b. The electronic actuators are not working. c. The vehicle is still moving. d. The ECM will not allow it to apply. 2. The parking brake is eliminated from the vehicle because the customer doesn’t want to fix it. What can the technician do to limit their liability? a. Nothing. b. Have the customer sign the refuse-to-repair section on the repair order and document the failure to fix the issue. c. Fix it for free. d. Do not take the car in for repair of anything else. 3. The wheel cylinder is leaking on a drum brake setup, and the parking brake will not hold the vehicle stationary. What could be the cause? a. The parking brake cables are stretched. b. The brake fluid is causing the shoes not to grab the drum.

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Chapter 7  Parking Brakes

c. The tires are slipping. d. The customer is not pulling the parking brake lever up enough. 4. The parking brake light is flashing on a vehicle when the parking brake button on the console is pushed. What could be the cause? a. The parking brake ECM detects a fault. b. The parking brake lever is stuck. c. The disc brakes in the front are not allowing the parking brake to operate. d. The operator is not pushing the button hard enough. 5. The top-hat parking brake design for rear brakes is not stock on the vehicle, but the customer wants it put on. What should be done? a. Nothing, and inform the customer that doing so not in accordance with factory specifications. b. Do the work and hope it works. c. Just put it on; it’s only the parking brake. d. Give it to a new technician to find out whether they can figure it out. 6. When completing a brake repair, the parking brake must always be actuated. True or false? a. True. b. False. 7. A vehicle with parking brake–integrated rear calipers has one caliper sticking, causing the wheel to lock up. How should this problem be fixed? a. Nothing. b. Re-bleed the system. c. Readjust the brake line. d. Replace the caliper. 8. A vehicle with a driveline parking brake mounted on the transmission has a transmission rear seal leak and the brake will not hold the vehicle still on a hill. What could be the cause? a. The brake is not designed for the application. b. The transmission fluid is causing the friction surface to slip on the drum surface. c. The transmission is broke. d. The driveshaft is broken. 9. When the technician checks the electric parking brake with the scan tool, they find that the code in the module is an internal fault. What should the technician do to fix this vehicle? a. Replace the brake pads. b. Check the brake fluid. c. Go through the diagnostic procedures located in the service manual. d. Replace the front calipers. 10. The electric parking brake caliper could have an electric motor attached to it, or it could be attached to cables that actuate the calipers. True or false? a. True. b. False.

ASE Technician A/Technician B Style Questions 1. Technician A says that the parking brake pedal is not actuating the parking brakes enough to keep the vehicle from rolling, so it needs replaced. Technician B says that the foundation brakes on the rear of the vehicle are worn out and will therefore affect the parking brake performance. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 2. Technician A says when working on a parking brake, the technician should always verify that the brake works once the work is complete. Technician B says that when the technician does an oil change, they should determine whether the parking brake works. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 3. Technician A says that the parking braking system should not be modified. Technician B says that diagnosing an electric parking brake should be done by using only a test light. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 4. Technician A states that if the lever for the parking brake is not pulled up enough, the brake may not apply to hold the vehicle. Technician B says that the parking brake could be affected by the front brake calipers not applying correctly. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 5. Technician A says that the electric-over-hydraulic parking brake system uses compressed air for pressure. Technician B says that a leaking wheel cylinder could hinder the ­performance of the parking brake. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 6. Technician A says that the electric parking brake module is on the vehicle network and may be controlled by another computer on the network. Technician B says disabling the parking brake is the proper thing to do if that’s what the customer wants. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B



7. Technician A says parking brake cables should be used periodically to keep the moisture out of the housings. Technician B says every brake change should have the parking brake cables replaced. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 8. Technician A says that a technician should inspect the parking brake system before doing any brake work to a vehicle. Technician B says that most parking brake systems are not used on vehicles that have an automatic transmission. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B

Wrap-Up

205

9. Technician A says every caliper on the rear of a vehicle has a parking brake built into it. Technician B says that a technician should always replace the parking brake components that have failed, with quality parts that meet the original equipment manufacturer’s (OEM) specifications. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 10. Technician A says the top-hat parking brake design is a combination type of brake layout that has both a drum and a rotor. Technician B says parking brakes can be used in an emergency situation, such as when the regular brakes fail. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B

CHAPTER 8

Power-Assist Systems Learning Objectives ■■ ■■

8-1 Explain vacuum booster operation. 8-2 Summarize the operation of hydroboost power-assist systems.

■■

8-3 Diagnose a power-assist system failure.

You Are the Automotive Technician The customer comes in with a 2013 Chevy Silverado 2500HD with a Duramax engine in it.They report that the truck does not seem to have the power assist that it normally has, which is causing the driver to barely stop the vehicle. Along with the braking issue, the driver reports needing to increase their steering effort at low speeds. What should the technician check first?

1. The service brakes. 2. The serpentine belt tension and condition. 3. The power steering pump operation.





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Chapter 8  Power-Assist Systems

▶▶ Introduction The need for increased braking pressure was apparent with the introduction of the disc brakes to the automotive world. Previously, the drum brakes on vehicles would ­naturally multiply the force that was applied to brakes since the servo action would help with the application of the brake shoe. The limitations of drum brakes resulted in the design of disc brakes. This progression in design is a direct result of the increasing r­equirements of safety and the National Highway Traffic Safety Administration (NHTSA). The NHTSA regulates the stopping distance of vehicles that operate on roads in the United States. As the ­distance the ­vehicle needed to stop within decreased, the technology to get the vehicle to stop needed to be f­urther developed. With these technologies, there are tradeoffs that must be dealt with as they become apparent, which gave rise to power assist.

▶▶ Vacuum

Booster Operation

As the need for power assist rose, the design for a simple, compact component that would meet the needs of the increased pressure to operate the disc brake system was developed. The vacuum brake booster uses the engine vacbrake booster chamber brake fluid uum created by the operation of the engine to help with in reservoir boosting the force applied to the master cylinder. The term poppet valve master cap spring “vacuum” here refers to any pressure that is lower than atmocylinder reservoir spheric pressure. A  ­vacuum booster uses the difference in poppet valve seat atmospheric pressure and the vacuum created by the engine master (FIGURE 8-1). cylinder A vacuum booster uses a diaphragm to isolate ­atmospheric pressure from vacuum pressure. Vacuum or negative pressure is created by the operation of the engine. The negative pressure builds up on the engine side of the booster. The other side secondary primary filter piston piston of the diaphragm fills with atmospheric pressure when the hydraulic brake pedal is depressed. Once the brake pedal is depressed, pushrod primary secondary the atmospheric pressure contained inside the vehicle, which check brake brake valve is positive pressure, floods into the booster, creating a positive circuit circuit pressure on the opposite side of the ­diaphragm (FIGURE 8-2). to engine Once the pressure has built up to the tipping point, it starts to vacuum source push the ­diaphragm toward the negative pressure side, causdiaphragm ing the brake push rod located inside the booster to push the FIGURE 8-1  A vacuum booster at rest with no pressure on it from piston in the master cylinder with the force of the driver’s foot the driver. 8-1 Explain vacuum booster operation.

brake booster chamber contains constant vacuum

brake fluid in reservoir master cylinder reservoir

cap

piston return spring resists vacuum

poppet valve (shown open) allows air into pressure chamber

poppet valve seat follows poppet and closes automatically until brake pedal is moved farther

master cylinder

FIGURE 8-2  A vacuum

booster actuated with the driver pushing the brake pedal, which moves the valve within the booster, allowing atmospheric pressure to build on the opposite side of diaphragm.

secondary piston secondary brake circuit to engine vacuum source

primary piston hydraulic pushrod primary check brake valve circuit

filter

air enters through filter

brake pedal

piston is pushed brake booster pressure by atmospheric pressure, adding force chamber contains atmospheric diaphragm ensures pressure only when poppet valve is to pedal pressure an airtight seal opened by brake pedal movement



Vacuum Booster Operation

on the pedal and the force of the diaphragm. This is how to gain the mechanical advantage to allow the driver to stop the v­ ehicle with less effort. To help with engine shut off and having power brakes for a few pumps after the engine stops running, a check valve is put on the brake booster so that the negative pressure/ vacuum will not be expended once it is no longer supplied by the engine ­(FIGURE 8-3). This feature will allow the driver to operate the braking system with assist for a couple of applications.

Dual (Tandem) Diaphragm Booster The normal way to increase the force that a vacuum booster made was to make the booster larger so that the diaphragm had a larger surface area. As vehicles became more aerodynamic and more compact, this theory no longer worked, because of the l­imited space underneath the hood. To increase the force made by the vacuum booster, two smaller diaphragms were placed next to each other so that the total area of the diaphragms was bigger than a larger single-diaphragm booster (FIGURE 8-4). When the tandem diaphragm booster is actuated, the air valve assembly allows the atmospheric pressure to flow into each diaphragm’s cavity to allow the movement of the master cylinder push rod toward the master cylinder (FIGURE 8-5).

209

▶▶TECHNICIAN TIP The vacuum booster will not cause a low pedal on the braking system. Some people blame the power assist for their brake pedal not being where it is supposed to be, when in reality it is the hydraulic system that has an issue. When diagnosing a low brake pedal concern, make sure to verify the reported problem and inspect the foundation brakes before replacing any components.

Vacuum Booster Pushrod Adjustment The vacuum booster’s pushrod is sometimes an adjustable component that must be inspected and adjusted, if it is adjustable, before it is installed in the vehicle (FIGURE 8-6). When replacing the vacuum booster, the technician must make a measurement of how much the pushrod is protruding out of the booster so that they can adjust the new booster to be the same (FIGURE 8-7). Adjusting the push rod is usually as simple as holding the push rod with a pair of pliers and moving the self-locking nut into the required position. If this measurement is not checked and adjusted, the pushrod may push the piston too far into the master cylinder, causing it to bottom out and eventually break. If the push rod is too short after adjustment,

FIGURE 8-3  The check valve allows the booster to retain vacuum

pressure once the engine is off and will also allow the power assist to operate until the vacuum is exhausted.

FIGURE 8-4  The dual-diaphragm vacuum booster has two separate

FIGURE 8-5  The dual-diaphragm (tandem) vacuum booster consists

diaphragms that when combined, are larger than a single, larger vacuum booster. This increases the force that is made inside the booster to facilitate the driver in applying the brakes.

of two boosters that have been sized down to fit together in one housing. This design allows for a more forceful vacuum booster in a smaller space.

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Chapter 8  Power-Assist Systems

FIGURE 8-6  The pushrod on the vacuum booster must be inspected

FIGURE 8-7  Taking a measurement from the old booster will

before the component is replaced so that it is adjusted to the necessary position.

allow the technician to adjust the replacement booster to the same measurement so that no component breakage will happen.

it may not fully apply to the brakes when the driver presses firmly down on the brake pedal, which may cause an accident.

Vacuum Pump Some engines require the use of a vacuum pump, which creates a vacuum to operate the booster and other vacuum accessories (FIGURE 8-8). These types of engines make very little vacuum when they run, because either they have a special application ­camshaft or they are a diesel engine. A diesel engine usually does not use a venturi or butterflies to create the vacuum required to operate a vacuum-powered booster. The vacuum pump can be either engine driven or electric, depending on the application. Engine-driven vacuum pumps have started to dwindle as electricand hydraulic-assisted power brake units have come into favor by most original equipment ­manufacturers (OEMs). FIGURE 8-8  A vacuum pump is just a simple pump that creates

negative pressure (vacuum) by rotating the pump, creating suction inside the pump.

Checking a Vacuum-Type Power Booster Unit for Leaks and Inspecting the Check Valve

Vacuum leaks in the power booster require increased driver foot pressure to activate the brakes. Because the vacuum normally comes from the intake manifold, a leaky power booster can affect the operation of the engine by changing the air-fuel mixture. Power boosters can leak internally or externally. Perform the external leak test before the internal leak test to avoid confusion in identifying the cause of the leak. To perform an external leak test, follow these steps: 1. Start the engine, and allow it to run for at least 10 seconds. 2. With the foot off the brake pedal, turn the engine off. 3. Wait at least 10 minutes, and then apply the brake pedal with moderately firm pressure (20–30 lb [9.1–13.6 kg]). Note the feel and pedal reserve height. 4. Apply the brake pedal a couple more times with the same moderately firm pressure. Each application should result in a higher and firmer brake pedal as the vacuum is released from the booster.



Vacuum Booster Operation

211

If it does operate properly, then there are no substantial external leaks in the system. If it does not hold vacuum, inspect the booster for external vacuum leaks. The cause could be any of the following: ■■ ■■ ■■ ■■ ■■

the vacuum check valve and/or grommet the front or rear seals of the unit the atmospheric valve at the rear of the booster the case where the halves are crimped together a hole worn or rusted through the case.

Upon verifying that there are no external leaks, move on to the second step, which is to perform an internal leak test: 1. Begin by starting the engine and letting it idle. 2. Apply the brake pedal with firm pressure (30–50 lb [13.6–22.7 kg]). 3. Without moving the foot on the pedal, shut off the engine and observe the brake pedal for approximately one minute. If the pedal stays steady, there are no internal leaks. If the brake pedal rises, there is an internal leak in the diaphragm, the control valve, or the check valve. The third step is to perform a test for check valve operation: ■■ ■■

Start the engine, and allow it to run for 10 seconds to evacuate the booster. Turn off the engine, wait at least 10 minutes, and then remove the check valve from the booster.

There should be a large rush of air into the booster. If there is, the check valve is holding a vacuum and is OK. If there is not, test the check valve by blowing air through it. Air should flow from the booster side of the check valve to the engine side only. If the check valve is OK and the tests indicate a leak, use a stethoscope and listen for leaks around the outside of the booster, including the control valve at the rear of the booster, which is under the dash. To inspect the vacuum-type power booster unit for leaks and inspect the check valve for proper operation, follow the steps in SKILL DRILL 8-1.

SKILL DRILL 8-1 Checking Vacuum-Type Power Booster Unit for Leaks and Inspecting the Check Valve 1. The first step is to perform an external leak test. Start the engine, and allow it to run for at least 10 seconds. With the foot off the brake pedal, turn the engine off. Wait at least 10 minutes, and then apply the brake pedal with moderately firm pressure (20–30 lb [9.1–13.6 kg]). Note the feel and pedal reserve height. Apply the brake pedal a couple more times with the same moderately firm pressure. Each application should result in a higher brake pedal as the vacuum is released from the booster.

Continued

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2. The second step is to perform an internal leak test. Begin by starting the engine and letting it idle. Apply the brake pedal with firm pressure (30–50 lb [13.6–22.7 kg]). Without moving the foot on the pedal, shut off the engine and observe the brake pedal for approximately one minute. If it stays steady, there are no internal leaks. If the brake pedal rises, there is an internal leak.

3. The third step is to perform a test for check valve operation. Start the engine, and allow it to run for 10 seconds to evacuate the booster.

4. Turn off the engine, wait at least 10 minutes, and then remove the check valve from the booster. There should be a large rush of air into the booster if the check valve is holding a vacuum properly. If there is not, test the check valve by blowing air through it.

▶▶ Hydraulic 8-2 Summarize the operation of hydroboost power-assist systems.

Power-Assist Operation

Hydraulic power-assist brake boosters are usually sold under the name hydroboost. This type of power assist gets its pressure from the power steering pump. This hydraulic pressure is used to assist in the application of the brakes (FIGURE 8-9). This system is usually used on vehicles that create too little vacuum to operate a conventional vacuum booster style of power assist. These vehicles include those that are equipped with a diesel engine or a forced induction version of a gasoline engine. The design of diesel engines does not allow for a lot of vacuum, because of the absence of throttle plates, so if the manufacturer would like to use a vacuum booster in this application, they will have to install a vacuum



Hydraulic Power-Assist Operation Typical General Motors Hydroboost System Hydroboost to Pump

Master Cylinder

Pump

Hydroboost Unit Pump to Hydroboost

Hydroboost to Gear

Gear to pump

Gear

FIGURE 8-9  A typical

hydroboost system, which consists of a power steering pump, power steering gear, distribution block, hydraulic lines and hoses, and the hydroboost unit.

pump. In a forced induction engine application, the engine does not make enough vacuum as the vehicle accelerates, but it makes more than enough as the vehicle decelerates. This uneven application of ­vacuum could cause a power-assist-related issue, which is why some ­applications use hydroboost.

Hydroboost Operational Stages The hydroboost has three operational stages: unapplied, applied, and hold positions. ■■

Unapplied: The unapplied position is when the brakes are not being applied, the spool valve within the unit is in a neutral position, and the pressure is building up inside the accumulator (FIGURE 8-10). The lever attached to the input pushrod that is connected to the brake pedal keeps the spool valve in a position where the pressure from the power ­steering pump is being redirected to the power s­ teering gear. This is the bypass condition where there is no ­power-assist function happening. ACCUMULATOR PISTON ACCUMULATOR CAP

RESERVE SYSTEM PRESSURE

NITROGEN GAS

2 FUNCTION VALVE BALL CHECK

FILL IN ACCUMULATOR

PUMP PRESSURE ACCUMULATOR DUMP VALVE PUMP PRESSURE

TO STEERING GEAR SPOOL & SLEEVE ASSEMBLY BOOST PRESSURE CHAMBER

RETURN TO PUMP RESERVIOR SPOOL PLUG

LEVER PEDAL ROD

OUTPUT ROD

INPUT ROD END INPUT ROD PISTON

HOUSING

HOUSING COVER

FIGURE 8-10  When the power assist is not needed, the unit allows the power steering pressure to bypass the unit and be applied exclusively to the steering gear box.

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Chapter 8  Power-Assist Systems ACCUMULATOR PISTON ACCUMULATOR CAP

RESERVE SYSTEM PRESSURE

BALL CHECK

2 FUNCTION VALVE

NITROGEN GAS

PUMP PRESSURE ACCUMULATOR DUMP VALVE PUMP PRESSURE

TO STEERING GEAR SPOOL & SLEEVE ASSEMBLY

RETURN TO PUMP RESERVIOR

BOOST PRESSURE CHAMBER

SPOOL PLUG

LEVER PEDAL ROD

OUTPUT ROD

INPUT ROD END INPUT ROD PISTON

HOUSING

HOUSING COVER

FIGURE 8-11  When power assist is required, the power steering pressure is applied to the power piston inside the

unit. The power piston moves the reaction rod toward the master cylinder, causing the brakes to be applied with increased force.

■■

■■

Applied: When the brake pedal is pressed and the input push rod moves the lever inside the hydroboost unit, pressurized power steering fluid is applied to the lever attached to the power piston (FIGURE 8-11). As the pressure pushes the power piston, it moves the reaction rod into the master cylinder, thus increasing the applied pressure to the master cylinder. This is how the brake system gets power assist. Hold: In the hold position, the hydroboost unit directs the flow of the pressure from the power steering pump through the spool valve, bypassing the actuation lever (­ FIGURE 8-12). This function allows for pedal feedback to the driver and also does not allow for pressure increase unless the pedal moves.

Hydroboost Diagnosis When diagnosing a hydroboost system for poor brake assist, the first step is to determine whether the power steering pump is operating correctly. Since the hydroboost gets its power from the power steering pump, a missing belt, low fluid, or line restriction could cause the hydroboost to not operate correctly, which could cause a no-assist condition (FIGURE 8-13). Once it is determined that the power steering pump is operating, the technician must then determine whether the pump is putting out any pressure. This could be as simple as determining whether the steering assist is working correctly. If it is not, the steering wheel will be hard to turn from lock to lock. With the engine off, the accumulator on the hydroboost unit should have enough reserve capacity to allow for power assist for two to three braking events. If the accumulator does not hold enough pressure to allow for power brake application, it has failed and the unit should be replaced (FIGURE 8-14).



Hydraulic Power-Assist Operation ACCUMULATOR PISTON ACCUMULATOR CAP

2 FUNCTION VALVE

RESERVE SYSTEM PRESSURE

BALL CHECK NITROGEN GAS

PUMP PRESSURE ACCUMULATOR DUMP VALVE PUMP PRESSURE

TO STEERING GEAR SPOOL & SLEEVE ASSEMBLY BOOST PRESSURE CHAMBER

RETURN TO PUMP RESERVIOR SPOOL PLUG

LEVER PEDAL ROD

OUTPUT ROD

INPUT ROD END INPUT ROD PISTON

HOUSING

HOUSING COVER

FIGURE 8-12  When the hydraulic booster is in the hold position, the spool valve directs the flow through the unit without increasing the pressure on the master cylinder.

FIGURE 8-13  Sometimes the obvious issues with the power steering system can cause the symptoms that the customer reported. A thorough visual inspection is the first step in diagnosis.

Inspecting and Testing a Hydraulically Assisted Power Brake System Operation problems can be caused by a number of issues, such as leaks inside the booster unit, a worn power steering pump, a slipping or broken pump drive belt, badly contaminated power steering fluid, or leaky hose connections. Most of these issues can be verified with a visual inspection. To inspect and test a hydraulically assisted power brake system for leaks and proper operation, follow the steps in SKILL DRILL 8-2.

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TO RESERVOIR

FROM PUMP

TO GEAR

ACCUMULATOR CHARGING CHECK VALVE

PRIMARY VALVE

ACCUMULATOR VALVE BOOST CAVITY SECONDARY VALVE

HOUSING

RATIO LEVER

OUTPUT ROD

INPUT BRACKET TRAVEL LIMITER

NITROGEN GAS

ACCUMULATOR BODY

POWER PISTON

INPUT ROD

INPUT ROD SPRING

HOUSING COVER

ACCUMULATOR PISTON

FIGURE 8-14  The accumulator feeds the built up pressure into the hydroboost unit to provide power assist for brake application.

SKILL DRILL 8-2 Inspecting a Hydraulically Assisted Power Brake System 1. First, do the performance test. Begin with the vehicle engine off. Apply and release the brake pedal five or six times. Hold the brake pedal down with moderately firm pressure (20–30 lb [9.1–13.6 kg]). Start the engine and observe the brake pedal. It should drop or rise an inch or so if the booster is providing boost.

2. Second, perform the accumulator leak test. Start the engine, apply the brake pedal, and note the pedal feel and applied height. Release the brake pedal, turn off the engine, and wait at least 10 minutes.

Continued



Diagnosis and Service

217

3. Apply the brake pedal with moderately firm pressure (20–30 lb [9.1–13.6 kg]). Note the feel and brake pedal height. Apply the brake pedal a couple more times with the same moderately firm pressure. Each application should result in a higher brake pedal as the accumulator pressure is released from the booster.

Power Booster Testing in the Electrohydraulic Braking System Testing the operation of this system can be performed just as the hydraulic booster ­system was tested. Carry out the same steps to performance test the electrohydraulic braking (EHB) system. Follow these tests up with the accumulator leak test, as described previously. These tests will identify any operational issues with the system. If issues are found, follow the manufacturer’s recommended procedure to diagnose the fault.

▶▶ Diagnosis

and Service

When determining what needs to be done to the vehicle’s braking system, the technician must look at the whole system to determine how to fix it. Working in a systematic fashion will increase the possibility of correctly diagnosing the problem the first time and will decrease the time it takes to discover the underlying issue. Starting inside the vehicle cabin will lay the foundation for a proper repair.

8-3 Diagnose a power-assist system failure.

Inspecting the Brake Pedal Brake pedal height, free play, and travel are critical for proper brake operation. Having the proper brake pedal height helps to ensure that the brake pedal has enough starting height to fully apply force to the brakes, even if one-half of the hydraulic system is rendered useless by a leak. In other words, there has to be a specified distance in which the brake pedal can travel before it contacts the floor or anything else. To measure brake pedal height, follow the steps in SKILL DRILL 8-3. Free play is the amount of clearance between the brake pedal linkage and the master cylinder piston. To measure it, apply very light hand pressure to the brake pedal, measuring how far the pedal travels before resistance starts to be felt (FIGURE 8-15).

FIGURE 8-15  Measuring free play.

SKILL DRILL 8-3 Measuring Brake Pedal Height 1. Research the procedure and specifications for measuring brake pedal height, travel, and free play for the vehicle being worked on. Some manufacturers specify how much travel the brake pedal should have, whereas others specify how much reserve pedal should remain when the brake pedal is fully applied. Know which process applies to the vehicle being worked on.

2. Remove any removable floor mats or anything lying on the floor near the brake pedal. 3. With the engine off, measure the brake pedal height between the two specified points, using a measuring stick. 4. Compare this reading to the specifications, and determine any necessary actions.

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SKILL DRILL 8-4 Measuring Brake Pedal Free Play 1. Research the specified procedure for measuring pedal free play. 2. Use light hand pressure to apply the brake pedal until all clearances are taken up, all the while measuring the distance the brake pedal moved.

3. Compare this reading to the specifications, and determine any necessary actions.

To measure brake pedal free play, follow the steps in SKILL DRILL 8-4. Brake pedal travel is sometimes called reserve pedal. Travel is the distance the brake pedal travels from its rest position to its applied height. Travel is measured by reading brake pedal height and subtracting the height from the floor (FIGURE 8-16). For example, if the brake pedal height is 8" (203.2 mm) and the travel takes it down to 5" (127 mm) off the floor, then the travel is 3" (76.2 mm). Reserve pedal is the measurement from the floor to the height of the applied brake pedal and represents how much reserve is left for the brake pedal to travel if needed (FIGURE 8-17). To measure brake pedal travel, follow the steps in SKILL DRILL 8-5.

Diagnosing Power Brake Systems

▶▶TECHNICIAN TIP Be sure to understand and observe all legislative and personal safety procedures when carrying out tasks. If a technician is unsure of what these procedures are, they should ask their supervisor.

All power brake systems should be inspected and tested whenever the customer reports that the brakes are dragging, the brake pedal is harder than normal to push, or the pedal height has changed or if the engine operation changes more than a minimal amount when the brake pedal is applied. Vacuum boosters should also be tested if it is determined that the vehicle has an unlocated vacuum leak. Single-diaphragm and dual-diaphragm vacuum brake boosters are diagnosed in the same manner. The following tests can be performed on single-diaphragm and dual-diaphragm boosters: ■■

■■ ■■ ■■

Brake pedal free travel: Test to determine whether there is proper brake pedal linkage clearance. Performance/operation test: Test to determine whether the booster is operational. External leak test: Test for leaks to the atmosphere. Internal leak test: Test for leaks between the booster chambers.

FIGURE 8-16  Travel is measured by reading brake petal height and subtracting the height from the floor.

FIGURE 8-17  Reserve pedal is the measurement from the floor to the applied brake pedal.

SKILL DRILL 8-5 Measuring Brake Pedal Travel 1. Research the specified procedure for measuring the brake pedal travel or reserve pedal height. 2. Start the engine. This allows the booster to operate normally. 3. Apply the brake pedal with the specified force.

4. Measure the brake pedal travel or reserve height. 5. Compare this reading to the specifications, and determine any necessary actions.



Diagnosis and Service

Testing the Power Booster

219

▶▶TECHNICIAN TIP

Power booster testing starts with a brake pedal free travel test and then follows up with a performance test to determine whether the booster is operating properly. If not, then an external leak test and internal leak test can determine why the booster isn’t working ­properly. The brake pedal free travel is critical for proper brake operation. The proper amount of free travel ensures that the brake pedal linkage allows the master cylinder pistons to return to their proper rest position and uncover the compensating ports. Insufficient free travel can cause the brakes to drag due to trapped fluid pressure in front of each piston and not being able to return to the reservoir through the blocked compensating ports. Excessive free travel is not good either, because it reduces the amount of reserve pedal for braking in the event of a hydraulic brake system leak. As important as brake pedal free travel is, it normally does not need to be adjusted, because the components are locked in place. The situations that would call for adjusting it include the following: Someone changed the adjustment setting; the brake pedal linkage has been repaired or adjusted; the linkage has worn over time, leading to increased free travel; or the power booster is being replaced. Just changing the master cylinder does not ­normally require brake pedal free travel adjustment; however, it is good practice to verify that it is within specifications. If adjustment is needed, the manufacturer usually incorporates a locking adjustment rod between the power booster and the brake pedal. After verifying that the free travel is correct, the power booster needs to be performance tested. To do so, operate the booster to test its ability to provide boost to the master cylinder. To test pedal free travel and to performance test the vacuum booster, follow the steps in SKILL DRILL 8-6.

Holding the brake pedal in a steady manner should not affect the operation of the engine. If the engine runs rough when the brake pedal is held down or changes substantially when the brake pedal is released, it could indicate a vacuum leak in the booster. Perform the external and internal leak tests to identify any faults.

SKILL DRILL 8-6 Perform Brake Pedal Free Play and Performance Test on a Vacuum Power Booster 1. To test brake pedal free travel, start with the engine off and depress the brake pedal several times to remove any vacuum or hydraulic pressure from the power booster. Measure the distance of the brake pedal free travel by depressing the brake pedal by hand until all of the slack is taken up.

2. Performance test the booster by beginning with the vehicle engine off. Apply and release the brake pedal five or six times to bleed off any vacuum or hydraulic pressure in the power booster. Hold the brake pedal down with moderately firm pressure (20–30 lb [9.1–13.6 kg]).

Continued

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3. Start the engine and observe the brake pedal. On vacuumassisted vehicles, if the pedal drops about an inch, the booster is providing boost. If not, the booster is not providing boost, and thus the following tests must be performed. On hydraulically assisted vehicles, when starting the engine with a foot on the brake pedal, the pedal should either rise or fall about an inch (depending on the vehicle) if the booster is providing assist.

Checking Vacuum Supply to Vacuum-Type Power Booster The brake booster must have an adequate amount of vacuum to operate correctly. ­Insufficient vacuum requires the driver to increase foot pressure to activate the brakes. Excessive vacuum is not usually a problem, because the booster is designed to work using maximum engine vacuum. Many manufacturers specify a minimum of 16" of mercury (16 inHg; 406 mm of Hg) of intake manifold vacuum. If the reading is insufficient, check for vacuum leaks or restrictions in the supply hose or for an improperly tuned engine. To check vacuum supply to a vacuum-type power booster, follow the steps in SKILL DRILL 8-7.

SKILL DRILL 8-7 Solve Vacuum Supply Issues on a ­Vacuum-Type Power Booster 1. With the engine off, remove the inlet hose from the vacuumtype booster.

2. Connect a vacuum gauge to the vacuum supply end of the hose.

Continued



Diagnosis and Service

221

3. Start the engine, and read the vacuum supply available to the vacuum-type booster. Vacuum should be greater than 16 inHg (406 mm of Hg) on most vehicles.

Lack of Power Assist in the Braking System When diagnosing a power-assist failure on a braking system, the technician must first make some basic vehicle checks. Verifying that the engine is in operational condition is one of the main issues with power-assist failure (FIGURE 8-18). Inspecting the belts, hoses, ­fluids, and accessory components will help in diagnosing the root cause of the power-assist issue ­(FIGURE 8-19). Before elevating the diagnostic routine, the technician must verify the sources of the motivation for the various power-assist systems. The vacuum booster power-assist system needs a strong vacuum source from the engine to make it operate correctly. If the vacuum feed line from the intake manifold is broken, collapsed, or disconnected, the assist function will not operate (FIGURE 8-20). After verifying the vacuum source to the vacuum booster, the technician can move on to diagnosing the internal issue with the booster. The hydroboost power-assist system is a fed with the power steering system pressure to supply the force needed to boost the brake pressure (FIGURE 8-21). Verify that the power steering system is producing enough pressure to operate the hydroboost system and the steering system. To verify the pressure, the technician can use a pressure flow analyzer to determine the output of the power steering pump (FIGURE 8-22). Once it is determined that the power steering pump is outputting enough pressure, the technician can move on to diagnosing the hydroboost unit.

FIGURE 8-18  When inspecting the brake system, the technician needs to look at the various pieces of the engine compartment to make sure that they are in operational condition.

▶▶TECHNICIAN TIP Engines with modified camshafts regularly have decreased amounts of manifold vacuum due to the high-duration camshaft. This lowered vacuum results in higher foot pressure required to stop the vehicle. An auxiliary vacuum pump may be needed to provide adequate braking. Also, vehicles operated at high altitude always have less vacuum.The general rule of thumb is that 1 inHg (25 mm of Hg) is lost for every 1,000'' (305 m) of altitude gained.

FIGURE 8-19  A broken belt can cause a brake-assist issue inside a hydroboost system as the pressure is created within the power steering system.

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Chapter 8  Power-Assist Systems

High Side Port In

Low Side Port

Low side Power Steering Pump

High Side Port Out

T Fitting Driver Side

FIGURE 8-20  A broken vacuum line to the power booster will cause it to become inoperative.

Passenger Side

High Pressure Low Pressure

FIGURE 8-21  The hydroboost system is a form of brake-assist system that is run off of the power steering system.

FIGURE 8-22  The power steering pressure analyzer is used to verify that the power steering pump produces output high enough to operate both the power steering system and the hydroboost system at the same time.

To test the pre the pressure inside a hydroboost power-assist system and a vehicle power steering system, follow the steps in SKILL DRILL 8-8.

SKILL DRILL 8-8 Testing the Pressure Inside a Hydroboost Power-Assist System and a Vehicle Power Steering System 1. Locate the power steering gear box or rack and pinion on the vehicle.



Diagnosis and Service

223

2. Remove the pressure line from the steering gear box or rack and pinion.

3. Install the power steering pressure and flow analyzer in-line between the pressure line and the steering box.

4. Start the vehicle, and read the pressure output from the power steering pump on the gauge without moving the steering wheel.

5. Rotate steering wheel and watch the pressure build as the technician gets closer to the steering stop.

Continued

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Chapter 8  Power-Assist Systems

6. Once the pressure under the conditions that are explained in the service information has been read, compare the reading with the specification.

▶▶Wrap-Up Ready for Review ▶▶ ▶▶ ▶▶

▶▶ ▶▶

Verify the vacuum source to booster before diagnosing the fault. Verify power steering pressure to the hydroboost before diagnosing it as faulty. Inspect brake pedal free play so that if a mechanical problem is detected, it can be remedied before replacing non-faulty components. Engine inspection is recommended before diagnosing a brake power-assist fault. The power braking system is used to increase the potential pressure applied to the hydraulic brakes.

Key Terms Freeplay  Is the amount of movement before the actuation of the master cylinder. hydroboost  A power-assist system that uses the power steering pump to help with the power assist of the hydraulic braking system. National Highway Traffic Safety Administration (NHTSA)  A US federal agency that is in charge of making sure manufactures comply with safety protocols when developing their vehicles. tandem diaphragm booster  A brake booster that uses two diaphragms inside of it so that the amount of pressure created will equal that of a very large single-diaphragm booster. vacuum pump  An engine-driven or electric pump that generates vacuum so that vacuum-run components will be able to operate on a vehicle.

Review Questions 1. Why was the power-assist unit introduced to the modern automobile? a. The manufacture wanted to sell more parts. b. The use of disc brakes requires more pressure to stop the vehicle. c. The driver of the vehicle couldn’t operate the vehicle without it. d. So that 16-year-olds could drive the vehicle. 2. A vacuum pump is used when the engine does not _______________. a. create enough positive pressure to operate b. run correctly without suction within the engine c. create enough vacuum, because it does not have a butterfly or throttle body d. have a serpentine belt drive system 3. When the technician is replacing a vacuum booster, how far should they adjust the pushrod? a. Adjust it all the way out so that it will be sure to contact the master cylinder. b. Adjust it all the way in so that it will not cause component breakage. c. Leave it the way it came. d. Measure the old one and adjust the new one to match the old one. 4. A dual-diaphragm vacuum booster is used instead of a single-diaphragm vacuum booster because ______________________. a. there is limited room inside the engine compartment b. the vehicle designer thought it looked better



c. it’s more futuristic d. a single-diaphragm vacuum booster is meant only for big vehicles 5. The customer reports that their power steering and power brakes do not work on their vehicle. What could potentially be the cause? a. The hydraulic brake system. b. The power steering belt. c. The alternator. d. The water pump. 6. What is the first step in checking the vacuum booster? a. Look at the color of the booster. b. Determine whether the booster is the correct one for the vehicle. c. Check the vacuum source to the booster. d. Put a pressure gauge on the master cylinder. 7. When replacing the vacuum booster on a vehicle, the technician must first remove __________________. a. the steering wheel b. the master cylinder c. the brake light d. the brake pads 8. When measuring the brake pedal height, the brake must be applied first in order to _________________. a. take up the slack in the brake pedal mechanism b. make sure the brakes operate correctly c. set the parking brake d. adjust the brake drums 9. The hydroboost system shares a component from what other system? a. The transmission. b. The engine. c. The power steering system. d. The suspension system. 10. When replacing the hydroboost unit, what should also be done? a. Flush the power steering system. b. Flush the cooling system. c. Replace the brake pads. d. Replace the brake lights.

ASE Technician A/Technician B Style Questions 1. Technician A says that a low pedal condition could be caused by the power booster. Technician B says that hydroboost is used on heavy-duty vehicles and vehicles with engines that do not make a lot of vacuum. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 2. Technician A says that older vehicles that had four-wheel drum brakes did not need a booster to help with brake application, because the design of drums increased the pressure with the rotation of the wheel. Technician B says

Wrap-Up

225

that only those people who are of small stature need a power brake booster. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 3. Technician A says that the vacuum booster uses brake fluid pressure to increase the pressure inside the system. Technician B says that a vacuum booster has a check valve built into it so that when the engine is shut off, the driver will still have some power brakes for a few actuations. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 4. Technician A says that low vacuum to the booster may cause the power assist to not work. Technician B says that the check valve inside the power booster is used to retain some vacuum inside the booster so that driver will have some power assist in the event that the engine dies. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 5. Technician A says that when diagnosing a hydroboost system, the technician should flush the power steering system. Technician B says that a failure of a power steering pump can cause the power assist on a hydroboost system to fail as well. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 6. Technician A states that power-assist brakes are not needed on modern vehicles. Technician B says that power steering gearbox leaks can affect the hydroboost system. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 7. Technician A says that the vacuum line to a power booster is disconnected, and that could cause an engine performance issue. Technician B says that the vacuum line for the power booster is collapsed, which is the source of the ­power-assist condition. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 8. Technician A says that if the master cylinder is leaking into the power booster, this could cause power boost failure. Technician B says that using the wrong fluid in

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Chapter 8  Power-Assist Systems

the power steering system could cause hydroboost failure. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B 9. Technician A says that to check the vacuum booster, the technician must depress the brake pedal until the assist is depleted. Technician B says that to check the booster, the technician needs to depress the brake pedal and start the vehicle and, the pedal should then drop if the booster is OK. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B

10. Technician A says that a failing water pump or belt tensioner may cause the hydroboost to operate sporadically. Technician B says that using the wrong power steering pump can cause the vacuum booster to work poorly. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A or B

CHAPTER 9

Wheel Bearing Service Learning Objectives ■■ ■■ ■■

9-1 Summarize how wheel bearings affect the braking system. 9-2 Categorize the wheel bearing types. 9-3 Explain wheel bearing arrangements for rear-wheel-drive axles.

■■ ■■

9-4 Interpret wheel bearing failures. 9-5 Service serviceable wheel bearings.

You Are the Automotive Technician A customer pulls their 2008 Ford F250 pickup truck into your shop, reporting that the vehicle is making a howling sound when driving faster than about 20 mph. The noise seems to gradually increase as the speed of the vehicle increases. The customer is concerned that the vehicle might break down and leave them stranded.You ask them where the noise is coming from, and they explain that it is coming from the center or rear of the vehicle.You notice that the truck is equipped with a solid live rear axle and that the tires are in good shape, with a fairly smooth highway tread on them.

1. On a test drive, how can you determine whether the noise is coming from a wheel bearing or a transmission bearing? 2. What wheel bearing arrangements can be used on a solid live rear axle, and how do they differ? 3. How are rear wheel bearings lubricated on each type of solid live rear axle? 4. How is a sealed bearing different from a serviceable bearing?





227

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Chapter 9 Wheel Bearing Service

▶▶ How Wheel

Bearings Affect Braking Components

9-1 Summarize how wheel bearings affect the braking system.

Wheel bearings are a commonly overlooked component. All wheel bearings used to be the serviceable type, requiring periodic maintenance every 24,000–30,000 miles (38,624–48,280 km). The periodic service of a wheel bearing consisted of disassembling, cleaning, inspecting, lubricating, reassembling, and adjusting it. Sealed wheel bearing that do not need periodic service are used by most manufacturers on their lighter-duty vehicles. As a result, many technicians do not give the wheel bearings as much consideration as they once did. However, both types of wheel bearings can fail, and vehicles with serviceable wheel bearings still need maintenance, so it is important for technicians to become familiar with wheel bearings. This chapter discusses each wheel bearing component and the role wheel bearings play in braking system operation. In order to help prevent wheel bearing failure, it is important to know how to accurately diagnose common issues and recommend the proper service or repair to the customer. The chapter also discusses how to properly maintain and, if necessary, replace today’s wheel bearings.

Wheel Bearings Overview Wheel bearings play a critical role in the vehicle by allowing the wheels to roll with a minimum of friction while still maintaining accurate wheel positioning under all driving conditions. Most wheel bearing assemblies include the following components: an outer race, an inner race, roller bearings or ball bearings, and a bearing cage to hold the rollers or balls in place (FIGURE 9-1). The rollers or balls are made of hardened metal and roll between the two races. The races are also made of hardened metal and are carefully formed to match the contour of the rollers or balls, so all of the components roll easily against each other. When diagnosing a braking issue, the wheel bearings must be checked because they can potentially cause a runout condition that will affect brake pad operation. The technician should focus on looking for looseness, rough spots within the rotation of the wheel, or a component that is not in proper operational condition as they diagnosis a braking issue. Understanding how the wheel bearings are designed will help with pinpointing the issues affecting the braking system. Each race either fits firmly within a housing or fits on a shaft. In many situations, ▶▶TECHNICIAN TIP the races are held in place by an interference fit with the housing or shaft, which means Roller and ball bearing assemblies are they must be pressed into place with a high amount of force. In other words, the races are called antifriction bearings because designed so that when they are installed, they should not rotate in or on their respective the components are in rolling contact components. The rollers, or balls, are specifically designed to provide the rolling function. with one another and therefore have Because the races, rollers, and balls are made of hardened metals, they are designed to resist minimal friction. A sleeve-type bearing, or wear and damage. However, overloading the vehicle can put the wheel bearings under a bushing, such as a clutch pilot bushing, is called a friction bearing because the greater load than they are rated for, which can cause them to fail. In the same way, using components are in sliding contact with improper or insufficient lubricant can cause them to fail. This and other failures will be one another. Antifriction bearings run discussed in detail in the Wheel Bearing Failures section. much more freely than friction bearings. There are two categories of wheel bearings: serviceable bearings and sealed bearings. Serviceable bearings are designed so that they can be disassembled and serviced, whereas sealed bearings are not. Serviceable bearings must be serviced periodically by disassembling, cleaning, Outer Race inspecting, repacking them with the specified lubricant, reinCage stalling, and adjusting them. Sealed bearings are designed so they cannot be disassembled or adjusted. Sealed bearings are manufactured with the proper clearance and filled with the specified lubricant from the factory. They are designed to last Inner Race the life of the vehicle, but bearings do wear out or fail occasionally and have to be replaced. In these two categories, the technician must determine whether the wheel bearing is equipped Rollers with a speed sensor and establish whether it is internal or FIGURE 9-1  Components of a typical wheel bearing. ­external to the wheel bearing.



Wheel Bearing Types

229

Wheel Speed Sensors If wheel bearings on vehicles are equipped with an antilock brake system (ABS), the wheel speed sensor is sometimes incorporated within the wheel bearing. As with any component on the vehicle, failure of the sensor will happen and the wheel bearing will have to be replaced. Diagnosing this failure will help the technician determine what the next step to repair the vehicle is. This applies only to those wheel bearings that have integrated sensors (FIGURE 9-2). In applications that have an external sensor used to take a reading off of the reluctor wheel on the back of the wheel bearing, the sensor must be inspected to determine whether the air gap is too great, causing it to not read correctly (FIGURE 9-3). To determine what the failure is, the technician must employ the use of a scan tool to watch the wheel speed sensor operate, and they must be able to use a multimeter to verify the voltage output of the sensor as the wheel is rotated. Along with voltage output, the technician must verify the correct voltage output, because some vehicles have more than one option of wheel speed sensors, depending on the build date of the vehicle. Verifying the vehicle to be worked on by using the vehicle identification number (VIN) and or the build date will help in obtaining the correct parts.

▶▶ Wheel

Bearing Types

Wheel bearings are commonly of the following types: cylindrical roller bearing, tapered roller bearing, ball bearing, double-row ball bearings, and double-row tapered roller bearings. Each one is designed for a particular application. For example, roller bearings support the load over a large surface area so that they can carry heavier loads than ball bearings can. Manufacturers determine the type of wheel bearing they will use based on the particular application and its requirements. Technicians need to be familiar with each type of wheel bearing to successfully service a variety of vehicles.

9-2 Categorize the wheel bearing types.

Cylindrical Roller Bearings Cylindrical roller bearing assemblies use rollers that are cylindrical in shape so that the races are parallel to each other with the rollers between them (FIGURE 9-4). This type of wheel bearing assembly is used in situations where the wheel bearing is not subject to side loads. It is common in rear axles of rear-wheel-drive vehicles, where the rear axles are held from moving side to side by means other than the wheel bearings, such as a differential assembly that uses thrust bearings to prevent the axle from moving side to side. As all side-to-side movement is controlled within the differential assembly, the cylindrical roller bearing assemblies solely support the weight of the vehicle.

FIGURE 9-2  A wheel bearing with integrated sensors has a connector

FIGURE 9-3  Some ABS sensors are externally mounted where

that is external to the wheel bearing so that it can be hooked into the vehicles wiring harness.

the reluctor ring is exposed to the elements. These types of sensors sometimes have rust buildup underneath the sensor, which can increase the air gap, causing the sensor not to read. If this is the case, the technician must verify the gap setting and correct any one that is too wide.

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Chapter 9 Wheel Bearing Service

Cylindrical Roller Bearings

In many instances, cylindrical roller bearing assemblies use the surface of the axle as the inner bearing race. In this case, the cylindrical roller bearings ride directly on the axle shaft. If bearing assembly fails, the axle shaft will most likely be damaged and have to be replaced, along with the bearing assembly. However, some bearing manufacturers have designed replacement cylindrical roller bearing assemblies that ride farther out on the axle shaft than the original bearing, so the axle may not have to be replaced in this situation. Cylindrical roller bearing assemblies are manufactured to have the proper running clearance between the rollers and races, so no adjustment of this type of wheel bearing is needed. But like all bearings discussed in this chapter, they do require lubrication to ­cushion and cool the rollers and races while operating.

Tapered Roller Bearings Tapered roller bearings have races and rollers that are tapered in such a manner that all of the tapered angles meet together at a common point (FIGURE 9-5). This design allows the tapered rollers to freely roll between the angled inner and outer races. The tapered rollers are contained in a bearing cage that holds the tapered bearings to the inner race as a unit. The inner race is called the cone, and the outer race is called the cup. Together, the cone and cup make up a tapered roller bearing assembly (FIGURE 9-6). Outer Inner Race Tapered roller bearing assemblies are commonly used where heavy loads and side loads Race (thrust) have to be supported. Wheel bearing side load conditions occur when the vehicle FIGURE 9-4  Cylindrical roller bearing is cornering. When the wheel is being turned, the vehicle wants to keep going straight. This assembly. pushes the bottom of the outside wheel inward and the bottom of the inside wheel outward, which puts them both under a side load condition. In this situation, cylindrical roller bearing assemblies would allow only the wheel and axle to slide sideways, which means they cannot control the side load (thrust) condition. But tapered roller bearing assemblies can. Because the components are on an angle to the centerline of (and not parallel to) the axle shaft, they can control side movement (thrust) in one direction. Because of this, tapered roller bearing assemblies are generally used in opposing pairs, so they can control side movement (thrust) in both an inward and outward direction. When used in pairs, the individual tapered roller bearing assemblies are generally referred to as inner (or inboard) Tapered and outer (or outboard) bearings (FIGURE 9-7). The inner bearRoller Bearings ing assembly is closer to the centerline of the wheel and supports most of the vehicle weight. Because of this, the inner bearing FIGURE 9-5  In tapered roller bearings, the common axis of bearings and races provides minimal rolling resistance. assembly is typically larger than the outer bearing assembly.

Bearing Hub Outer Bearing Cotter Pin

Inner Bearing Seal Axle Shaft

Castle Nut

A

FIGURE 9-6  A. Cone. B. Cup.

B

FIGURE 9-7  Tapered roller bearings used to control thrust in

both directions.



Wheel Bearing Types

Some manufacturers have designed double-row tapered roller bearing assemblies, which combine two opposing tapered roller bearing assemblies into one sealed unit. This design provides excellent side thrust–carrying capacity and at the same time excellent load-carrying capacity. For more information, see the Sealed Wheel Bearings section. Because tapered roller bearing assemblies use tapered components, the wheel bearing assembly must be adjusted to have the proper running clearance between the tapered rollers and races when the bearing assemblies are installed. The running clearance is the amount of space between components during operation. If the tapered roller bearing clearance is too tight, the components will bind and overheat due to increased pressure and because the rollers squeeze out too much grease, making the lubricating film too thin. If the tapered roller bearing is adjusted with too much clearance, excessive side-to-side and up-and-down movement will occur, which can cause the components to hammer against each other, damaging the surfaces of the tapered roller bearings and races. Inner Race

Ball Bearings

Rivet

231

Outer Race

Ball bearing assemblies consist of an inner race, an outer race, ball bearBall ings, and a ball bearing cage (FIGURE 9-8). The balls roll in deep channels in the races. Deeply grooved ball bearing assemblies are used as wheel Cage bearings on a lot of light-duty vehicles because they have lower rolling resistance. The much smaller contact area between the balls and races prevents a ball bearing assembly from being used on larger vehicles, which experience higher loads. Side loads are controlled by the balls rolling against the sides of the channels. Because there is much less surface area FIGURE 9-8  A typical ball bearing assembly. between the balls and the sides of the channels, compared to the tapered roller bearing, ball bearing assemblies are limited in how much side load they can handle. At the same time, the small surface area allows them to roll more freely than roller bearings, and they therefore help manufacturers decrease drag on a vehicle, which increases fuel efficiency. Ball bearing assemblies also come in a double-row ball bearing assembly configuration (FIGURE 9-9). The double-row configuration gives the ball bearing assembly twice the surface contact area, so it can control greater amounts of loads and side loads than a single-row bearing assembly. Almost all wheel bearings using a ball bearing assembly are of the double-row ball bearing variety, and they are commonly used in automotive light-duty vehicle applications. The outer race is usually a one-piece unit, whereas the inner race usually consists of FIGURE 9-9  A double-row ball bearing two separate pieces. The inner races are manufactured to butt up against each other to creassembly. ate the correct running clearance when the double-row ball bearing assembly is torqued in place.

Sealed Wheel Bearings Some vehicles use sealed wheel bearings. These bearing assemblies are designed and manufactured as completely sealed units. They can be single row or double row, depending on the vehicle application, and can be made up of ball, cylindrical roller, or tapered roller types of bearings. The bearing assemblies are prefilled with lubricant and have integrated grease seals built into them to contain the lubricant (FIGURE 9-10). They also are manufactured with the proper running clearance, so they do not need to be adjusted. This saves the vehicle manufacturer money by decreasing the time it takes to install them during vehicle assembly. It also makes for a more reliable and consistent installation process, since every unit comes preset at the proper clearance. This style of wheel bearing assembly is also beneficial to vehicle owners because it is designed to last the life of the vehicle

FIGURE 9-10  Sealed bearing.

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Chapter 9 Wheel Bearing Service

during normal use and does not need periodic maintenance, which saves owners money. Another application for sealed wheel bearings is the unitized wheel bearing hub (­FIGURE 9-11). These usually include either a double-row ball bearing assembly or a double-row tapered roller bearing assembly installed in a housing that is bolted onto the knuckle or axle housing. In many cases, the wheel bearing hub being used also has the wheel flange pressed into the inner race of the wheel bearing. In this way, the old bearing assembly can be unbolted and a new one bolted in its place with minimal labor and no adjustments needed. This is almost a zero maintenance system until it wears out, and then it is usually replaced as a unit. On some vehicles equipped with ABS brakes, the ABS sensor is integrated into the unitized wheel bearing assembly. In automotive wheel bearing applications, the double-row ball bearing assembly may come as a unitized wheel bearing hub with the bearing assembly installed in the hub and the wheel flange installed in the center of the bearing assembly. FIGURE 9-11  A unitized wheel bearing hub assembly.

Grease Seals and Axle Seals

All wheel bearings rely on some sort of seal to keep the lubricant in and contaminants out. Some wheel bearings, such as sealed bearings, have the seal built right into the bearing ­assembly. Other wheel bearings rely on a completely separate grease seal. Wheel bearing grease seals seal against a rotating surface so that lubricants cannot leak out and dirt and contaminants can’t get in. In some applications, the grease seal is press fit into the axle housing, which is stationary, and seals against the axle shaft, which is rotating. In other applications, the grease seal is press fit into the wheel hub, which rotates and seals against the spindle, which does not rotate. Most axle seals consist of a stamped sheet metal case and flexible sealing lip with an internal garter spring (FIGURE 9-12). The metal seal case is constructed to be a press fit when installed in the housing. The outside surface of the metal case usually comes pre-coated with a thin layer of sealer compound to seal minor surface imperfections between the seal case and the ­housing. The sealing lip is carefully designed and precisely manFIGURE 9-12  Components of a typical seal. ufactured so that it will seal the specified lubricant. Many seals use a garter spring to help hold the lips of the seal in contact with the shaft it is sealing. The garter spring helps maintain an adequate seal if the parts are slightly out of alignment or if there is a small amount of runout or clearance between the seal and shaft. Because the garter spring holds the seal against the rotating component, the seal can wear a groove in that component, so always inspect the sealing surface of the rotating component to verify that it doesn’t have excessive wear. In some cases, it is possible to purchase ▶▶TECHNICIAN TIP a very thin metal repair sleeve that fits snugly over the sealing surface and provides a new Grease seals are designed to be used surface for the seal to ride against. If a repair sleeve isn’t available, the grooved component only one time. If a seal needs to be may have to be replaced if the wear is great enough. removed for any reason, replace it with a Seals come in a variety of configurations, so it is critical to use the specified seal for new one. Failure to do so will likely result a given application. The sealing lip can also be made from a variety of materials. Always in a leaky seal. Also, if a seal is damaged purchase seals from a reputable manufacturer to be confident that they will do their job and during installation, replace it with a new last a long time. When a seal is being installed, always remember to pre-lube the sealing lip one. Always use care when installing seals, with a small amount of oil or grease. This will prevent it from overheating during the initial and use the correct installation tool. use; such overheating would result in the seal failing prematurely.



Wheel Bearing Arrangements for Rear Drive Axles

▶▶ Wheel

Axles

233

Bearing Arrangements for Rear Drive

Rear drive axles come in three different designations: full floating, semi-floating, and three-quarter floating. Each designation refers to how the axle and wheel are supported by the wheel bearings. It is also important to understand the differences in order to be able to properly service each style. In a full floating axle arrangement, the axle carries only a twisting force. The weight of the vehicle is fully carried by a pair of tapered roller bearing assemblies, which ride between the hub and axle tube (FIGURE 9-13). The axle does not carry any of the vehicle load, because the wheel is bolted directly to the bearing hub. The hub also controls side thrust. Full floating axles handle heavy loads better than the other styles, so they are used in heavy-duty applications such as many one-ton pickups, trucks, and vans. In a semi-floating axle, the wheel flange is part of the axle, which is supported by a single bearing assembly (usually a ball or cylindrical roller bearing style) near the flange end of the axle (FIGURE 9-14). The bearing assembly rides between the axle and the axle tube, so all of the weight is put on the axle flange, which transfers the weight to the wheel bearing assembly. In this case, the axle and bearing assembly together carry the full weight of the vehicle. The axle also provides the twisting force for the wheel. This arrangement is generally considered the lightest duty of the three types of axle designations. In a three-quarter floating axle design, there is a single bearing assembly (usually a ball or cylindrical roller bearing style) between the outside of the axle tube and the hub (FIGURE 9-15). The axle has a wheel flange that bolts to the hub and provides lateral support for the hub and wheel, and the bearing assembly supports the weight of the vehicle. The axle also provides the twisting force for the wheel. This arrangement is generally considered heavier duty than the semi-floating axle, but lighter duty than the full floating axle.

9-3 Explain wheel bearing arrangements for rear-wheel-drive axles.

Lubrication All wheel bearings require lubrication to extend their useful life. There are two common types of lubricants used: gear lube and bearing grease. Each application specifies one or the other. They cannot be substituted, due to the different housing and seal designs each lubricant demands. Gear lube is somewhat thicker than engine oil. Because gear lube flows much more easily than grease, the bearing assembly runs in a housing partially filled with gear lube. The gear lube is thin enough to flow

FIGURE 9-14  Semi-floating axle—bearing located between

the axle and housing.

FIGURE 9-13  Full floating axle—two bearings between the axle housing and hub.

FIGURE 9-15  Three-quarter floating axle— one bearing between the outside of the axle housing and the hub.

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Chapter 9 Wheel Bearing Service

Axle Seal Wheel Bearing

Lubricant, Level with Fill Plug

FIGURE 9-16  Gear lube at the proper level lubricates some types of axle bearings.

around and between the rollers, lubricating them and helping to prevent the wheel bearing assemblies from overheating. In many rear-wheel-drive vehicles, the wheel bearing assemblies are open to the axle housing, which is partially filled with gear lube. In this case, the gear lube lubricates the wheel bearings and also the final drive assembly (FIGURE 9-16). Maintenance of this system involves draining the old gear lube and refilling the system with the specified new gear lube. In most cases, this can be performed without disassembling the wheel bearings, by removing a drain plug or bolt and filling the system through the fill plug (FIGURE 9-17). A fill plug is usually threaded and can be removed to allow the level of a fluid to be checked and filled. Some manufacturers have chosen to use a rubber fill plug that snaps into the fill hole. The level of gear lube should normally be within 0.25" (6.35 mm) of the bottom of the fill plug hole. FIGURE 9-17  Drain and fill points for rear axle assembly. In order to properly perform maintenance on wheel bearings, a technician needs to understand the different characteristics of gear lube and bearing grease. This helps ensure that the proper lubricant is selected ▶▶TECHNICIAN TIP for the task at hand. Gear lube is classified by the Society of Automotive Engineers (SAE) according to its viscosity and by the American Petroleum Institute (API) according to its If the level of gear lube is lower than it service grade. Viscosity refers to the thickness of the gear lube; the higher the number, the should be, suspect a leaking axle shaft thicker the gear lube. Vehicle manufacturers specify a certain viscosity of gear lube based on grease seal. This can usually be verified by looking at the inside of each tire and the climate the vehicle is operated in or the load it is carrying, so understanding viscosity is the back side of the brake backing plate. very important when servicing vehicles. If gear lube is present, the grease seal is Standard viscosities for gear lube are 70W, 75W, 80W, 85W, 90, and 140. The “W” leaking and needs to be replaced. Also stands for “winter,” or the gear lube’s cold temperature viscosity. The non-W ratings are check the wheel bearing to make sure it the viscosity of the gear lube at a predetermined hot temperature. Similar to engine oil, is not faulty; to do so, follow the bearing gear lube is available in multi-viscosity configurations such as 75W-90, 80W-90, or 85Wdiagnosis procedure listed later in this 140. Unlike engine oil, the temperature for the “W” rating varies according to the standard chapter. being met. For example, 70W has a maximum allowable temperature for its viscosity of −67°F (−55°C), and 85W has a maximum allowable temperature for its viscosity of −10°F (−23°C). The manufacturers have gone to great lengths to specify the proper gear lube for their vehicles. Be sure to follow their recommendations when choosing the gear lube for a particular vehicle. Current ratings of the API service grade are GL-4 and GL-5. Generally, the higher the number, the better the lubricant. GL-4 is intended for use with bevel-type gears operating under moderate speeds and loads, such as in many manual transmissions. GL-5 has about twice as much extreme pressure additive as GL-4, so it is intended for most differentials that use hypoid-type gears operating under high-speed/low-speed, high-torque, and shock-load conditions. It can also be used in some manual transmissions. Always check the manufacturer’s specifications to determine the proper gear lube for the application being worked on.



Wheel Bearing Arrangements for Rear Drive Axles

235

Most serviceable wheel bearings require grease as their lubricant. TABLE 9-1  NLGI Rating System Grease is made of a base oil, plus a thickening agent, and specific addiNLGI Number Consistency tives to meet the requirements of the application. Lithium soap is a common thickening agent in automotive grease. Other greases use calcium or 00 Semifluid molybdenum thickening agents. Some add small amounts of copper and/ 0 Very soft or lead to enhance the grease’s ability to withstand extreme pressures. 1 Semisoft Automotive wheel bearing grease is a thickened lubricant, designated 2 Semifirm as a plastic solid. This means that although it is thick enough at room tem3 Soft—common wheel bearing perature to maintain its shape if left undisturbed, it is thin enough to be grease squeezed into and out of small spaces. Its consistency is similar to a glob of 4 Firm gel toothpaste. The thickness of grease is graded by the National Lubricating Grease 5 Very firm Institute (NLGI) (TABLE 9-1). Because it does not flow at room temperaCourtesy of the National Lubricating Grease Institute. ture, the grease has to be packed into the spaces around the rollers when the wheel bearing assemblies are installed. This also has to be done when the grease wears out and must be replaced. Packing a wheel bearing assembly can be done by hand or with a bearing packer, which is a tool that forces grease into the spaces between the bearing rollers. Packing a bearing assembly should be done only after thoroughly cleaning and inspecting the rollers and races for wear, damage, and corrosion. If any of these is present, the bearing assembly should be replaced. To remove and install rear axle wheel bearings, follow the steps in SKILL DRILL 9-1.

SKILL DRILL 9-1 Removing and Installing Rear Axle Wheel Bearings 1. To remove roller wheel bearings in a c clip type rear differential, the technician needs to obtain a slide hammer and the proper-sized bearing puller adapter.

2. Install the proper-sized wheel bearing puller into the wheel bearing, then screw the slide hammer into the adapter. Make sure to screw the slide hammer all the way in so that the threads bottom out in the adapter. This is to prevent stripping the threads when using the slide hammer.

Continued

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Chapter 9 Wheel Bearing Service

3. Using a smooth constant motion, slide the 5 lb (2.27 kg) slide hard against the end of the slide hammer to transfer the force so that the tool will pull the bearing out of the axle tube. While the hammer is sliding, pay attention to the angle to ensure that it is as straight as possible.

4. As the bearing starts to move the hammer, it will move more with each hit, so pay attention so that the slide hammer is not thrown across the shop when the bearing comes all the way out.

5. To install the new wheel bearing, take the bearing and obtain the proper bearing installer from the installer kit.

6. Tap the new bearing slightly into the axle tube to start it straight.

Continued



Wheel Bearing Failures

237

7. Once the bearing is started straight, use the bearing installer and a hammer to drive the bearing until it is seated within the axle tube.

8. Once the bearing is fully installed, verify that it is seated correctly and reinstall the axle and wheel assembly.

▶▶ Wheel

Bearing Failures

Wheel bearings can be damaged from excessive loads, such as overloading the vehicle; shock loads, such as hitting a large pot hole; improper adjustment; or just plain wear over time. The wheel bearings must hold up under difficult circumstances, so suspect faulty bearings whenever an unusual rumbling, whirring, howling, or rough sound comes from the wheel areas when the vehicle is being driven. The noise usually can be heard once the vehicle gets up to 15–20 mph (24.1–32.2 kph) and gets louder as the vehicle speeds up. Loose or worn wheel bearings can also cause the vehicle to wander, shimmy, or vibrate. For these concerns, it is best to lift the wheels off the ground and check the wheel bearings for looseness by grabbing the tire at the 6 o’clock and 12 o’clock positions and lightly wiggling it back and forth. Watch the inside of the wheel to verify that the play is coming from the wheel bearings and not the ball joints. Then grab the wheel at the 3 o’clock and 9 o’clock positions, and again lightly wiggle the wheel back and forth. Watch to verify that the play is coming from the wheel bearings and not the tie rod ends. If there is play in the wheel bearings and the vehicle uses sealed bearings, they will have to be replaced. If the vehicle uses serviceable bearings, the maximum allowable play is approximately 0.010" (0.254 mm). If it is greater than that and they are still in good condition, they will have to be disassembled, cleaned, inspected, repacked, reinstalled, and readjusted. One way to isolate a wheel bearing noise from a transmission noise is to drive the vehicle at the speed at which it is making the noise and then shift into a higher or lower transmission gear while maintaining the same speed. If the noise speeds up or slows down, then it is a transmission-related issue. If it stays relatively the same, then accelerate, coast, and decelerate the vehicle. If the noise changes, suspect the differential or

9-4 Interpret wheel bearing failures.

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Chapter 9 Wheel Bearing Service

FIGURE 9-18  Wheel bearing failure chart.

universal joints. If the noise stays relatively the same, it is most likely related to the wheel bearings. To determine which side the bearing noise is coming from, the vehicle can sometimes be driven at the speed at which it is making the noise, and then lightly rock the car side to side, using the steering wheel. If the noise gets louder when the car is steered right, then it is usually the left side with the bad bearing, and vice versa. Perform this test only in a safe place, such as an abandoned parking lot. Another challenge is distinguishing a wheel bearing noise or a tire noise. The best approach to this problem is to drive over different road surfaces, such as asphalt and concrete. If the noise changes, it is likely a tire problem. If the noise does not change, it is likely to be a faulty wheel bearing. If the source of the noise cannot be determined on a test drive, try placing the vehicle on a hoist and rotating the wheels. If the vehicle has a MacPherson strut suspension, raise and support the vehicle, and spin the wheel by hand while holding onto the coil spring with the other hand, feeling for a vibration or roughness. The spring tends to magnify the wheel bearing roughness, which can be felt with some practice. If the suspect wheel is a drive wheel, under close guidance of a supervisor have an assistant drive the vehicle in gear while on the hoist, and listen to the wheel bearings with a stethoscope. Because the vehicle is running on the hoist, this can be a hazardous situation and must be performed only under the close guidance of a supervisor.

Failure Analysis Wheel bearings can be inspected and determinations made about why the wheel bearing failed. Getting to the source of the problem is important to ensure that the same thing does not happen to the new wheel bearings. Failure analysis starts with removing the faulty wheel bearing and cleaning it and the races thoroughly. Once they are clean, visually inspect the wheel bearing components, and compare the findings to the wheel bearing manufacturer’s failure analysis chart (FIGURE 9-18). This information should lead to what caused the wheel bearing failure. Once the cause of the wheel bearing failure has been determined, take measures to ensure that the failure does not repeat itself. For example, if the bearing and race surfaces show signs of rust or corrosion, inspect the sealing surfaces of the hub to determine whether water is getting past the dust cover or grease seal, and replace the wheel bearing assembly. Also, if the wheel bearing is so damaged that there are metal shavings in the wheel hub and grease, a thorough cleaning of the wheel hub will be required so that all of the metal shavings are removed and will not ruin the new bearing assembly. Some wheel bearing failures involve the races spinning in either the machined bore of the wheel hub or on the surface of the spindle/axle. This can lead to wear of the wheel hub or spindle/axle, which means that these components must be replaced. Be sure to inspect the wheel hub and spindle/axle closely for any damage every time the wheel bearings are removed and serviced.

▶▶ Serviceable Wheel 9-5 Service serviceable wheel bearings.

Bearings

Maintenance consists of disassembling, cleaning, inspecting, repacking, installing, and adjusting the bearings. This is commonly performed during brake shoe/pad replacement or at intervals specified by the vehicle manufacturer, usually around 24,000–30,000 miles (38,624–48,280 km). A normal part of servicing wheel bearings includes replacing the old grease seal with new ones—along with replacing the cotter pin (if used), which retains the wheel bearing adjusting nut. A cotter pin is a soft metal pin that can be bent into shape and is used to retain the bearing adjusting nut. Ensure that the correct replacement parts and grease are available before starting the job.



Serviceable Wheel Bearings

239

Tools Here is a list of common tools used to maintain and repair wheel bearings (FIGURE 9-19): ■■ ■■ ■■ ■■ ■■ ■■

bearing packer seal puller wheel bearing race installer/seal installer set wheel bearing locknut sockets cotter pin removal tool dust cap pliers.

A

B

C

D

E

F

FIGURE 9-19  A. Bearing packer. B. Seal puller. C. Wheel bearing race/seal installer set. D. Wheel bearing locknut sockets. E. Cotter pin removal tool. F. Dust cap pliers.

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Chapter 9 Wheel Bearing Service

Serviceable Wheel Bearing Adjustment All wheel bearings need the proper end play or preload to operate correctly. End play, in the context of wheel bearings, refers to the amount of inward and outward movement of the hub due to the clearance within the bearing assembly. Preload refers to the absence of clearance in the bearing and the specified amount of pressure forcing the bearing components together. Sealed bearings and double-row bearing assemblies come from the manufacturer with the proper clearance machined into them. These wheel bearings are designed so that when the components are tightened together, the races butt up against each other in such a way that the proper clearance is maintained. For these wheel bearings, it is only critical that the retaining bolt or bolts are torqued to the proper specification. This is usually quite high and can typically exceed 200 ft-lb (271.1 N·m). Be sure to check the manufacturer’s torque specifications for the application being worked on. On adjustable wheel bearings, the proper clearance must be set using the adjusting nut. In this case, the adjusting nut is initially tightened to about 20 ft-lb (27.1 N·m) to squeeze out any grease between the races and rollers. It is then loosened one-sixth to one-quarter turn and tightened only lightly (usually about 15–25 in-lb [1.69–2.82 N·m]) so that a small amount of clearance or preload is maintained between the rollers and races, depending on the manufacturer’s specifications. The adjusting nut is then locked in place by a locking mechanism so that it cannot loosen. This is critical to prevent the wheel from falling off and causing injury. The most common locking mechanism uses a keyed washer (hardened), adjusting nut, lock cage, and cotter pin (FIGURE 9-20). On four-wheel drive vehicles, the locking mechanism commonly includes a keyed washer (hardened), adjusting nut, keyed lock washer (or tang washer), and locknut (FIGURE 9-21).

Non-serviceable or Sealed Wheel Bearings

FIGURE 9-20  Typical cotter pin–style wheel bearing locking

mechanism.

Adjusting Nut Washer Bearing Adjusting Nut Lock Washer

When servicing sealed wheel bearings, the process is straightforward. Refer to service information on how to remove the wheel bearing, and reverse the removal procedure to install. The technician must keep in mind that as they are disassembling the braking components, the technician should verify that the components are serviceable once they are reinstalled. If the technician finds that some of the braking components are not in serviceable condition. they must be replaced because reinstalling such components will not serve the customer properly. When replacing the wheel bearing, the technician must verify that the wheel bearing is the same as the old unit and that it installs the same way it was disassembled. On vehicles that have an axle shaft that goes through the wheel bearing, the technician must torque the axle nut to the specification. Once the sealed wheel bearing is installed, there are no other adjustments that can be preformed on it.

Repacking and Adjusting Wheel Bearings

Lock Nut Lock Nut Socket FIGURE 9-21  Typical locknut-style wheel bearing locking mechanism.

Serviceable wheel bearings should be serviced periodically, according to the manufacturer’s scheduled maintenance chart, whenever brake work is being performed, or if a faulty wheel bearing is suspected. When servicing wheel bearings, it is critical to use the proper grease. Also avoid mixing different types of grease, by thoroughly cleaning all old grease from the wheel bearings. Another option is to use a bearing packer, with the same kind of grease, to force the old grease out of the wheel bearings



Serviceable Wheel Bearings

241

FIGURE 9-22  A bearing

packer.

(FIGURE 9-22). Make sure the grease in the bearing packer is not contaminated with dirt or debris and is correct for the vehicle being serviced. When reinstalling wheel bearings, it is critical to follow the manufacturer’s adjustment procedure. Always use new grease seals and cotter pins (if used). Doing so will prevent grease leaks and ensure that the adjusting nut does not back off, causing an unsafe driving situation. To remove, clean, inspect, repack, and install wheel bearings, and to install the locking mechanism, follow the steps in Skill Drills 9-2 through 9-8. To remove, clean, and inspect the wheel bearings, follow the steps in SKILL DRILL 9-2.

SKILL DRILL 9-2 Removing, Cleaning, and Inspecting Wheel Bearings 1. Remove the wheel bearing dust cap by using dust cap pliers or a narrow cold chisel and hammer.

2. Remove the locking mechanism. Remove the adjusting nut, keyed washer, and outer bearing. Reinstall the adjusting nut approximately five turns back onto the spindle.

Continued

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Chapter 9 Wheel Bearing Service

3. Grasp the drum/rotor at the 1 o’clock and 7 o’clock positions or the 11 o’clock and 5 o’clock positions. While holding downward pressure, quickly pull the drum/rotor away from the wheel bearing. The adjusting nut should catch the inner bearing race and pop the grease seal and bearing out of the hub, leaving them sitting on the spindle.

4. Wipe any old grease off of the wheel bearings, races, and spindle with a rag and give them a quick visual inspection. Consult the bearing diagnosis chart to identify any faults.

5. If the wheel bearings are in serviceable condition, completely clean the wheel bearings, races, and hub. If solvent is being used to clean any of the components, make sure there is no solventcontaminated grease left on the parts.

6. Give the parts a final inspection, and consult the bearing diagnosis chart if there are any signs of damage. Using the specified grease, pack both wheel bearings, being careful to keep dirt and debris out of the grease (see Skill Drills 9-2 and 9-3).



Serviceable Wheel Bearings

243

To pack grease by hand, follow the steps in SKILL DRILL 9-3.

SKILL DRILL 9-3 Packing Grease by Hand 1. Using a pair of latex or nitrile (nitro) gloves, the technician places a small glob of grease in the palm of their nondominant hand. They then place the index finger of their other hand through the bearing center hole, with the larger diameter facing down.

2. Push the large diameter of the bearing down the edge of the grease into a palm. This should force grease into the space between the bearings and races. Continue this process until grease comes out of the top of the bearing.

3. Carefully turn the bearing as a unit to a new space, and keep forcing grease between the bearings. Do this until all of the spaces are full.

4. Smear some grease around the outside of the bearing. Repeat this process on the other bearing.

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To pack grease with a bearing packer, follow the steps in SKILL DRILL 9-4.

SKILL DRILL 9-4 Packing Grease with a Bearing Packer 1. Make sure the bearing packer has the proper grease and that it is uncontaminated. Place the wheel bearing with the narrow side down.

2. Place the packer cone on the top of the wheel bearing. Pack the wheel bearing, following the packer’s instructions.

3. Remove the wheel bearing. Wipe off any old grease.

4. Smear some new grease around the outside of the wheel bearing. Repeat this process on the other wheel bearing. Be sure to set a packed wheel bearing down on a clean surface.



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To install wheel bearings, follow the steps in SKILL DRILL 9-5.

SKILL DRILL 9-5 Installing Wheel Bearings 1. Place a small amount of extra grease in the center of the hub. Do not fill it completely. Place the inner wheel bearing in its race, with the narrow side toward the race.

2. Using a hammer or seal installer, install the new grease seal, being careful not to damage it. Place a small amount of grease on the lip of the seal to provide it with initial lubrication.

3. Make sure the spindle is clean, including the mating surface for the seal.

Continued

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Chapter 9 Wheel Bearing Service

4. Without getting grease on the drum/rotor, carefully install it on the spindle, making sure the inner bearing fully seats against the spindle flange.

5. Install the outer bearing on the spindle and into the race.

6. Install the keyed washer and adjusting nut on the spindle, and tighten until finger tight.

7. Tighten the adjusting nut to the specified seating torque (usually about 20 ft-lb [27.1 N·m]) while turning the drum/rotor. This squeezes the excess grease out from between the wheel bearings and races while seating the bearings. Do not leave the bearing this tight!

Continued



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8. Loosen the adjusting nut approximately one-sixth to one-quarter turn without turning the drum/rotor.

9. Tighten the adjusting nut to the specified preload torque. This is usually about 15–25 in-lb (1.69–2.82 N·m).

▶▶TECHNICIAN TIP Some technicians simulate the preload torque by placing a 12" (304.8 mm) crescent wrench on the nut and using the hanging weight of the crescent wrench when it is parallel to the ground. Do not let it drop into position; just lower the handle to where it stops turning the nut.This should be when the handle is approximately level. If not, reposition the crescent wrench on the nut, and allow it to lower until it stops parallel.

To install the locking mechanism, follow the steps in SKILL DRILL 9-6.

SKILL DRILL 9-6 Installing the Locking Mechanism 1. If the locking mechanism is a cotter pin, insert the new cotter pin through the castellated nut, or locking cage, and the spindle. The short leg of the cotter pin should be against the castellated nut, and the long leg should be toward the technician. With the cotter pin fully engaged in the notch, the technician must bend the outer leg toward themselves and up over the end of the spindle. Cut it off just beyond the spindle. Also cut the short leg off at the nut or cage. Make sure the cotter pin will not hit the inside of the dust cap.

Continued

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2. If it is a pin and hole style, or a bendable tang locking style, then place the washer against the adjusting nut (with the pin lined up, if that style) and thread the locking nut up against it. Torque the locking nut to the specified torque, and if it’s a bendable tang style, the technician should bend the appropriate tang out toward themselves and against the flat side of the locking nut by using a small pry bar to lock the adjustment in place.

3. If it is a locking nut style, then tighten the locknut to the specified torque. This is usually a substantial torque of 50 ft-lb (67.79 N·m) or way more.

4. Install the dust cap, being sure it is fully seated in the hub. Make sure the drum/rotor turns freely without binding or making any unusual noises.

Replacing Wheel Bearings and Races Wheel bearings and races have to be replaced only when they are damaged. If one part of the wheel bearing is damaged, all parts must be replaced. So if the tapered roller bearings are damaged, both the bearing and the race of that bearing have to be replaced at the same time. On serviceable bearings, the inner race, roller bearings, and bearing cage are one unit, which generally slips off of the spindle. The outer race is usually press fit into the hub and has to be driven or pressed out and a new one press fit back in. It is critical that the seat in the hub be spotless, with no burrs; otherwise, the bearing race will not seat properly and the bearing will fail prematurely. To replace a wheel bearing and race, follow the steps in SKILL DRILL 9-7.



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SKILL DRILL 9-7 Replacing Wheel Bearings and Races 1. With the wheel bearings removed from the wheel hub, clean and inspect the bearing and race for damage. Determine which bearing and race have to be replaced. Using a hydraulic press or a hammer and punch from the opposite side of the hub, carefully force the race from the hub. Keep it as straight as possible while removing it.

2. Remove any burrs by using a fine file or Dremel, and remove any debris from the seat. Lightly lubricate the outside surface of the new race, and set it with the thick side down in the hub.

3. Using a hydraulic press or a hammer and bearing race installer, carefully drive the race until it is fully seated in the hub. When using a hammer and punch, a distinct sharp metallic sound should be produced when it seats. Inspect the race to verify that it is fully seated. Also check for any damage caused by installation. If everything is good, pack the new bearing and install it according to Skill Drills 9-2 through 9-5.

Removing and Reinstalling Sealed Wheel Bearings Sealed wheel bearings come in two configurations. The first is a replaceable sealed bearing only. On most front wheels, this wheel bearing is pressed between the hub and wheel flange and is the more difficult of the two to replace. The second configuration consists of a unitized wheel bearing hub, including a sealed wheel bearing, a removable wheel bearing hub, and possibly the wheel flange. In most cases, this type can be unbolted from the suspension system and a new one bolted in its place, and then it is ready to go. This is the most common arrangement on recent vehicles. The replaceable bearing style needs to be pressed apart with a hydraulic press or a special sealed bearing removal/installing tool. If using the hydraulic press method, the steering

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knuckle will have to be removed from the vehicle so that it can be placed on the hydraulic press. If the special sealed bearing tool is used, most bearings can be removed while the steering knuckle is still installed on the vehicle, which can save the technician a fair amount of time. To remove and reinstall a sealed wheel bearing assembly that is using the wheel bearing hub style, follow the steps in SKILL DRILL 9-8.

SKILL DRILL 9-8 Removing and Reinstalling Sealed Wheel Bearings That Are Using the Wheel Bearing Hub Style 1. Loosen the axle hub nut, if equipped, while the tire is still on the ground. Remove the wheel and brake assembly, following the specified procedure. Also disconnect the ABS connector and/or sensor if mounted to the hub.

2. If the wheel being worked on is a drive wheel, remove the axle hub nut and tap the drive axle loose with a dead blow hammer.

3. Unbolt and remove the hub assembly from the steering knuckle. Clean the knuckle assembly and check the hub seat for nicks, burrs, or other damage.

Continued



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4. Carefully compare the new hub to the old one; then fit the new hub assembly (over the axle shaft, if equipped) to the knuckle, making sure it is fully seated in place, and torque the mounting bolts to the specified torque.

5. Reassemble the brake assembly and ABS sensor, if removed, following the specified procedure; install the wheel and torque the lug nuts. Be sure the correct ends of the lug nuts are facing the wheel.

6. Install the drive axle nut, if equipped. Use a new hub nut if called for by the manufacturer, and torque to specifications.

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▶▶Wrap-Up Ready for Review ▶▶ ▶▶ ▶▶ ▶▶

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▶▶ ▶▶ ▶▶ ▶▶ ▶▶ ▶▶ ▶▶ ▶▶ ▶▶

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Wheel bearings allow wheels to roll with minimum friction. Wheel bearing assemblies include the outer race, inner race, roller or ball bearings, and a bearing cage. Categories of wheel bearings are serviceable and sealed. Types of wheel bearings are cylindrical roller bearing, ­tapered roller bearing, ball bearing, double-row ball ­bearing, and double-row tapered roller bearings. Cylindrical roller bearing assemblies have parallel races flanking the rollers and are most commonly used in rear axles of rear-wheel-drive vehicles. Cylindrical roller bearing assemblies rely on lubrication to cushion the rollers and transfer heat to the atmosphere. Tapered roller bearing assemblies have tapered rollers housed in a bearing cage, are used for heavier loads, are used in pairs, and control side movement. Tapered roller bearing assemblies must be adjusted for proper running clearance. Ball bearings are used in light-duty vehicles, are designed as a manufacturer-sealed bearing assembly, and should be replaced as a unit. Wheel bearings are sealed to keep out contaminants and contain the lubricant. Serviceable wheel bearings need periodic maintenance, including replacing old grease seals and the cotter pin. Wheel bearings are lubricated by gear lube or bearing grease. Gear lube is classified by viscosity (thickness) and service grade. Bearing grease is graded by thickness; it holds its shape at room temperature. For correct operation, wheel bearings must be adjusted for proper end play/preload. Wheel bearing locking mechanisms include a keyed ­washer, adjusting nut, lock cage, and cotter pin. Damage to wheel bearings may be from vehicle overload, shock loads, improper adjustment, or wear over time. Diagnosing a wheel bearing assembly may include wiggling the tire (with vehicle lifted) by hand, by test-driving, or by placing it on a hoist and rotating the wheels. Determine the cause of wheel bearing failure so that the new bearings are not damaged for the same reason.

Key Terms adjusting nut  The nut used to adjust the end play or preload of a wheel bearing. antifriction bearing  Wheel bearing assemblies that use surfaces that are in rolling contact with each other to greatly reduce friction, compared to surfaces in sliding contact. ball bearings  The rolling components of a wheel bearing, consisting of hardened balls that roll in matching grooves in the inner and outer races.

bearing cage  The component in a wheel bearing that maintains the proper spacing between the roller bearings or ball bearings. bearing packer  A tool that forces grease into the spaces between the bearing rollers. castellated nut  An adjusting nut with slots cut into the top such that it resembles a castle. It is used with a cotter pin to prevent the nut from loosening. cotter pin  A single-use soft metal pin that can be bent into shape and is used to retain bearing adjusting nuts. cylindrical roller bearing assembly  A type of wheel bearing with races and rollers that are cylindrical in shape and roll between inner and outer races, which are parallel to each other. double-row ball bearing assembly  A single ball bearing assembly using two rows of ball bearings riding in two channels in the races. fill plug  Usually a threaded plug that can be removed to allow the level of a fluid to be checked and filled. This could also be a rubber snap fit plug. friction bearing  A bearing that uses sliding motion between components, such as a clutch pilot bushing. garter spring  A coiled spring that is fitted to the inside of the sealing lip of many seals, used to hold the lip in contact with the shaft. gear lube  A type of lubricant used primarily to lubricate transmission and differential gears but also used to lubricate some wheel bearings. grease  A lubricating liquid thickened to make it suitable for use with many wheel bearings. grease seal  A component that is designed to keep grease from leaking out and contaminants from leaking in. inner race  The inside component of a wheel bearing that has a smooth, hardened surface for rollers or balls to ride on. interference fit  A condition in which two parts are held together by friction because the outside diameter of the inner component is slightly larger than the inside diameter of the outer component. keyed lock washer  The washer that fits between the adjusting nut and the locknut. The face of the washer is drilled with a series of holes that mate to a short pin from the adjusting nut, locking it to the spindle. keyed washer  The washer that fits between the adjusting nut and the wheel bearing and that has the center hole keyed to fit a slot on the spindle or axle tube. lithium soap  A thickening agent for grease to give it the proper consistency. lock cage  The stamped sheet metal cap that fits over the bearing adjustment nut and is secured by a cotter pin going through it and the spindle/axle.



locknut  The nut that holds the adjusting nut from turning, which is usually tightened much more tightly than the adjusting nut. molybdenum thickening agent  A compound used in some greases to give it the needed consistency. National Lubricating Grease Institute (NLGI)  An organization that grades the thickness of automotive and industrial grease. outer race  The outside component of a wheel bearing that has a smooth, hardened surface for rollers or balls to ride on. preload  A condition where the wheel bearing components are forced together under pressure and therefore have no end play. roller bearings  The rolling components of a wheel bearing, consisting of hardened cylindrical or tapered rollers. running clearance  The amount of space between wheel bearing components while in operation. sealed bearings  Wheel bearings that are assembled by the manufacturer with the proper lubrication and sealed for life. They normally cannot be disassembled. serviceable bearings  Wheel bearings that can be disassembled, cleaned, inspected, packed, reinstalled, and adjusted. tapered roller bearing  A type of wheel bearing with races and rollers that are tapered in such a manner that all of the tapered angles meet at a common point, which allows them to roll freely and yet control thrust. unitized wheel bearing hub  An assembly consisting of the hub, wheel bearing(s), and possibly the wheel flange, which is preassembled and ready to be installed on a vehicle. viscosity  The measurement of the thickness of a liquid. wheel bearing  A component that allows the wheels to rotate freely while supporting the weight of the vehicle, made up of an inner race, outer race, rollers, and a cage.

Review Questions 1. Which component of wheel bearings holds the rollers or balls in place? a. An outer race. b. An inner race. c. A bearing cage. d. An interference fit. 2. Which of the following statements describing a cylindrical roller bearing is correct? a. They are cylindrical in shape. b. Cylindrical roller bearing assemblies use rollers that are cylindrical in shape. c. They are used in situations where the wheel bearings are subject to side loads. d. They are used only in rear axles of rear-wheel-drive vehicles. 3. Which type of bearing has an inner race called a cone and an outer race called a cup? a. Cylindrical roller bearings. b. Double-row ball bearings. c. Ball bearing. d. Tapered roller bearing.

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4. Which of the following prevents a single-row ball bearing assembly from being used on larger vehicles? a. Its smaller size. b. Its smaller surface area. c. The smaller contact area between the balls and races. d. Its deep groove. 5. All of the following statements describing the difference between single-row and double-row ball bearings are true, except: a. A single-row has one inner and one outer race, whereas a double-row bearing has two separate inner and outer races. b. A single-row has less surface contact area than a double row. c. In a double-row bearing, the inner race is usually two separate pieces, unlike a single row. d. A double-row bearing controls a greater number of loads and side loads than a single-row bearing assembly. 6. All of the following statements regarding sealed and serviceable bearings are true, except: a. Sealed bearing assemblies are prefilled with lubricant and have integrated grease seals, unlike serviceable bearings. b. In serviceable bearings, proper running clearance needs to be adjusted, whereas sealed bearings are manufactured with clearance. c. Sealed bearing assemblies do not need periodic maintenance, unlike serviceable bearings. d. Sealed bearing assemblies cannot fail at any time, whereas serviceable bearings can fail. 7. Choose the correct statement: a. All wheel bearings have the seal built right into the bearing assembly. b. A garter spring helps maintain an adequate seal if the parts are slightly out of alignment. c. Grease seals seal against a rotating surface so that lubricants cannot leak out, and they can be re-used. d. When installing a seal, the sealing lip need not be prelubed, because it comes with a small amount of oil or grease. 8. In which of the following rear drive axle designations is a pair of tapered roller bearing assemblies used? a. Full floating axle. b. Semi-floating axle. c. Three-quarter floating axle. d. One-half floating axle. 9. All of the following statements with respect to lubricants and their applications are true, except: a. Gear lube is somewhat thicker than engine oil. b. The level of gear lube should normally be within 0.25" (6.35 mm) of the bottom of the fill plug hole. c. The higher the viscosity number, the thinner the gear lube. d. Bearing packer is a tool that forces grease into the spaces between the bearing rollers.

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10. In the case of serviceable wheel bearings, the maximum allowable play is _________. a. 0.001" (0.025 mm) b. 0.010" (0.254 mm) c. 0.015" (0.381 mm) d. 0.020" (0.508 mm)

ASE Technician A/Technician B Style Questions 1. Technician A says that cylindrical roller bearings can carry more weight than similarly sized ball bearings. Technician B says that tapered roller bearings used in opposing pairs control side thrust in both directions. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that a tapered roller bearing assembly has less rolling resistance than a similarly sized ball bearing assembly has. Technician B says that the bearing assembly in a unitized wheel bearing assembly can normally be disassembled, cleaned, and repacked. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that over time a grease seal can wear a groove in the sealing surface of the axle or shaft that may mean that the axle or shaft needs to be replaced. Technician B says that grease seals need to be replaced every time the bearing is removed. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that in a full floating axle, the axle does not support the weight of the vehicle. Technician B says that when adjusting tapered wheel bearings, the final torque should be about 20 ft-lb (27.1 N·m). Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that serviceable wheel bearings can be repacked by removing the dust cap, filling it with grease, and reinstalling it. Technician B says that the cotter pin must be replaced with a new one every time it is removed. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

6. Technician A says that the gear lube level in the final drive is OK as long as the level can be touched with a finger. Technician B says that the gear lube level should normally be no more than one-quarter inch below the bottom threads on the fill plug hole. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that wheel bearings must be replaced as a set: bearing and race. Technician B says that the wheel bearings and races on both sides of the vehicle must be replaced if one side fails. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says that unitized hubs have a wheel nut with a higher installation torque than serviceable wheel bearings have. Technician B says that unitized hubs have the proper bearing end play designed into the assembly once they are torqued properly. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says that when installing a bearing race, it should take only finger pressure to install it. Technician B says that on a tapered roller bearing, the race is also called the cup. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that when a race is fully seated, a sharp metallic sound is produced when installation is complete. Technician B says that to be sure a race is fully seated, the wheel bearing adjusting nut should be tightened to at least 100 ft-lb (135.6 N·m) of torque, which will finish seating it. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

CHAPTER 10

Hybrid Vehicle Braking Systems Learning Objectives ■■

■■

10-1 Demonstrate knowledge of the science behind  regenerative braking. 10-2 Explain the types of hybrid systems that use an ICE.

■■

10-3 Execute a diagnosis strategy on a regenerative braking  system.

You Are the Automotive Technician A new customer is having problems with their 2012 Toyota Prius’ braking system making noise. The customer said that their vehicle started making noise when they were on a trip through the mountains of Colorado. They said that when they apply the brakes the display in the vehicle said that there was a fault with the regenerative braking system. You as the technician should do what first?

1. Check the foundation brakes first 2. Use a scan tool to verify a regenerative braking system fault 3. Disable the high voltage battery





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▶▶ The 10-1 Demonstrate knowledge of the science behind regenerative braking.

▶▶TECHNICIAN TIP Hybrid Safety Considerations When working on hybrid vehicle systems, the technician must constantly be aware of the various systems that use high voltage so that they know where not to touch. Being aware at all times will minimize the possibility of injury or death.

Science Behind Regenerative Braking

Hybrid vehicle braking systems are very similar to the conventional vehicle braking system, except that most hybrid electric vehicles (HEV) employ some form of regenerative braking. Regenerative braking is the vehicle turning the electric motors that operate the wheels into generators, which will increase the resistance within the motor, thus slowing the vehicle. This chapter is a brief overview of the process of regenerative braking and will go into some detail on the basics of the various systems that affect an HEV’s ability to recapture the energy that was used to propel the vehicle. Most series or parallel hybrid vehicles are driven by an electric motor that is typically integrated with the vehicle’s transmission. These motors are sometimes referred to as traction motors, which propel the vehicle with electricity stored in the high-voltage battery pack. This chapter focuses on the regenerative braking system and the proper procedures to follow when servicing the system.

The Principles of Physics in Regenerative Braking Systems The electrical motors found in the vehicle convert electrical energy into a forward or reverse motion of the vehicle. The movement of the vehicle creates an inertia as the weight of the vehicle moving down the road becomes a force that must be managed when the driver wants to stop the vehicle. Inertia is the resistance of an object to change the state it is in. Overcoming a vehicle that is stopped is easier than overcoming a vehicle that is moving down the road at 60 mph (96.56 kph). Another term that helps explain what is happening when the vehicle is accelerated to speed is kinetic energy. Kinetic energy is the term used for the energy that the vehicle is creating when it is moving down the road. The regenerative braking system must take this kinetic energy and convert it back into electrical energy that is then stored in the battery bank. Determining the amount of energy that the regenerative braking system could recapture on deceleration would require figuring out the amount of force (shortened to F in formulas) applied to the vehicle as it is moving. To calculate force, use the formula associated with Isaac Newton’s second law of motion: force = mass × acceleration. For example, F = weight of the vehicle × speed of the vehicle. 200,000 N = 8,818 lb (4,000 kg) × 164 f/s2 (50 m/s²) Or 44,962 lbf Approximately 45,000 lbf (pounds of force) is generated by an 8,818 lb (4,000 kg) vehicle at 164 feet (50 meters) per second squared. The unit lbf applied to a vehicle means that the vehicle is applying a force of one pound-force through a linear displacement of one foot. Recapturing some of this energy is the main purpose of regenerative braking. The laws of physics govern how everything operates on earth. They cannot be overcome. Energy cannot be used up; it just changes form. This is how the vehicle is able to recoup some of the expended energy used to get the vehicle’s motion and maintain that motion. Regardless of the vehicle type or design, the normal friction brakes do that same process, except they transform the energy into heat that will dissipate into the atmosphere. Friction brakes use the resistance between the brake pads or shoes and the rotor or drum to create that energy transfer.

▶▶ Hybrid 10-2 Explain the types of hybrid systems that use an ICE.

Systems That Use an ICE

The hybrid regeneration systems are usually broken down into two different systems: the series system and the parallel system. Determining which type of system the vehicle has helps with diagnosing what the issue is with the system. Before the technician determines whether there is an electrical fault, they need to determine whether the service braking system is in operable condition.

Series Regeneration Systems In a series regeneration system, the controller for the regenerative braking system is applied proportionately to the amount of braking pressure that is applied to the service brakes. Thus, the harder the brakes are being applied, the more the controller will apply



Hybrid Systems That Use an ICE

Hybrid Vehicle Power Management Discharging Mode

Charging Mode

Braking Mode

• Battery is the primary power source.

• Engine/generator is the primary power source.

• Regenerative braking is activated to absorb braking power.

• When power demand exceeds battery capacity, the engine is activated to supplement power demand.

• When battery SOC is lower than limit, engine supplies additional power to charge the battery.

• When the braking power is larger than motor or battery limits, friction braking is used.

SOC

• Once the power demand is determined, engine is operated at most efficient point.

SOC Low Limit Charge

Discharge

Charge

Wheel

Wheel

Efficiency ( engine + generator )

Engine Torque

SOC High Limit

Wheel

Power Flow

Battery

Engine

Power Bus Controller

Generator

Battery

Engine

Generator

Motor

Motor

Motor

Power Bus Controller

Engine Speed

Power Bus Controller

Battery

Wheel

Wheel

Motor

Generator

Inactive

Motor

Engine

Conditionally active

Motor

Active

Wheel

FIGURE 10-1  In a series hybrid drive layout, the force applied by the friction brakes and the hybrid motors in regeneration mode are directly related to each other. The regenerative braking system is used to enhance the effort being inputted by the driver inside the vehicle.

the regenerative braking system (FIGURE 10-1). The issues with this system stem from the fact that most hybrid vehicles have only two wheels that are driven, so the brake controller must modulate the other two wheels so that the driver will be able to maintain control of the vehicle when it is in a braking event. Because most hybrid vehicles are only two-wheel drive, the regenerative brake controller must interface with the electrohydraulic brake unit so that they may work in concert with each other to control braking events. This requires each unit to communicate over the CANbus network so that each module in the vehicle can operate together to successfully stop the vehicle while generating power to charge the battery.

Parallel Regeneration Systems In a parallel regeneration system, the hybrid drivetrain is connected directly to the internal combustion engine (ICE), which can bypass the electric motors that help ­propel the vehicle. This type of hybrid drive system is more of a “helper” system since it enhances the power produced by the ICE (FIGURE 10-2). Because it only helps to propel INTERNAL COMBUSTION ENGINE

TORQUE COUPLER

FUEL TANK TRANSMISSION POWER ELECTRONIC CONVERTER

TRACTION MOTOR

BATTERY PACK FIGURE 10-2  In a parallel drive layout on a hybrid, the engine does most of the propulsion, and the hybrid portion is present to help the engine not work as hard. This means there are fewer parts, smaller batteries, and a lesser ability to recapture the kinetic energy that has been created by movement as the series system.

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the vehicle, the amount of energy that can be recouped when the brakes are applied is less than the series version. P ­ arallel systems tend to be simpler since they still rely on the ICE to create the power required to move the vehicle. The battery packs in these types of hybrids are smaller than the series types, so there is less to charge.

Brake Master Cylinder Differences

FIGURE 10-3  Master cylinders on hybrid vehicles are very similar to a conventional master cylinder. The master cylinder must be able to operate with the ICE off, which means that an alternate form of power assist must be used. To achieve the assist, the driver can use an electric motor or an accumulator to create the desired effect.

Conventional master cylinders are very basic in that they convert mechanical leverage into hydraulic pressure that is then used to actuate the calipers and wheel cylinders. When a master cylinder is used on a hybrid vehicle, the complexity of it must be increased so that it can be monitored by the hybrid drive system, which then must adjust to allow the driver to come to a safe stop ­(FIGURE 10-3). The master cylinder in a hybrid vehicle must monitor the application of the brake pedal, provide power assist when the ICE is not operating, and modulate the slowdown of the vehicle when the operator requests that. Increasing the complexity of the hydraulic braking system requires the use of a braking electronic control module (ECM) that will allow precise control of all the functions of the braking systems. With this type of system becoming the norm in hybrid braking, the drive-by-wire approach is evolving into the easiest way to control the braking system so that it meshes well with the regenerative braking system. But the increased monitoring sensors and electronic controls on a hybrid master cylinder require more in-depth diagnosis when a failure is experienced.

Electronics to Manage a Regenerative Braking System The great thing about the hybrid’s electronic control for propulsion is that the regenerative braking system uses the same components to slow down the vehicle. A simple change of program will allow the traction motors (or trac motors, for short) to convert into generators to help recoup some of the kinetic energy that is trying to be recaptured to charge the battery in the vehicle (FIGURE 10-4). This system requires all the modules that are present

FIGURE 10-4  When the vehicle is in hybrid drive mode, the electricity is directed to the traction motor to propel the vehicle. This drawing shows a conventional type of circuit: it features a power source and load (traction motors).

When the vehicle is in regenerative braking mode, the motor controller switches the polarity of the windings inside the motor to make them opposite to the permanent magnets inside the housing. This causes power generation inside the motor and at the same time causes the magnetic field inside the traction motor, slowing the armature, which is connected to the drive wheels. This alone cannot stop the vehicle quickly enough, so the conventional friction brakes must be used as well.



Diagnosis Strategy on a Regenerative Braking System

Energy Monitor E

OUTSIDE TEMP

259

–13°F

ENGINE

BATTERY

ELEC.MOTOR Consumption

Current

20.6 MPG

FIGURE 10-5  When the vehicle is in regenerative braking mode, the display on the dashboard will show the flow of electricity back into the battery pack so that the driver knows what is happening at all times in the hybrid system.

inside the vehicle to talk to each other so that they can all be used to control the actions of the vehicle when it is in regeneration mode. The braking systems module is at the forefront of the network directing the hybrid ECM to switch the polarity of the trac motors to generate electricity instead of consuming it. Everything working together is what makes this process work; if one component fails, the process fails.

Battery Charging This chapter has discussed how the hybrid controller changes the drive motors to generators for use in the regenerative ­braking ­system. When this action happens, the motor controller changes the polarity of the electromagnets inside the drive motors to c­ onvert them into generators. As these motors slow down the vehicle with the electromagnetic field that they are now ­producing, they create energy that is being put back into the battery pack (FIGURE 10-5). Some applications use permanent magnet motors for propulsion and r­ egeneration. The polarity of permanent magnets cannot be changed, so the field coils around the armatures must be quickly switched to maintain the movement of the armature. This can be maintained only by a module that is able to switch the polarity at a very high rate. By continuing to constantly vary the polarity of the field coil or the magnetic field around the armature, controlled deceleration of the vehicle will result. Along with the deceleration control, the modulated control of the traction motors recharges the battery pack.

▶▶ Diagnosis

Strategy on a Regenerative Braking System

When diagnosing the regenerative braking system, the technician should first scan the vehicle with a compatible scan tool to pull any codes relating to the hybrid drive or braking system. These diagnostic codes can direct the technician to where to start diagnosing the issues with the vehicle. Understanding the system is the first key to determining where to start the repair, and the second step is to look up the service information for the vehicle. This includes verifying whether there is a technical service bulletin (TSB) that relates to the complaint. Some TSBs are simple repairs that fix everyday issues with the vehicles. Other TSBs are in-depth engineering fixes that require a complex repair. A hybrid vehicle is not much different from a conventional vehicle: because the principles of electricity still apply, proper education on how to safely operate around the high-voltage system will result is a quick repair and will help prevent electrocution.

10-3 Execute a diagnosis strategy on a regenerative braking system.

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Chapter 10  Hybrid Vehicle Braking Systems

Disabling HEV Battery Pack To disable the HEV battery pack on hybrid vehicles, a proper procedure must be f­ ollowed so that it is properly disabled. This procedure does not necessarily need to be completed for every repair. It must, however, be done when working near or on the high-voltage components. Always refer to the factory service information to minimize the possibility of injury. To disable an HEV battery pack, follow the steps in SKILL DRILL 10-1.

SKILL DRILL 10-1 Disabling HEV Battery Pack 1. After the vehicle is pulled into the bay, look up the hybrid battery pack disabling procedure.

2. Remove the keys from the vehicle, tag them, and put them on the work bench.

3. Disable the high-voltage battery by following the procedure listed in the service information.

Continued



Wrap-Up

261

4. Verify that the high-voltage battery pack is disabled, by measuring the high-voltage circuits with a CAT V multimeter.

5. Once it is verified that the vehicle hybrid battery is disabled, work can commence on the high-voltage systems.

As electrical vehicles become more prevalent in the automotive repair universe, the technician will have to understand how electric motors affect the braking system. The ECMs that control electric motors are able to help slow down the vehicle as the driver tries to decelerate, which allows the service braking system to live longer. With this longer life, the braking system must be maintained because it could potentially damage the electric motors if components become too worn. The precision of hybrid and electric vehicles requires all systems to operate as they are supposed to. If they don’t, the vehicle could be permanently damaged. When servicing a hybrid vehicle, proper tool usage and application is key to repairing the vehicle without breaking more parts. Working on the internal parts of a hybrid requires specialized meters, personal protection equipment (PPE), special high-voltage component removal tools, and knowledge to complete the repair. This type of vehicle should not be given to a new technician, but instead to one that has been trained, is experienced, and is not afraid to work on the complex systems in the vehicle. Proper training and use of the proper equipment results in increased work output and job quality.

▶▶Wrap-Up Ready for Review ▶▶ ▶▶

When servicing a hybrid system, the high-voltage system must first be disabled. The series regeneration system works in-line with the vehicles drive system.

▶▶ ▶▶ ▶▶

The parallel regeneration system works separately from the drive system. The hybrid regenerative system works with the service brakes to slow the vehicle down. A hybrid drive system fault disables the regenerative b ­ raking system.

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Chapter 10  Hybrid Vehicle Braking Systems

Every moving object creates kinetic energy. The force of a vehicle is determined by the size of the vehicle.

Key Terms electrohydraulic brake unit  An electric hydraulic motor that creates pressure so that the braking system can act independently of the input from the brake pedal. inertia  The resistance of an object to change the state it is in. kinetic energy  Energy that a mass has based on the motion it is in. force  The measurement of the amount of pressure created by a mass accelerating. The equation for force is F = m × a. series regeneration system  In this system, the braking system works directly in-line with the propulsion system, which means the amount of regeneration is directly related to the amount of braking input from the driver. parallel regeneration system  The drive system is separate from the regeneration system, which means that it is variable in the amount of power it can recoup.

Review Questions 1. In a hybrid vehicle, when removing the battery pack for service, the technician ___________. a. must first look up the procedure for disabling the pack b. must first look up the information on the brakes on the vehicle c. must first talk to the supervisor about how unsafe this is d. simply unplugs the battery pack, because there is no procedure. 2. All the following statements about the regenerative braking system are true, except: a. The regenerative braking system is used to help recharge the hybrid battery. b. Replacing a charging controller does not require disconnecting the hybrid power system. c. It helps slow down the vehicle. d. It helps increase fuel efficiency. 3. A hybrid vehicle uses the same type of foundation brakes as a conventional vehicle. True or false? a. True. b. False. 4. Kinetic energy in a hybrid vehicle can be eliminated by ____________. a. burning it off b. converting the energy into heat and electricity c. saving it in a containment vessel d. allowing it to rest 5. When replacing the master cylinder on a hybrid vehicle, the technician can use _____________. a. only a master cylinder that has three ports b. only a master cylinder that is made for this application c. any master cylinder that fits the bolt pattern d. any master cylinder that has worked on other hybrids

6. Once the vehicle is put into regenerative braking mode the braking ECM changes the motors into generators. These generators are used to _______. a. power the lights b. power the radio c. power the heat d. recharge the battery 7. On a hybrid vehicle, the dash display of the hybrid system shows that the power flow is moving toward the traction motors away from the battery. What mode is it in? a. Drive mode. b. Regenerative mode. c. Plug-in mode. d. Stationary mode. 8. Hybrid vehicles do not have typical friction brakes, since all of the braking comes from the regenerative function. True or false? a. True. b. False. 9. If there is a fault within the hybrid vehicle regenerative braking system, the whole system ______________. a. goes into limp mode b. applies 100% c. becomes inoperative d. stops the vehicle from operating 10. When diagnosing a hybrid electronic control module (ECM) issue, the technician must do which of the following first? a. Replace the battery pack. b. Verify the issue. c. Scan the vehicle. d. Look for replacement parts.

ASE Technician A/Technician B Style Questions 1. Technician A says that if the battery pack for a series hybrid regenerative braking system is full the regenerative braking shuts off. Technician B says that a parallel regenerative brake system doesn’t work when the vehicle is being accelerated. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that monitoring the braking pressure from the driver is necessary to determine the amount of potential regeneration. Technician B says that some hybrid vehicles are brake by wire, which does not have a mechanical link to the brake master cylinder. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B



3. Technician A says that a series regeneration system can recapture more energy than a parallel system. Technician B says that a parallel regeneration system is more complex than a series regeneration system. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that hybrid vehicles have different modes, which are available for the ECM (electronic control module) to change to as the needs of the driver change. Technician B says that all hybrid drive systems are the same. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that using different sizes of tires on the drive wheels of a hybrid will not cause any issues with the regeneration mode. Technician B says that changing the wheel size on a hybrid vehicle will increase the ability to recapture energy in regenerative braking mode. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that regeneration can happen only when the throttle is closed and the vehicle is decelerating. Technician B says overriding the regeneration system will allow it to function when the vehicle is stationary. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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263

7. Technician A says that regenerative braking works better on a series hybrid vehicle than on a parallel one. Technician B says that the bigger the vehicle is, the more kinetic energy is created. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says that not all hybrid vehicles have a mechanical connection between the engine and the drive wheels. Technician B says that only hybrid vehicles have kinetic energy. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says that when performing a brake pad and rotor replacement, the parts must be balanced to the vehicle. Technician B says that the foundation brakes are the main stopping force of the vehicle. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that the regenerative braking system works independently of the other systems on the vehicle. Technician B says that the regenerative braking system uses inputs from every system in the vehicle to maintain control of the vehicle. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

CHAPTER 11

Advanced Braking Systems: Electronic Brake Controls Learning Objectives ■■ ■■ ■■ ■■

11-1 Explain the basic operation of EBC systems. 11-2 Outline ABS components. 11-3 Translate ABS operation. 11-4 Identify ABS hydraulic components.

■■ ■■ ■■ ■■

11-5 Explain how wheel speed sensors operate. 11-6 Summarize what the electrical components do inside an ABS. 11-7 Develop a diagnostic process for ABSs and EBC systems. 11-8 Diagnose wheel speed sensors and tone wheels.

You Are the Automotive Technician A longtime customer brings their 2012 Ford Explorer into the shop to get their brakes inspected. They claim that the brake pedal pulsates when they apply the brakes.You ask them whether they have had any recent work done on the vehicle.The customer explains that he recently had new tires put on and that after leaving the shop, a car pulled out in front of them, so they had to lock up the brakes to avoid hitting the other vehicle. The customer was startled by very heavy brake pedal pulsations and says that ever since then, the brakes have had a small pulsation that seems to be getting worse. They wonder if there is a problem with the antilock brakes.

1. Is there a problem with the ABS? What will you say to the customer? 2. What do you suspect is causing the pedal pulsations, and how did they occur? 3. How would you diagnose a problem when the ABS warning lamp is illuminated? 4. In what two primary ways does traction control reduce wheel slip?





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▶▶ Introduction Electronic brake control (EBC) systems have greatly increased the safety of vehicles over the years by integrating computer-­ controlled hydraulics into the braking system (FIGURE 11-1). ­Standard hydraulic brake systems have limitations on how effecYikes, brakes tively they can stop a vehicle. The driver can input braking force to Oh No.......... don't fail me now!! the system only through the brake pedal, which applies hydraulic pressure at predetermined ratios to the front and rear brakes. In a panic situation, the driver is unable to apply the exact amount of force required to maintain the maximum amount of braking. With too little force, the vehicle does not stop as quickly. With too much force, the tires skid, making the vehicle’s stopping distance even longer. At the same time, if the front wheels skid, the driver loses the ability to steer the vehicle. If the rear wheels skid, the car could spin out and possibly roll over. Even if the driver could apply the perfect amount of braking force, there is no way to accommodate different amounts of traction at each wheel under all driving conditions. When tires on one side of the vehicle are on dry pavement and the other tires are on wet FIGURE 11-1  Brakes must be fully functional in case of an emergency. pavement, braking the vehicle is likely to lead to a loss of control. In the quest for increased safety, manufacturers developed a series of EBC systems. The first-generation EBC system was the antilock brake system (ABS), which was designed to prevent wheels from locking up under braking conditions, to help the driver maintain steering control of the vehicle. Maintaining steering control allows drivers to be better able to avoid collisions. ABS also shortens most panic-stop distances for the same reason. ABS was a great start, but manufacturers found that by adding a few components to the system and modifying the computer’s software, they could provide traction control capability to the vehicle. This helps the driver maintain control of the vehicle while driving, compared to braking the vehicle. Then, with a few more components and advancements in the software, they were able to provide vehicle stability control, which helps prevent loss of driving control and vehicle rollovers. And the enhancements keep coming, with systems that provide engine braking control, trailer sway control, crash avoidance braking, and other features.

▶▶ Electronic 11-1 Explain the basic operation of EBC systems.

Brake Control Systems

ABSs use a computer that monitors the speed of each wheel as the brakes are applied. If one or more wheels begin to lock up, the computer sends electrical signals to solenoid valves that momentarily hold or release hydraulic pressure to that wheel until it speeds up. Once that happens, the computer allows hydraulic pressure to be applied to that wheel again, slowing it down. This process is repeated very rapidly as the vehicle is brought to a stop. Because the tires remain in rolling contact with the road surface, the vehicle can be steered, allowing the driver to maintain directional control (steerability) (FIGURE 11-2). These actions are completely dependent on the driver applying pressure to the brake pedal. Although the basic ABS does a good job of managing the braking effort of the driver in a panic-stop situation, it is limited to using the hydraulic pressure the driver exerts on the system. This means that the standard ABS by itself cannot increase the hydraulic pressure in the ABS; nor can it apply hydraulic pressure separate from the driver. As long as the driver is exerting firm pressure on the brake pedal, ABS can work fully. The second-generation EBC system was the traction control system (TCS). With the addition of a high-pressure pump and a few isolation valves to the basic ABS, manufacturers found that they could assist the driver in minimizing wheel slip while the vehicle is being accelerated. This is especially effective on slippery road surfaces, such as gravel, snow, and ice. In most vehicles, the vehicle’s traction is only as good as the traction on the tire with the least traction. So if one tire is on a patch of ice, the vehicle may not have enough traction



Electronic Brake Control Systems

267

Wheel Slipping With ABS

Ice

Brake Applied

Control Unit Hydraulic Unit

TCU

Without ABS

Torque Increased to Non-Slipping Wheel Wheel Speed Sensors FIGURE 11-2  ABS helps the driver avoid collisions and accidents in a panic-stop situation by maintaining steering control over the vehicle.

FIGURE 11-3  If a wheel is slipping, the TCS system applies the brake pressure to the slipping wheel. This causes torque to be sent to the wheel that has more traction.

to move and will ultimately become stuck. The TCS system applies brake pressure to the slipping tire, which causes more of the engine’s torque to be transmitted to the wheel or wheels with the most traction (FIGURE 11-3). If necessary, the TCS system can also request that the engine’s powertrain control module (PCM) reduce the power output of the engine to further enhance traction. These actions are controlled by PCM and do not require any input from the driver. The next-generation EBC system was the electronic stability control (ESC) system. ESC takes the ABS and TCS systems one step further. By adding sensor information regarding the driver’s directional intent (from the steering wheel position sensor) and sensor information regarding the vehicle’s actual direction (from the yaw sensor), the EBC module (EBCM) can detect the start of an understeer, an oversteer, or a potential rollover condition. Understeer and oversteer are conditions that happen when a vehicle is traveling too fast for a particular corner. During understeer, the vehicle’s front wheels are turned more sharply than the vehicle’s path (FIGURE 11-4A). The front tires are actually sliding somewhat sideways toward the outside of the corner. The greater the understeer, the more the tires slide. Understeer is also referred to as “push,” as in “the vehicle is pushing in the corners.” Oversteer is just the opposite. It occurs when the vehicle is turning more sharply than the front wheels are being steered (FIGURE 11-4B). This happens when the rear tires are sliding sideways toward the outside of the corner. Oversteer is also referred to as “loose,” as in “the vehicle is getting loose in the corners.” The rear tires lose traction while cornering during oversteer. Most passenger vehicles are designed to have a bit of understeer because this condition is easier for a driver to recover from than an oversteer is. Using information provided by the sensors of the ESC system, plus the wheel speed sensors, the EBCM monitors the stability of the vehicle and can command individual brakes to be applied and can request decreased engine torque, as necessary. For example, if a vehicle is traveling too fast around a right-hand corner and the front wheels are starting to lose traction (understeer), the control system can apply the right rear brake to help pivot the vehicle around the right rear tire. This assists the vehicle in turning and at the same time slowing down the vehicle slightly. If additional measures are needed, additional brakes can be applied and the engine torque reduced. The control system performs these functions automatically without any driver input other than steering the vehicle in the desired direction.

▶▶TECHNICIAN TIP It is important for customers to know that ABS, TCS, and ESC are not guarantees of avoiding a collision or an accident. These systems are designed for drivers who are driving in a responsible manner, to help them avoid an accident. Drivers can easily exceed the ability of these systems.

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Chapter 11  Advanced Braking Systems: Electronic Brake Controls Rear Slip Angle

Rear Slip Angle

Front Slip Angle

Front Slip Angle

Actual Direction Intended Direction

Intended Direction Actual Direction Cornering Force

A

Cornering Force

B

FIGURE 11-4  A. Understeer condition. B. Oversteer condition.

▶▶ Antilock 11-2 Outline ABS components

Brake System Components

The ABS is designed to prevent wheels from locking or skidding, no matter how hard the brakes are applied or how slippery the road surface, to maintain steering control of the vehicle, and to shorten stopping distances. The primary components of the ABS are shown in FIGURE 11-5 and listed here: ■■

■■ ■■

■■

■■

■■

ABS master cylinder: a system that creates hydraulic pressure for each of the two hydraulic brake circuits. Power booster: a device that boosts driver brake pedal force on the master cylinder. EBCM or electronic control unit (ECU): an onboard computer that is programmed to monitor sensor data and send output control signals to electronic solenoid valves, which modify brake pressure to individual wheel brake units. Hydraulic control unit (HCU) or modulator: a device that contains electric solenoid valves controlled by the EBCM to modify hydraulic pressure in each hydraulic circuit (FIGURE 11-6). Most systems also contain an accumulator to store brake fluid under pressure. Wheel speed sensor: a device that monitors wheel speed and sends that signal to the EBCM. Brake switch: an on/off switch mounted at the brake pedal that informs the EBCM whether the driver is applying the brakes.

Electronic Brake Control Module Hydraulic Control Unit Wheel Speed Sensors

Wheel Speed Sensors FIGURE 11-5  A typical ABS.

Brake Switch Power Booster Master Cylinder FIGURE 11-6  An HCU.



Antilock Brake System Operation

The EBCM may be located inside the vehicle or mounted near the HCU, or it could be integrated into the HCU. In many cases, it is a separate module from the PCM and may be part of the vehicle’s body control module (BCM). The body control module is the computer that controls the electrical system in the body of the vehicle. The EBCM receives input signals from the ABS sensors, compares these data to information stored in its memory, decides what actions are necessary, and sends output commands to the HCU. The HCU or modulator is connected in line with the brake lines between the master cylinder and the wheel brake units. It houses electric solenoid valves that control the flow of brake fluid to each wheel. The HCU receives operating signals from the EBCM to control the brakes under ABS conditions. The power booster and master cylinder assembly are mounted on the firewall. In most current applications, these FIGURE 11-7  A wheel speed sensor and tone wheel. components operate similarly to non-ABS power boosters and ­tandem master cylinders. Some manufacturers use a portless master cylinder to allow brake fluid to return to the master ­cylinder ­reservoir more easily than would occur using a master cylinder fitted with a compensating port. When the brakes operate without ABS action, the brake pressure is controlled by the driver’s foot pressure, which is assisted by the power booster. In other words, the ABS affects brake pressure only when one or more wheels start to skid. The wheel speed sensor consists of a toothed tone wheel (or tone ring) that rotates with the road wheels and a pickup assembly that generates a speed signal. In many applications, the wheel speed sensor is located near the wheel hub (FIGURE 11-7). The wheel speed sensor sends to the EBCM an electrical signal that varies with the speed of the wheel. Wheel speed sensors can be variable reluctance sensors (magnetic induction), generating an analog AC (alternating current) sine wave signal (FIGURE 11-8). FIGURE 11-8  An oscilloscope pattern from a wheel speed sensor. Wheel speed sensors can also be of the magneto-resistive or Halleffect type, generating a digital square wave signal. These signals can be used by the EBCM to determine the speed of each wheel. These sensors are covered in much greater depth in the ABS Components section.

▶▶ Antilock

Brake System Operation

When the ignition switch is turned on, the ABS controller illuminates the yellow ABS ­warning lamp and performs an automatic self-check of the system. If the system check passes, the controller will extinguish the warning lamp, indicating to the driver that the ABS is functional. Some ABSs perform an additional self-check once the vehicle is traveling more than approximately 3–5 mph (4.8–8 kph). Failures in the ABS cause the controller to ­illuminate the ABS warning light in the instrument panel. If the light is illuminated, the ABS is shut down and won’t operate. As the wheels start to turn, the wheel speed sensors generate small electrical signals and send them to the EBCM. When the brakes are applied, the wheels’ rotational speed is reduced. As the speed changes, the signal sent to the EBCM changes in like manner. If the control unit detects that a wheel might be slowing too quickly and starting to lock, it sends output signals to the appropriate solenoid valve in the HCU to modify the hydraulic pressure to the affected wheel brake unit.

Principles of ABS Braking Braking force and the tendency of the wheels to lock up are affected by a combination of factors, such as the friction of the road surface; the type, condition, and loading of each tire;

11-3 Translate ABS operation.

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Chapter 11  Advanced Braking Systems: Electronic Brake Controls

and the difference between the vehicle speed and the speed of the wheels. It should be noted that maximum traction happens with approximately 10–20% Dump Solenoid Isolation Solenoid tire slip. Thus, maximum braking traction occurs when the wheels are rotating 10–20% slower than the vehicle speed. In the same way, maximum traction during acceleration occurs when the wheels are rotating 10–20% faster than To the vehicle speed. At the same time, traction falls off quickly above approxiWheel mately 20% wheel slip, which is why ABS is so effective. It allows just enough Brake slip to keep the tires at close to their maximum traction. It does so by rapidly modulating the hydraulic pressure in the vehicle’s brake system. From During normal braking, as the rotational speed of each wheel falls equally, Master no ABS intervention is needed. In this condition, the EBCM does not enerCylinder gize the solenoid valves in the hydraulic unit. The master cylinder hydraulic ­pressure is applied to the wheel brake units, and the ABS is not involved. HowHigh-Pressure High-Pressure Accumulator ever, even though the ABS is passive during normal braking, the EBCM is conPump Discharge Pump Suction Valve stantly m ­ onitoring the speed of each wheel, looking for any wheel that begins to ­decelerate more rapidly than any of the other wheels. FIGURE 11-9  HCU solenoid valve arrangement for modulating ABS hydraulic pressure. If one wheel speed sensor signals more severe wheel deceleration, which means the wheel is beginning to slip, the EBCM sends current to the appropriate solenoid valve (FIGURE 11-9). The first level of valve action isolates that brake circuit ▶▶TECHNICIAN TIP from the master cylinder. This stops the braking pressure at that wheel from rising and Drivers should be taught to expect ABS keeps it constant. If the wheel speed sensors indicate that the wheel is still decelerating too brake pedal pulsation when in a panic rapidly, the EBCM commands the appropriate solenoid valve to release braking pressure. stop. Some drivers who have never The solenoid valve opens a passage from the brake circuit, releasing the hydraulic pressure experienced this actually let up on the to that brake unit. Brake fluid is released from the specific brake circuit back to the master brake pedal because of the rapid pulsacylinder. Pressure in the brake circuit is reduced so that the wheel is not being braked. tions and accompanying noise. When in If the wheel speed sensors indicate that reducing the brake pressure is allowing the wheel a panic stop, drivers should push hard to roll again, the EBCM deenergizes the solenoid valves. This lets the hydraulic p ­ ressure from on the brake pedal and not let up until the master cylinder be applied to the brake again. This cycle repeats itself at up to 16 times per the vehicle is stopped or out of danger. second. It is normal in an ABS for the valves in the HCU to keep changing position as they Brake-by-wire systems are not subject modulate the applied brake pressure. These changes in valve position normally cause rapid to this brake pedal pulsation situation hydraulic pulsations, which on some vehicles can be felt by the driver through the brake pedal. because the brake pedal is not part of the hydraulic system. The solenoid valves also make a fairly loud clicking noise as they cycle on and off.

▶▶ ABS 11-4 Identify ABS hydraulic components.

Hydraulic Components

The ABS hydraulic components work in tandem with the service braking system components so that it is a seamless application. Having a redundant braking system is necessary to ensure the safety of the driver. Use of electronics for vehicle monitoring and brake control is necessary for the operation of the ABS.

ABS Master Cylinder ABS master cylinders come in two major configurations: integral and non-integral (­ FIGURE 11-10). Integral ABSs are found mostly on older vehicles. They combine the tandem master cylinder, HCU, and power booster in one unit. The power booster consists of a high-pressure electric pump and accumulator that operates the integrated master cylinder. Brake fluid passes from the master cylinder portion of the assembly to the HCU portion, where pressures are modified by the computer-controlled solenoid valves. Non-integral ABSs use a fairly standard tandem master cylinder and a typical vacuum or hydraulic power booster. The booster assists the driver in applying force to the master cylinder. The master cylinder sends fluid under pressure to the HCU, which is a separate assembly that is installed in line with the brake lines between the master cylinder and the wheel brake units. If the pressure has to be modified, the computer-controlled solenoid valves in the HCU will carry out the commands.

Purpose and Operation of the ABS Master Cylinder Non-integral ABS master cylinders are usually identical to non-ABS master cylinders. They both use primary and secondary pistons in a common housing with a common bore. Some



ABS Hydraulic Components

271

MAX

Reservoir

MIN

Primary Piston

Stopper Pin

FIGURE 11-10  Non-integral and integral master cylinder assemblies.

Secondary Piston

Primary Piston Seal

FIGURE 11-11  Portless ABS master cylinder.

of these master cylinders use a portless ABS master cylinder design, which does not use a compensating port on the secondary circuit. Instead, the secondary piston incorporates a center valve (FIGURE 11-11) that controls the opening and closing of a supply port in the piston. At rest, the supply port is open and connects the reservoir with the front brake circuit. The primary piston still uses an inlet port and a compensating port; therefore, the portless design is used only on the secondary circuit. When the brake is applied, the primary piston moves and closes its compensating port. Fluid pressure in the primary circuit rises. It acts with the primary piston spring to move the secondary piston forward, closing the center valve. Pressure builds in the secondary circuit, keeps building in both circuits, and applies the brakes in both circuits. If braking conditions are such that the hydraulic modulator must return brake fluid to the master cylinder, then for the front brake circuits, brake fluid is returned to the front section. This forces the secondary piston back against the force of the primary piston spring and the rear brake pressure. If enough brake fluid returns, the center valve opens and allows the brake fluid to return to the master cylinder reservoir. If brake fluid is returned from the rear brake circuit, the secondary and primary pistons tend to be forced apart, which generally moves the primary piston rearward. If it travels far enough, brake fluid will return to the reservoir through the compensating port. The amount of brake fluid that returns to the master cylinder is determined by the degree of antilock braking control. With as many as 16 ABS control cycles per second, the rapid changes in hydraulic pressure cause brake fluid pulsations to be sent back to the master cylinder; these pulsations can be felt by the driver at the brake pedal.

How the Hydraulic Control Unit Operates The ABS control module (or EBCM) sends commands in the form of electrical signals to the HCU. The HCU executes the commands, using one or two solenoid valves for each hydraulic circuit, depending on the type of HCU. Because the control valves are situated between the master cylinder and the wheel brake units, they can apply, block, or release hydraulic pressure going to the brake units. In a normal non-ABS braking scenario, brake pedal force is transmitted to the master cylinder and then through the nonenergized open isolation valves to the brake units at the wheel. When the signals from the wheel speed sensors show no tendency for the wheels to lock up, the hydraulic pressure from the master cylinder flows freely through the HCU to the brake units at each wheel. When the control unit detects any lockup tendency, it sends a command current to the isolation solenoid valve for that brake circuit. This current causes the solenoid valve to close, isolating the brake circuit from the master cylinder. That holds the hydraulic pressure between the solenoid valve and the brake circuit constant, regardless of whether the master cylinder hydraulic pressure rises or falls.

▶▶TECHNICIAN TIP Many, but not all, HCUs are sealed units and cannot be serviced. Whenever a sealed HCU is faulty, it will have to be replaced. This can be quite costly.

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Chapter 11  Advanced Braking Systems: Electronic Brake Controls Front

Front

ABS ECU

A

Single Channel

ABS ECU

B

Two Channel

Front

Front

If the wheel speed sensor signals that the excessive wheel deceleration is continuing, the control module commands the dump valve for that brake circuit to open. This reduces the braking pressure by opening a passage from the brake circuit to the accumulator. A pump in the HCU sends brake fluid back to the master cylinder, pushing one or both pistons rearward in the bore and venting to the reservoir. If the sensor indicates that the lower pressure has allowed the wheel to speed up, the EBCM signals the dump valve to close and the isolation valve to open. The hydraulic pressure from the master cylinder is again allowed to apply the brakes, and the wheel is again slowed down. This process continues until the vehicle comes to a stop or until the driver lifts their foot from the brake pedal. In most standard ABSs, the hydraulic pressure in the brake circuits can never rise above the master cylinder pressure.

Types of Hydraulic Control Units There are a number of HCUs that vehicle manufacturers use, and they generally fall into a few categories. The first category relates to how many channels the system has. A channel generally means the number of electrical wheel sensor circuits and hydraulic circuits that a system has (FIGURE 11-12). A single-channel system uses one sensor circuit, with the speed sensor typically located in the differential and one hydraulic control circuit to control both rear wheels. Since it has only one sensor, the ABS has nothing to compare the C D Four Channel Three Channel deceleration to. So maximum deceleration is programmed into the control unit. If the deceleration of the differential exceeds the maxiFIGURE 11-12  The four types of ABS channels. A. Singlemum deceleration, then the HCU will be activated. channel system. B. Two-channel system. C. Three-channel system. A two-channel system is similar, but it uses two separate speed D. Four-channel system. sensors and hydraulic control circuits, one for each rear wheel. The two hydraulic control circuits apply brake pressure to the rear wheels, separately. A three-channel system is configured so that each front wheel has its own speed sensor and hydraulic control circuit, whereas the rear brakes use a single speed sensor with a single hydraulic control circuit. A four-channel system uses separate speed sensors and hydraulic control circuits for each of the four wheels. Since these systems have more than one sensor, the control unit watches not only for wheel deceleration that is faster than the programmed maximum but also for individual wheels decelerating faster than others. This gives a more robust ABS function. Another difference among types of HCUs is the number of solenoid valves per hydraulic control circuit. Some HCUs use a single, three-position solenoid valve per circuit; others use dual, two-position valves per hydraulic circuit (FIGURE 11-13). The first position of the single, three-position valve allows brake fluid to flow through the apply port while blocking the release port. The second position blocks the apply port and the release port. The third position blocks the apply port and opens the release port. Thus, the single, three-position valve has all three conditions: apply, hold, and release. The dual, two-position valve style of HCU uses one solenoid valve to open and close the apply port. This is commonly called the isolation valve. When this valve is not energized, the apply port is open. The second solenoid valve opens and closes the release port. When this valve is not energized, the release port is blocked. The EBCM operates each of these valves independently to obtain the apply, hold, and release functions. Because there are twice as many solenoid valves and because each valve needs its own electrical control circuit, the EBCM is more complicated and costly to build. Therefore, EBCMs are specifically designed to work with only the specified type of HCU, and therefore also, EBCMs and HCUs cannot be randomly interchanged. Another difference between HCUs is the type of accumulator used: low pressure or high pressure. Low-pressure accumulators hold brake fluid in a spring-loaded chamber ABS

ABS ECU



Wheel Speed Sensors Operate Dump Valve (normally closed)

From Master Cylinder Isolation End

273

Isolation Valve (normally open)

Pump

Solenoid To Wheel Brake

Three-Way Valve Assembly (Apply Position)

To Wheel Brake A

Dump End

Accumulator

B

To Accumulator Valve

From Master Cylinder

FIGURE 11-13  A. Single, three-position valve. B. Dual, two-position valves.

when it is released by the dump valves during an EBC event. The hydraulic pressure remains fairly low because an electric pump returns the released brake fluid to the master cylinder when brake fluid in the accumulator reaches a certain point. When the electric pump turns on, the fluid returning to the master cylinder pushes the brake pedal toward the driver’s foot, causing the brake pedal to rise. This can be confusing to drivers because it feels like someone is under the dash pushing the brake pedal back toward them. High-pressure accumulators are used to store brake fluid under high pressure for one of two purposes: to be used as a power booster for applying the integrated master cylinder or to be used to independently to apply the wheel brake units when the EBCM commands it. When used as a power booster, pressure in the accumulator is maintained by a high-­pressure electric pump. The pump is activated by a pressure switch and relay when the hydraulic pressure falls below a certain point. When the pressure reaches the specified upper pressure limit, the pressure switch opens and deactivates the electric pump. The hydraulic pressure is then used to boost the driver’s foot pressure on the master cylinder when the driver depresses the brake pedal. If the high-pressure pump fails for any reason, the accumulator holds enough brake fluid at high pressure to apply the brakes 10 to 20 times before the boost is used up. If that occurs, the brakes will still operate but will require much higher foot pressure. The accumulator used to supply brake pressure to the HCU also uses a high-pressure pump, pressure switch, and relay to maintain an operating pressure of approximately 1,200– 2,700 psi (8,274–18,616 kPa), depending on the system (FIGURE 11-14). The high-pressure pump pushes the brake fluid against a high-pressure nitrogen chamber, which holds pressure on the brake fluid. The hydraulic pressure is used to independently apply the brakes during a TCS or ESC event. If the high-pressure pump fails while driving and the hydraulic pressure falls below the pump’s specified “on” pressure, the EBCM will disable the ABS and illuminate the yellow warning lamp, alerting the driver to an ABS fault.

▶▶ Wheel

▶▶SAFETY TIP Be sure to carefully follow the manufacturer’s procedures when working on EBC systems. High-pressure brake fluid stored in the accumulator is dangerous.

Speed Sensors Operate

Wheel speed sensors create electrical signals based on the rotational speed of each wheel they monitor. Wheel speed sensors do so by using principles of electromagnetism to generate an analog or digital electrical signal. This signal is read by the EBCM to determine the speed of each wheel as well as the rate of deceleration of each wheel. This information is used to determine whether a wheel is starting to lock up and skid. A wheel sensor assembly consists of a toothed tone wheel (or tone ring) that rotates with the wheels, and a stationary pickup assembly attached to the hub or axle housing. The pickup assembly and tone wheel do not touch each other; a small gap, called an air gap,

11-5 Explain how wheel speed sensors operate.

274

Chapter 11  Advanced Braking Systems: Electronic Brake Controls Dump Valve Isolation Valve Accumulator Valve Control Unit

To RHF Brake

To LHR Brake Pump Drive Eccentric (driven by ABS motor)

IN (from master cylinder)

ABS Pump Element

Check Valves IN (from master cylinder) To LHF Brake

To RHR Brake

Control Unit

FIGURE 11-14  A high-pressure accumulator.

must be maintained at the specified clearance. Because there is no mechanical connection, there is almost no wear unless a foreign object gets between them.

Types of Wheel Speed Sensors The three most common types of wheel speed sensors are the variable reluctance (magnetic induction style), magneto-resistive, and Hall-effect styles. The variable reluctance type is simpler and usually less expensive for manufacturers to use. This style is sometimes called a passive system because it is self-contained and needs no outside power to function. Magnetic induction occurs when the teeth on the tone wheel pass the sensor, creating an analog AC voltage signal. As each tooth of the tone wheel approaches the pickup, the magnetic field creates a small voltage that pushes current flow in one direction inside the pickup assembly. As each tooth leaves the pickup assembly, voltage is generated that pushes current flow in the opposite direction. This process creates a full-cycle sine wave for each tooth on the tone wheel (FIGURE 11-15). The faster the wheel is turned, the faster the sine wave rises and falls. The speed at which the sine wave Low Speed (A) rises and falls is referred to as frequency. Frequency is measured in hertz, where one hertz equals one full-cycle sine wave per second. The height of the sine wave, called its amplitude, also tends to change Moderate Speed (B) with speed. At very slow vehicle speeds, when the vehicle is just creeping along, the amplitude of the sine wave is very low. As the speed increases, so does the amplitude, along with the frequency. This AC signal is sent to High Speed (C) the ECU, where it is processed and then compared to the AC signals from the other wheels to determine wheel lockup. Most variable reluctance wheel speed sensors are two-wire sensors, which complete the circuit back to the ECU. The variable reluctance FIGURE 11-15  Wheel speed sensor sine wave. A. Signal sensor assembly consists of a coil of wire around a permanent magnet, during low vehicle speed. B. Signal during moderate vehicle speed. C. Signal during high vehicle speed. with each end of the coil connected to one of the wheel speed sensor



Wheel Speed Sensors Operate

275

ABS ECU Sensor Signal

Inductive Sensor Coil Soft Iron Core

Tone Wheel

Magnetic Field Permanent Magnet

Air Gap (critical for correct operation)

FIGURE 11-16  Variable reluctance wheel speed sensor assembly.

terminals, which connect directly into the EBCM (FIGURE 11-16). Because this type of sensor operates on principles of magnetism, the air gap between the toothed tone wheel and sensor is critical. If the air gap is too small, the parts could contact each other, damaging them. If the air gap is too large, the sensor output signal to the ECU could be too weak and trigger a code or cause the sensor to work intermittently. One drawback to the variable reluctance sensor is that because it depends on the speed of movement of the tone wheel to create a signal, it does not function effectively below vehicle speeds of around 5 mph (8 kph). In other words, the amplitude of the sine wave it creates at slow speeds is not high enough for the EBCM to read it. This can prevent the ABS from functioning during the last part of a braking event. On a very slippery road surface, such as ice, the lack of ABS functionality at that speed could lengthen the stopping distance significantly. The magneto-resistive and Hall-effect sensor systems are called active systems because they require an outside power source to operate. If the sensor loses power or ground, it cannot generate an output signal. The power wire originates from the EBCM and normally supplies the magneto-resistive or Hall-effect sensor systems with a reference voltage of between 5 and 12 volts, depending on the manufacturer. This helps ensure that the sensor is not affected by changes in the vehicle’s electrical system voltage. A signal wire transmits the output signal from the sensor to the EBCM. The magneto-resistive and Hall-effect sensors can have a three-wire arrangement, with the third wire being a dedicated ground, or a twowire arrangement, with ground being provided by the chassis. The magneto-resistive speed sensor and Hall-effect wheel speed sensor types operate similarly to all Hall-effect sensors. A reference voltage and ground are supplied to the sensor assembly, where internal circuitry causes a small current to flow across the semiconductor bridge/Hall material (FIGURE 11-17). If the bridge/Hall material is exposed to a magnetic force, the magnetism forces the current to flow to one side of the bridge/Hall material. This produces a small difference in voltage across the sides of the bridge/Hall material (FIGURE 11-18). Voltage is then amplified and processed into a digital “on” signal (circuit is pulled to ground) and sent to the EBCM. As the magnetic field is removed, the small signal voltage across the bridge/Hall material falls to 0 volts. The signal sent to the EBCM will be a digital “off ” signal (reference voltage). As the magnetic field is alternately applied and removed, the sensor will send a digital square wave on/off signal corresponding to the changes in the magnetic field. Because the magnetic field does not have to be moving in order for the bridge/Hall-effect voltage to be created, the sensor works all the way down to 0 mph (0 kph). This allows the ABS to continue functioning until the vehicle comes to a virtual full stop.

▶▶TECHNICIAN TIP Testing wheel speed sensors depends on knowing which kind of sensor the vehicle uses. Do not assume all two-wire sensors are of the variable reluctance style. With the ignition switch set to the Run position and the wheels stationary, use a digital multimeter (DMM) to properly back-probe both sensor wires for v­ oltage. If neither wire has voltage, suspect a variable reluctance sensor. If one of the two wires has a reference voltage, the sensor at hand is likely a magneto-­resistive or Hall-effect type.

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Chapter 11  Advanced Braking Systems: Electronic Brake Controls

ABS ECU 12V

Magnetic Tone Wheel

Sensor Signal

5V

North Pole South Pole

Hall-Effect Sensor

Magneto-Resistive Element (simplified) IC Chip FIGURE 11-17  Hall-effect wheel speed sensor assembly.

v 15.0 13.5 12.0 10.5 9.0 7.5 6.0 4.5 3.0 1.5 0.0

ms 0

10

20

30

40

50

60

70

80

90

100

80

90

100

Road Speed Sensor (Hall Effect)—Slow speed

v 15.0 13.5 12.0 10.5 9.0 7.5 6.0 4.5 3.0 1.5 0.0

ms 0

10

20

30

40

50

60

70

Road Speed Sensor (Hall Effect)—High speed FIGURE 11-18  Hall-effect operation.



Electrical Components in an ABS

▶▶ Electrical

277

Components in an ABS

To operate the ABS on a vehicle, the use of electrical sensors, modules, and actuators is a requirement. The sensors tell the modules what is happening with the vehicle, and then the module can then use the actuators to help the driver to regain control the braking system. Everything works together to ensure that the vehicle is operable in all conditions.

11-6 Summarize what the electrical components do inside an ABS.

Brake Switch In addition to activating the rear brake lights, the brake switch sends an electrical input signal to the EBCM, telling it whether the driver is applying the brakes. If the brakes are being applied, the EBCM will activate the appropriate solenoid valves if the wheel speed sensors signal that the wheels are starting to lock up. If the brake switch indicates that the brakes are not being applied and the wheel speed sensors are showing unequal speeds, indicating a slippery road surface, then the EBCM on some vehicles will illuminate a low-traction warning lamp, alerting the driver to the low-traction condition. The brake switch is a normally closed switch, meaning that if the switch is not affected by any outside force, electrical current will flow through it. The brake pedal pushes the brake switch open when the brakes are released (FIGURE 11-19). As soon as the driver steps on the brake pedal, the spring in the brake switch closes the contacts and sends electrical current (signal) to activate the brake lights. This electrical signal is also sent to the EBCM, signaling to it that the driver is applying the brakes. In some systems, the brake switch is used only to signal the body control module, which then sends current to illuminate the brake lights. More advanced EBC systems may use a brake pedal position sensor, which indicates how far and fast the brake pedal is being pushed (FIGURE 11-20). It sends a variable signal based on the application of the brakes, which the EBCM uses to determine brake pedal travel and speed. This gives the EBCM additional information about the type of braking being performed, which can be used to modify the ABS intervention.

Antilock Brake System Electronic Brake Control Module The EBCM is made up of electronic circuitry to process input signals; an electronic data processor; computer memory; and output drivers to control the output devices, such as the electric solenoid valves. The EBCM is programmed from the manufacturer to make brake control decisions and send output commands to the controlled devices based on sensor input data, which are compared to the data maps in its memory. These maps are designed to From Ignition

Locknut Pedal Released

Adjustment Thread Switch Contacts

From Ignition

Pedal Applied

Spring FIGURE 11-19  The brake light switch is an important input

for the ABS. The switch is held open by the released brake pedal.

FIGURE 11-20  A typical brake pedal position sensor.

Chapter 11  Advanced Braking Systems: Electronic Brake Controls

account for all of the reasonable braking conditions that the vehicle could experience. The EBCM continuously monitors the sensor data for any indication that one or more wheels are about to lock up. The EBCM receives signals from several sources (FIGURE 11-21). A switch at the brake pedal provides a brake on/off condition or, on some vehicles, a brake pedal position signal. An input from the ignition switch signals that the driver has turned the ignition on. Some control units monitor the battery voltage and use the rise in charging system voltage to indicate that the engine is actually running. The vehicle speed sensor reports the speed of the vehicle. Each of these input signals is used by the EBCM to know whether a wheel starts to lock while the driver is applying the brakes and which ABS actions are necessary to prevent a full skid condition. KL 15

ESC Lamp

KL 30

EBCM Main Relay

!

ABS Lamp

KL 15

ABS

ESC Switch

ESC

Power Supply Power Supply

LF Pump

Wheel

Inlet (4X)

RF

Outlet (4X)

Speed

ESC III Hydraulic Unit

Changeover (2X)

LR

Pre-load (2X)

Sensors

ESC III

RR

Power Supply Ground

Brake Pressure Sensor

Rotation Rate sensor

Sensor Test Charge Pump Control

Steering Angle Sensor with Integral MicroProcessor

CAN Rotation Rate Sensor

AGS

+ Electronic Brake Light Switch

EML IIIs

5 4 3 2 1

5 4 3 2 1

DME 1

DME 2

Wheel Speed X4 AGS

278

To LCM EDC Park Brake Switch

FIGURE 11-21  An EBCM circuit.

DME II IKE &DME I

Diagnosis



Diagnostic Process for Antilock Brake and Electronic Brake Control Systems

Some ABS control modules have additional functionality designed into them. One example is electronic brake proportioning. This feature does away with the mechanical proportioning valve and duplicates that action electronically by using the ABS valves to reduce rear brake hydraulic pressure under moderate brake pedal application. The EBCM restricts pressure to the rear wheels, based on how hard the brake pedal is being applied. In this case, the EBCM does not wait until a wheel sensor reports that one or both rear wheels are locking up; instead, it gradually reduces rear brake pressure before lockup occurs. It does this during moderate and heavy braking because of weight being transferred from the rear wheels to the front wheels, reducing the traction at the rear wheels. The EBCM performs an automatic system self-check and warning lamp bulb check on the ABS every time the key is turned to the Run position. If the EBCM detects a fault, the ABS warning lamp will remain on and most, but not all, systems store the fault in the EBCM memory. The faults are stored as diagnostic trouble codes (DTCs) for retrieval by technicians when diagnosing ABS faults. Some older ABSs provide blink codes, also known as flash codes, through the ABS warning lamp when a specific terminal is grounded or two specific terminals are shorted together. If a code is stored in the EBCM memory, the EBCM will blink the ABS warning lamp in a manner that indicates a particular trouble code. For example, a code 12 would be one blink followed by a short pause, then two rapid blinks followed by a long pause. Each code is usually displayed three times before the next code is displayed. Once all codes have been displayed, the codes start at the beginning again. Most ABSs require a scan tool that connects to the EBCM or PCM data link connector to read the fault codes. Fault codes indicate which circuit is experiencing a fault, such as an open left front wheel speed sensor circuit. The fault codes can also indicate whether there is a condition that the EBCM determines is out of acceptable tolerances, such as wheel speeds that do not match within the specified tolerance. The cause could be as simple as having a tire of the wrong size installed on the vehicle or having properly sized tires that are not inflated to the same pressure. Once the codes have been retrieved, technicians use service information to diagnose the problem and locate the cause of the fault.

279

▶▶TECHNICIAN TIP It is important for the customer to know that the ABS is disabled when the ABS yellow warning lamp is on.The brakes will work normally, but without ABS function.

▶▶ Diagnostic

Process for Antilock Brake and Electronic Brake Control Systems

Diagnosing ABS, TCS, and ESC systems starts with a thorough understanding of the particular type of system being worked on. Refer to the service information to become familiar with the manufacturer’s description and operation of the system. Also check for any DTCs related to the customer’s concern. In cases where the system warning lamp is illuminated, access the DTCs by using a compatible scan tool or code retrieval key. DTCs can be diagnosed by following the steps laid out in the service information pertaining to the specific DTCs indicated. In cases where the system warning lamp is not illuminated, indicating there are no faults stored in memory, following the symptom charts listed in the service information will sometimes work. Just remember that the EBCM makes decisions based on the sensor information and software programming. Those decisions are then sent as output signals to the HCU, the vehicle’s PCM, and other controlled devices. The controlled devices then need to carry out the commands. Because of knowing that information, along with the verified customer concern and the service information, the cause of EBC faults can be diagnosed.

Tools The following tools are used to diagnose and repair ABS, TCS, and ESC, many of which shown in FIGURE 11-22: ■■

■■

ABS code retrieval key: A stamped sheet metal key used to access the ABS blink codes on earlier General Motors vehicles. Scan tool: a handheld electronic tool used for accessing ABS codes and live data from the EBCM. Some have bidirectional abilities used to command certain outputs, such as activating a solenoid during a diagnostic routine or when bleeding the brakes. They are also used to clear codes after repairs have been completed.

11-7 Develop a diagnostic process for ABSs and EBC systems.

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Chapter 11  Advanced Braking Systems: Electronic Brake Controls

A

B

C

FIGURE 11-22  Tools used to diagnose electronic brake systems. A. A digital storage oscilloscope/diagnostic scan tool. B. A handheld diagnostic oscilloscope. C. A digital volt-ohmmeter.

■■

■■

■■

■■

Oscilloscope: a handheld electronic tool used to display electrical waveforms relative to voltage signals over divisions of time, such as wheel speed sensor patterns. DMM: a handheld meter used to measure volts, ohms, and amps in electrical circuits. ABS proportioning valve depressor: a device that depresses the proportioning valve on some ABSs while bleeding the brakes. ABS pressure tester: a device that allows the technician to test accumulator and HCU pressure issues.

Diagnosing Braking Concerns Caused by Vehicle Modifications The EBC systems are engineered so that all of the parts and the programming work together for the purpose of assisting the driver in maintaining control of the vehicle. If any of the original components get replaced with nonstandard parts, the EBC systems will not perform as designed, putting the occupants at risk. Examples of this could be as simple as installing a smaller spare tire on the vehicle due to the original (correctly sized) tire going flat. The smaller tire turns at a faster rotation than the original tire, confusing the EBC system; consequently, the yellow warning lamp illuminates and the EBC system is disabled. In the same way, installing larger tires with a greater circumference will cause the wheel speed sensors to report a slower wheel speed than the vehicle is actually ­traveling. Because the EBCM is not calibrated to match this parameter, the system



Diagnostic Process for Antilock Brake and Electronic Brake Control Systems

cannot respond appropriately, and again a DTC will be reported and the EBC system will shut down. Changing curb height causes the vehicle’s center of gravity to change. Taller vehicles tend to transfer weight more easily. In the case of braking, the weight is transferred to the front wheels, unloading the rear wheels. This makes the rear wheels more prone to lockup. In the case of cornering, the weight transfers to the outside wheels, unloading the inside wheels. This makes the vehicle more susceptible to rollover. In both of these cases, the EBCM is not programmed to take into account the nonstock height of the vehicle. Therefore, the EBCM would not be able to properly anticipate the vehicle’s actions in these scenarios. Changing final drive ratios causes the same situation as changing tire size. The vehicle wheel speed sensors will report a different speed than the vehicle speed sensor, and the EBCM will set a code and disable the system. Also, mismatched final drive ratios on fourwheel-drive vehicles can affect the EBC systems, because they can cause the wheels to rotate at slightly different speeds, front to rear. In some cases, the EBCM can be reprogrammed with the new vehicle information, or an EBCM that does match the new parameters of the vehicle may be available for installation. In any case, technicians need to be aware of how these factors affect the EBC systems. They also need to know how to verify that the components match the information programmed into the EBCM.

Diagnosis EBC Systems If the yellow warning lamp is illuminated, indicating there is a fault in the system, retrieve any DTCs following the procedure listed in the service information. If the yellow warning lamp is not illuminated and the brakes are not functioning properly on a vehicle that has an EBC system, suspect a problem that is not monitored by the EBCM. This could be from a fault in the base brake system, such as warped rotors, contaminated friction lining, or a seized caliper piston. Diagnose these problems as if they were on a non-EBC system. It could also be a problem in the hydraulic system, including the HCU, such as a dump valve that is stuck partially open or an isolation valve that is stuck closed. Even systems that are monitored by the EBCM can cause operating concerns without the yellow warning lamp being illuminated. An example would be a wheel speed sensor that does not have the correct air gap. Using the manufacturer’s diagnostic symptom charts and system diagrams and having a good understanding of brake, hydraulic, and system theory will help diagnose the cause of the concern. Most EBC-related faults set DTCs and store them in memory. A good starting point for diagnosis is retrieving any stored DTCs with a scan tool or code retrieval key. The service information provides a testing procedure for each of the fault codes. Following the steps listed and using an understanding of brakes, electricity, and the circuit being tested will help lead to the cause of the fault. To diagnose EBC system electronic controls and components by retrieving DTCs, first connect the scan tool to the data link connector. Navigate to the code retrieval screen and record any DTCs. Research the TSBs and diagnostic procedure for the stored DTCs in the service information. Follow the diagnostic chart to diagnose the cause of the fault. Once the fault has been corrected, clear the diagnostic code and verify that it does not reset. This may involve a test drive.

Bleeding the EBC System Bleeding the EBC system can be easy or difficult. If care is taken to never allow air to enter the hydraulic system, most EBC systems can be bled just like a non-EBC system. However, if air is allowed into the EBC system, it can become trapped in the HCU, requiring a scan tool to operate the solenoid valves to help bleed the air from the EBC system. Many EBC systems also require that a detailed step-by-step process be followed to successfully purge the air. The best advice is to always make sure that air never enters the EBC system while bleeding the brakes. The easiest way to do so is to check the level of brake

281

282

Chapter 11  Advanced Braking Systems: Electronic Brake Controls

fluid in the master cylinder reservoir, often while bleeding the brakes, and to always add fluid well before it gets low enough to allow air to enter the master cylinder. Also, cap any open lines or components so that the brake fluid does not drain out of the lines while the system is open. Remember that DOT 5, the silicone brake fluid, should never be used in an EBC system unless the manufacturer clearly specifies it, because it is prone to aeration in the HCU. To bleed the EBC system’s hydraulic circuits, follow the steps in SKILL DRILL 11-1.

SKILL DRILL 11-1 Bleeding the EBC System’s Hydraulic Circuits 1. Research the bleeding procedure in the service information. Follow the specified procedure precisely. The following steps are given as an example of one manufacturer’s bleeding process.

2. Connect a pressure bleeder to the brake fluid reservoir, but do not pressurize it yet.

3. Install bleeder hoses over the bleeder screws at each wheel. If applicable, submerse the other end of each hose in a container partially filled with clean, specified brake fluid.

Continued



Diagnostic Process for Antilock Brake and Electronic Brake Control Systems

283

4. Connect the scan tool to the data link connector, and access the ABS bleed function.

5. Apply the specified amount of pressure from the pressure bleeder.

6. Follow the instructions on the screen of the scan tool.

7. Once the scan tool indicates that the HCU has been fully bled, it may recommend bleeding the brakes manually to remove any remaining air from the system.

Continued

284

Chapter 11  Advanced Braking Systems: Electronic Brake Controls

8. Remove the pressure bleeder, and fill the reservoir to the full mark.

9. Test the brake pedal feel.

Depressurizing High-Pressure Brake Components Depressurization of an EBC system’s high-pressure components is needed on some systems when specific tasks are being performed, such as during the removal or disassembly of certain EBC system components. Always check the service manual to determine whether depressurization is needed. In many vehicles, depressurization of the accumulator can be accomplished in one of the following ways: ■■

■■ ■■ ■■

verifying that the ignition switch is in the Off position (to disable the electric pump) and depressing the brake pedal 30–50 times pulling the ABS fuse or relay to prevent the electric pump from charging the system using a scan tool to depressurize the system following the manufacturer’s specific timeout process, which depressurizes the system automatically. To depressurize high-pressure components in the EBC system, follow the steps in

SKILL DRILL 11-2.

Removing and Installing Electric and Hydraulic Components of the Electronic Brake Control System Removing and installing electric and hydraulic components of the EBC system is normally required only when there is a fault in the HCU. Because many HCUs are non-serviceable, they will have to be replaced as a unit. This may require depressurizing the high-pressure



Diagnostic Process for Antilock Brake and Electronic Brake Control Systems

285

SKILL DRILL 11-2 Depressurizing High-Pressure Components in the EBC System 1. Research the procedure for depressurizing the accumulator in the service information. Be sure to carefully follow all instructions. The following steps are given as an example of one manufacturer’s process. Verify that the ignition switch is turned to the Off position.

2. Remove the ABS fuse from the fuse box.

3. Apply the brakes firmly at least 40 times. Verify that there is no power assist. If there is, pump the brakes an additional 10 times, then verify that there is no assist remaining.

accumulator on some of these units. If the HCU is serviceable and the faulty components are available, follow the manufacturer’s service procedure to remove and install the faulty component. Failure to do so could damage the HCU and render it inoperable. To remove and install an EBC system’s electrical/electronic and hydraulic components, follow the steps in SKILL DRILL 11-3.

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Chapter 11  Advanced Braking Systems: Electronic Brake Controls

SKILL DRILL 11-3 Removing and Installing an EBC System’s Electrical/Electronic and Hydraulic Components 1. Research the manufacturer’s procedure for removing and installing the component on the vehicle to be worked on. 2. Verify that the system has been depressurized.

3. Follow all instructions carefully, and be sure to replace all components and tighten all fasteners according to specifications.

▶▶ Wheel

Speed Sensors and Tone Wheel Diagnosis

11-8 Diagnose wheel speed sensors and tone wheels.

▶▶TECHNICIAN TIP When checking variable reluctance sensors, it is important to use a true-rms meter since the sensor outputs an AC signal. A true-rms meter is designed to accurately represent this signal. Nontrue-rms meters can accurately represent only clean AC signals, not faulty ones like true-rms meters can. If the meter is a true-rms meter, it will typically say that on its face.

Wheel speed sensors are a high-probability cause of illuminated EBC warning lamps and stored DTCs, because they are usually mounted where they are exposed to the elements and in harm’s way. Watching their speeds during a test drive will help determine whether they are reading the same speed or not. If not, then check for mismatched or improperly inflated tires. If the tires are not the cause, then testing the wheel cylinders for electrical faults is a common next step in EBC diagnosis. Measuring the output signal with a digital storage oscilloscope (DSO) or a graphing multimeter (GMM) and comparing it to known good signals is the most conclusive method of testing the sensors. Variable reluctance–type sensors (passive) create an analog AC sine wave. Magneto-­ resistive-type sensors (active) and Hall-effect sensors (active) create a digital square wave signal. The sensor electrical circuits can be tested with a DMM for opens, shorts, high resistance, and grounds. The tone wheel can usually be visually inspected for any faults, such as broken or damaged teeth. Just make sure to inspect the teeth all the way around the tone wheel, not in only one section. Also verify that the air gap is within specifications. To test and diagnose EBC system speed sensors (digital and analog), tone wheel, and circuits, first read any diagnostic codes by using a scan tool or other code retrieval method. Using the DTCs, determine which wheel speed sensor or sensor circuit is at fault. Research the service information to determine what type of speed sensors the vehicle is equipped with and the specified testing procedure. To test variable reluctance sensors, follow the steps in SKILL DRILL 11-4. To test magneto-resistive sensors, follow the steps in SKILL DRILL 11-5.

SKILL DRILL 11-4 Testing Variable Reluctance Sensors 1. Disconnect the suspect sensor, measure its resistance, and compare the reading to the specifications. If the resistance does not meet the manufacturer’s specifications, replace the sensor.

Continued



Wheel Speed Sensors and Tone Wheel Diagnosis

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2. If it is within specifications, test the two-wire circuit back to the EBCM for opens, shorts, high resistance, and grounds. Repair as necessary.

3. If the circuit is good, reconnect the speed sensor connector and attach a GMM or DSO to one of the sensor wires.

4. Spin the tone wheel and observe the pattern. It should be a clean AC analog pattern of sufficient amplitude (voltage).

5. If the pattern is not correct, inspect the tone wheel for damage, and replace as necessary.

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6. If the tone wheel is good, replace the wheel speed sensor. If the pattern looks OK, it may need to be compared to the other wheel speed sensor signals while driving the vehicle. After the repair, clear the diagnostic codes, if directed by the service information.

SKILL DRILL 11-5 Testing Magneto-Resistive Sensors 1. Make sure the ignition switch is in the Run position, and measure the available voltage at the suspect sensor. Also check the ground for voltage drop.

2. If the voltage does not meet the manufacturer’s specifications, test the circuit for opens, shorts, high resistance, and grounds. Repair as necessary.

Continued



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3. If the voltage to the sensor is within the manufacturer’s specifications, connect a GMM or DSO to the signal wire.

4. With the key in the Run position, spin the tone wheel, then observe the pattern. It should be a clean digital square wave signal of the appropriate height and shape.

5. If the pattern is not correct, inspect the tone wheel for damage, and replace as necessary. If the tone wheel is good, replace the wheel speed sensor.

Continued

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6. After repair, clear the codes if directed by the service information.

Applied Science AS1: Problem-solving: The technician can use computers, scan tools, and onboard data to diagnose problems. Vehicles presenting with ABS warning lights illuminated are now almost an everyday occurrence in most workshops. The first step in the diagnostic process is to read the DTCs stored on the ABS module. The most common cause of ABS malfunctions is faults in wheel speed sensor circuits. Common causes of wheel speed sensor circuit faults include corroded or poorly tensioned connectors, mismatched tire rolling diameters, and physical sensor failures.

Possibly the most effective way to diagnose wheel speed sensor circuit faults is to use the live data function of a scan tool. Start in the shop, looking at the speed readings and making sure they are all equal before even test-driving the car. An inconsistency indicates a fault. During the road test, look at the graphed data outputs from all of the sensors; they should all look consistent. A graph that differs from the others indicates a faulty circuit. Once the affected circuit has been identified, the next step is to test at the sensor with a GMM or an oscilloscope.

▶▶Wrap-Up Ready for Review ▶▶

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Electronic brake control (EBC) systems integrate computer controls to prevent wheel lockup, shorten panic-stop distances, help drivers maintain steering control, and improve vehicle stability. Basic antilock brake systems (ABSs) control hydraulic pressure by holding and releasing via solenoid valves, but they cannot function without the driver’s applied brake pressure. Traction control systems (TCSs) minimize wheel slip by automatically applying brake pressure to a slipping wheel’s brake unit and reducing engine output. Electronic stability control (ESC) systems use steering wheel position sensors, yaw sensors, roll-rate sensors, and wheel speed sensors to independently monitor vehicle stability and apply brakes as necessary. The primary components of an ABS are an ABS master cylinder, an EBC module/ECU, a hydraulic control unit/ modulator, a power booster, a wheel speed sensor, and a brake switch. Braking force and wheel lockup are affected by road surface friction and by the type, condition, and loading of each tire.

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Maximum traction occurs with 10–20% tire slip. Wheel speed sensors signal the EBCM, which sends current to the solenoid valve, which then holds or releases hydraulic braking pressure. ABS master cylinders are integral (mainly in older vehicles) or non-integral with the hydraulic control unit (HCU). Solenoid valves provide three operating conditions: apply, hold, and release. The HCU executes the commands of the ABS control module. HCUs differ by number of channels (one, two, three, or four), number of solenoid valves (single or dual), and type of accumulator (low or high pressure). Wheel speed sensors send electric signals to the EBCM to determine the speed and rate of deceleration for each wheel. Wheel sensor assemblies consist of a toothed tone wheel and a pickup assembly, separated by an air gap. Wheel speed sensors are available in the following types: variable reluctance, magneto-resistive, and Hall effect. The EBCM consists of electronic circuitry, an electronic data processor, computer memory, and output drivers.

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Wrap-Up

The EBCM receives input signals from the brake switch, ignition switch, vehicle speed sensor, wheel speed sensors, and sometimes the battery. The ESC system includes a yaw sensor (directional rotation), steering angle sensor (driver’s directional intent), and roll-rate sensor (rate and amount of vehicle roll). Some TCS and ESC systems can be manually deactivated by the driver. EBCM systems can self-diagnose and store faults as diagnostic trouble codes (DTCs) for technicians to retrieve. It is best not to allow air into an EBC hydraulic system, because they can be difficult to bleed. If the HCU needs to be bled, a scan tool will likely be needed to perform that task. The EBC system should be depressurized before servicing the hydraulic system. Follow the manufacturer’s procedures when depressurizing and servicing EBC systems. When diagnosing EBC systems, don’t overlook faults that could be in the base brake system. Exercise extreme care when test-driving the vehicle while diagnosing the EBC system. Most faults in the EBC system will store a DTC. Use a scan tool to retrieve the DTC, and then research the service information and technical service bulletins (TSBs). Variable reluctance–type sensors (passive) create an ­analog AC sine wave. Magneto-resistive-type sensors (active) and Hall-effect sensors (active) create a digital square wave signal. Variable reluctance–type sensor circuits can be checked for opens, shorts, grounds on the connecting wires, ­resistance, and a signal on the sensor. Magneto-resistive-type sensor circuits can be checked for reference voltage, signal quality, and ground. EBC systems are affected by vehicle modifications such as tire sizes, curb height, and drive ratios and need to be checked during diagnosis.

Key Terms antilock brake system (ABS)  This system helps the driver maintain control by helping the tire maintain traction with the road surface through actuating braking in a positive way. ABS master cylinder  A cylinder that creates hydraulic pressure for each of the two hydraulic brake circuits. ABS code retrieval key  A stamped sheet metal key used to access the ABS blink codes on earlier General Motors vehicles. ABS proportioning valve depressor  A device that depresses the proportioning valve on some ABSs while bleeding the brakes. ABS pressure tester  A device that allows the technician to test accumulator and HCU pressure issues. accumulator  A component that stores brake fluid under pressure. air gap  The space or clearance between two components, such as the space between the tone wheel and the pickup coil in a wheel speed sensor.

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blink codes  Codes used to communicate DTCs. They are given by the EBCM as a series of blinks illuminated by the ABS warning lamp. body control module (BCM)  A computer that controls the electrical system in the body of the vehicle. brake switch  An on/off switch mounted at the brake pedal that informs the EBCM of whether the driver is applying the brakes. channel  The number of wheel speed sensor circuits and hydraulic circuits that the EBCM monitors and controls. digital multimeter (DMM)  A handheld meter used to measure volts, ohms, and amps in electrical circuits. EBCM or electronic control unit (ECU)  An onboard computer that is programmed to monitor sensor data and send output control signals to electronic solenoid valves, which modify brake pressure to individual wheel brake units. fault codes  An alphanumeric code system used to identify potential problems in a vehicle system. Hall effect  An electrical effect where electrons tend to flow on one side of a special material when exposed to a magnetic field, causing a difference in voltage across the special material. When the magnetic field is removed, the electrons flow normally and there is no difference in voltage across the special material. This effect can be used to determine the position or speed of an object. high-pressure accumulators  A storage container designed to contain high-pressure liquids such as brake fluid. hydraulic control unit (HCU) or modulator  A device that contains electric solenoid valves controlled by the EBCM to modify hydraulic pressure in each hydraulic circuit. integral ABSs  Systems that combine all the following components into one larger unit—the master cylinder, HCU, and power booster are all mounted on the firewall. low-pressure accumulators  A storage container for brake fluid coming from the release valves, which is under relatively low pressure. magneto-resistive sensor  A type of wheel speed sensor that uses an effect, similar to what a Hall-effect sensor uses, to create its signal. non-integral ABSs  Systems that separate the master cylinder from the HCU so that each individual component can be serviced. This is the more prevalent than the combined unit. oscilloscope: a handheld electronic tool used to display electrical waveforms relative to voltage signals over divisions of time, such as wheel speed sensor patterns. power booster  A device that boosts driver brake pedal force on the master cylinder. pickup assembly  A component with a wire coil wrapped around a ferrous metal core; it is used to generate an electrical signal when a magnetic field passes through it. scan tool  A handheld electronic tool used for accessing ABS codes and live data from the EBCM. Some have bidirectional abilities used to command certain outputs, such as activating a solenoid during a diagnostic routine or when bleeding the

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brakes. They are also used to clear codes after repairs have been completed. tone wheel  The part of the wheel speed sensor that has ribs and valleys used to create an electrical signal inside the pickup assembly. variable reluctance sensors  Magnetic induction sensors that produce voltage as a metal-toothed wheel breaks the magnetic field from the sensors’ permanent magnet. vehicle speed sensor  The component that creates an electrical signal based on the speed of the vehicle, which is sent to the EBCM. wheel speed sensor  A device that monitors wheel speed and sends that signal to the EBCM.

Review Questions 1. How does changing the tire size on a truck change the ABS? a. It changes the speed at which the wheel speed sensor is spinning, thus mismatching it from the vehicle speed sensor, which could disable the system. b. It changes the effects of the braking distance. c. It changes the maneuverability of the vehicle. d. All of the above. 2. When removing ABS components, the technician must ______________. a. remove all of the fluid from the system b. remove all the lines from the system c. disable the system and vent any residual pressure from the accumulator d. start the vehicle and drive it around 3. If the vehicle has ABS, it will have a four-channel system. True or false? a. True. b. False. 4. If a valve in the HCU is broken, how can the technician go about changing it? a. Replace the valve individually. b. Replace the unit. c. Ignore it and disable the ABS. d. Bypass the valve. 5. The ABS will prevent a vehicle from colliding with another vehicle. True or false? a. True. b. False. 6. Choose the correct statement: a. Solenoid valves momentarily hold or release hydraulic pressure to a wheel until it speeds up and starts rolling again. b. The standard ABS can automatically increase the hydraulic pressure in it. c. The ABS assists the driver in minimizing wheel slip while the vehicle is being accelerated. d. ABS actions are automatic and do not require any input from the driver.

7. Which primary component in an ABS sends output control signals to electronic solenoid valves? a. The power booster. b. The hydraulic control unit (HCU). c. The wheel speed sensor. d. The electronic control unit (ECU). 8. All of the following statements referring to the principles of ABS braking are true, except: a. During normal braking, the EBCM does not energize the solenoid valves in the hydraulic unit. b. If the wheel speed sensors indicate that the wheel is still decelerating too rapidly, the EBCM commands the appropriate solenoid valve to release braking pressure. c. During normal braking, the EBCM does not monitor the speed of each wheel. d. Changes in HCU valve position normally cause rapid hydraulic pulsations. 9. The device used to measure volts, ohms, and amps in electrical circuits is the _________. a. oscilloscope b. DMM c. scan tool d. ABS code retrieval key 10. If the foundation brakes are not in good condition, will the ABS overcome their issues? a. Yes. b. No.

ASE Technician A/Technician B Style Questions 1. Technician A says that an antilock brake system (ABS) helps shorten the stopping distance during a panic stop. Technician B says that ABSs work by increasing the hydraulic pressure in the brake system so the brakes can be applied harder. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that ABS controls brake every time the brakes are applied. Technician B says that during an ABS event, it is normal for the brake pedal to pulsate. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that during antilock braking, brake fluid may be returned to the master cylinder. Technician B says that solenoid valves in the HCU will isolate the master cylinder from the brake circuit when it is in the Hold mode. Who is correct? a. Technician A b. Technician B



c. Both A and B d. Neither A nor B 4. Technician A says that mismatched tires may cause the ABS to register a fault code. Technician B says that a traction control system (TCS) may automatically apply brake pressure to a wheel brake unit even if the vehicle is not being braked. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that an ABS key-on system test checks for faults in the vehicle’s base brake system. Technician B says that on most vehicles, ABS DTCs are stored in memory for later retrieval. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that a scan tool may be required to bleed air from the ABS. Technician B says that some vehicles have a high-pressure accumulator that may need to have the pressure bled off before hydraulic brake repairs are made. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that a four-channel ABS uses four wheel speed sensors and three valves in the HCU. Technician B says when trying to determine what type of ABS the vehicle has, simply count the brake lines. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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8. Technician A says that a DTC in the ABS will allow the technician to change parts correctly. Technician B says that bleeding some ABSs requires the use of a scan tool to actuate the HCU. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 9. Technician A says that when testing wheel speed sensors with a multimeter they must be unplugged and the wheel must be spinning to generate an AC voltage. Technician B says that when testing a wheel speed sensor for resistance, the voltmeter must be set on V to get a reading. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that all vehicles with ABS have a four-channel system. Technician B says that when referring to a channel on an ABS, it means a wheel speed sensor. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

CHAPTER 12

Electronic Stability Control Systems Learning Objectives ■■ ■■

12-1 Illustrate ESC operation. 12-2 Explain TCS operation.

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12-3 Connect an ESC to other driver safety systems.

You Are the Automotive Technician A 2013 Ford Fusion was dropped off at your repair facility with a complaint of a electronic stability control light illuminated. They also have complained that at every stop sign the brake pedal pulsates and they roll through the stop. This have been happening more and more lately and they haven’t noticed anything else wrong with the vehicle. What should you do first as the automotive technician?

1. Replace the electronic stability control module 2. Replace the ABS sensors 3. Connect the scan tool to vehicle and retrieve codes from the ESC system





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▶▶ Introduction The electronic stability control (ESC) system is an integral part of maintaining control of a potentially out-of-control automobile. Using various sensors and electrohydraulic control modules helps the driver maintain control. This chapter will explore the different aspects of the ESC system.

▶▶ Electronic

Stability Control Operation

The antilock brake system (ABS) does a good job of preventing wheels from locking up under hard braking or poor traction conditions and allows the driver to maintain directional control of the vehicle. The traction control system (TCS) also does a good job of maintaining traction when the vehicle is driven in a relatively straight line. However, drivers can lose directional control of the vehicle while driving aggressively, while taking emergency steering actions, or when there are sudden changes in the traction of the road surface while in a turn. These situations can cause the vehicle to understeer (push) or oversteer (loose, or fishtail). It can also cause vehicles with a high center of gravity, such as an SUV (sport utility vehicle), to roll over (FIGURE 12-1). All of these situations can lead to serious collisions or accidents. If any of these situations are imminent, the ESC system can independently activate individual wheel brake units as necessary to help keep the driver from losing control of the ­vehicle. ESC uses both ABS and TCS, but with a few enhancements to more actively interface with the vehicle’s operation in maintaining directional stability while the vehicle is being steered. A U.S. Insurance Institute for Highway Safety 2006 study estimated that if all vehicles were equipped with ESC, approximately 10,000 fatal accidents in the United States could be avoided each year. This finding led the U.S. Department of Transportation to require that all vehicles of less than 10,000 lb (4,536 kg) gross vehicle weight, and manufactured after September 1, 2011, be equipped with an ESC system that meets their minimum specifications. The ESC system integrates a yaw sensor, a steering angle sensor, a ­lateral acceleration sensor, and sometimes a roll-rate sensor into the basic ABS and TCS (FIGURE 12-2). It also adds new programming parameters into the electronic brake control module (EBCM) to monitor the vehicle’s stability, as well as added output command capabilities to apply i­ndividual drive wheel and non–drive wheel brake units independent of the driver. The yaw sensor measures the amount of directional rotation of the vehicle on its v­ ertical axis. In other words, it tells the EBCM the rate at which the vehicle is turning. The steering angle sensor tells the comFIGURE 12-1  ESC helps the driver maintain control of the vehicle while driving. puter what the driver’s directional intent is. If equipped, the roll-rate 12-1 Illustrate ESC operation.

INPUTS LHF Wheel Speed Sensor RHF Wheel Speed Sensor

OUTPUTS

LHR Wheel Speed Sensor

Hydraulic Unit Solenoids

RHR Wheel Speed Sensor

ABS Warning Lamp

Steering Angle Sensor Brake Switch

EBO Warning Lamp

ABS/ESC Control Unit

ESC Warning Lamp

ESP OFF Sensor Yaw Rate Sensor Roll Rate Sensor Master Cylinder Pressure Sensors FIGURE 12-2  A typical ESC schematic.

ESC Function Lamp CAN

PCM



Electronic Stability Control Operation

sensor tells the computer the rate of roll and the amount of roll that the vehicle is experiencing. The lateral acceleration sensor is used to measure the lateral acceleration acting on the car to help calculate its actual position. The lateral acceleration acts on a car sideways to the direction of travel, usually when the vehicle is approaching a spin. The EBCM continuously monitors these signals and compares them to preprogrammed scenarios, and it decides which, if any, brake units need to be applied and whether engine torque needs to be reduced to keep the vehicle stable. This process is a good example of the following computer feedback loop: Input → Control Logic Process → Output.

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SAFETY TIP Most standard passenger vehicles are designed with a bias toward understeer. It is generally agreed that understeer is easier for the average driver to recover from. However, many performance vehicles are designed with a slight bias toward ­oversteer, which can be managed by an experienced driver while driving aggressively.

If the ESC system is activated, the EBCM (FIGURE 12-3) monitors the yaw sensor signal, the steering angle sensor signal, and the roll-rate sensor signal, as well as the wheel speed sensor signals. If the vehicle is beginning to understeer, oversteer, or roll, the EBCM detects it in ▶▶TECHNICIAN TIP the signal values. It then applies up to three wheel brake units to help bring the vehicle back The yaw sensor operates similarly to a within proper stability parameters. If that does not stop the stability issue, the EBCM will Wii or other video game controller. Its request a reduction in engine power through the powertrain control module to help slow internal circuitry senses movement and the vehicle further. On most vehicles, the EBCM illuminates a warning lamp on the dash sends a signal directly related to the or sounds a beeper that signifies when the ESC system has detected the start of a skid and movement it senses. reacted to it. This way the driver will be informed that he or she is on the verge of losing control of the vehicle. On most vehicles, the ESC system defaults to “on” so that it is always active. Some vehicles have a switch on the dash or center console to temporarily deactivate the system. This can be useful when driving in mud or sand when traction is nearly nonexistent and the ESC system cannot function effectively. Even though some ESC systems can be turned off, they may still monitor the operation of the vehicle and reactivate the ESC system under certain situations, such as driving above a specified speed or when a spin is detected while the brakes are being applied. Some ESC systems incorporate a switch that allows the driver to select one or more varying levels of assist from the ESC, such as “touring,” “track,” or “sport” (FIGURE 12-4). This option allows the driver to experience differing levels of wheel slip by being able to push the vehicle closer to the edge of control than when ESC is fully activated, while still having the ESC system available as a backup, but with limited assistance. When driving on a racetrack, for example, the driver may want full control of the vehicle instead of being limited by the ESC system. In the continuous search for new bells and whistles to impress FIGURE 12-3  The EBCM is a computer that takes inputs from various braking sensors and then uses those readings to operate the ABS. customers and enhance safety, manufacturers have designed other features into ESC systems, such as the following: ■■

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Hill assist—holds the brake pressure until the throttle is depressed and the vehicle starts to move forward. All-wheel-drive traction control—applies brake pressure as needed to any of the four individual wheels that may be slipping to maintain power to the wheels with the most traction. Engine braking control—increases the engine torque if the ESC system detects wheel slippage during deceleration. Panic-stop assist—detects a driver’s rapid throttle release and lightly applies the brakes to dry the rotors and prepare the brakes for a panic stop. Accident avoidance—works in conjunction with adaptive cruise control to monitor objects in front of the vehicle. If the ESC system detects an imminent collision, it can apply the brakes or boost the brake pressure above driver pressure.

FIGURE 12-4  The switch to select the desired level of ESC assist.

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Chapter 12  Electronic Stability Control Systems ■■

▶▶TECHNICIAN TIP Many ESC-equipped vehicles monitor signals from other sensors as well, to help prevent a loss of control of the vehicle; these sensors include the throttle position sensor, vehicle speed sensor, and brake pedal position sensor. When diagnosing an ESC system fault, research the sensors monitored by the EBCM.

12-2 Explain TCS operation.

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Hill descent control—works in conjunction with the ESC system to control the speed of the vehicle when going down loose, rough, or slippery slopes. Trailer sway control—detects trailer sway and uses the ESC system to keep it under control. Optimized hydraulic braking—monitors brake pressure in each brake circuit and increases it above boosted pressure if deemed necessary.

▶▶ Traction

Control System Operation

Although a basic ABS can prevent skidding by holding or releasing individual brake circuit pressure, it has no ability to apply the brakes, apart from the driver-created hydraulic pressure. This system works fine as long as the tire slippage is a result of the driver applying the brakes. However, tires also slip because the engine torque accelerating them exceeds their traction with the road surface; in this scenario, they can slip, spin, or break loose, causing a loss of control of the vehicle. The TCS was developed to prevent the drive wheels from slipping while the vehicle is being accelerated. It is active up to a manufacturer-specified speed. Above that speed, traction control is deactivated by the EBCM because further acceleration is unlikely to cause the wheels to lose traction. To obtain traction control capabilities, manufacturers have added a few design features to the basic ABS, one of which is the high-pressure pump and accumulator that was discussed earlier. This pressure is used to activate brake units on the drive wheels independently of the driver. The sensors are the same as in the ABS, but the ability to apply the individual drive wheel brakes is needed; thus, two to four extra solenoid valves, called boost valves, are added to the hydraulic control unit (HCU) (FIGURE 12-5). These

Dump Valve Isolation Valve Boost Valve Control Unit

To RHF Brake

To LHR Brake

IN (from master cylinder)

Inlet Check Valve ABS Pump Element

High-Pressure Switch

Pump Drive Eccentric (driven by ABS motor)

High-Pressure Accumulator Valve

IN (from master cylinder)

Suction Accumulator Valve

To RHR Brake

To LHF Brake

Control Unit FIGURE 12-5  An HCU with boost valves.



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boost valves direct hydraulic pressure from the accumulator to the ABS solenoid valves so that individual wheel brake units can be applied independently. Additional programming is added to the EBCM to control the high-pressure pump and extra HCU valves and to decide when each of them needs to be activated.

Operation of the TCS When the TCS is active, the EBCM monitors the speed of the individual drive and non-drive wheels, along with the vehicle speed from the vehicle speed sensor. If the driven wheels are accelerating at different speeds from each other or the non-driven wheels, the EBCM can identify which wheel or wheels are slipping. It will then take action to reduce the torque to the appropriate wheels by first applying the brake to any wheels that are slipping. It does this by activating the isolation valve to close off the supply port from the FIGURE 12-6  A switch for deactivating the traction control system. master cylinder. It then activates the boost valve to pressurize the brake circuit on the spinning wheel to slow it down. If that is not enough to prevent the slippage, the EBCM will request reduced power from the engine. This can be accomplished by reducing the throttle plate opening, shutting down one or more fuel injectors, reducing the engine timing, or selecting a higher gear in the transmission. Once the wheel speeds return to proper parameters, the EBCM will return the TCS to normal and continue to monitor the wheels for slippage. ▶▶TECHNICIAN TIP Some TCSs can be temporarily deactivated by a TCS function switch located on the Manufacturers use various strategies dash or center console (FIGURE 12-6). If the driver deactivates the TCS, the system will in their TCSs, and not all of them apply not intervene during wheel slip. Drivers will disable the TCS for a variety of reasons. the brakes as a first step. Some of them They might be climbing a long hill on a rough gravel road, which would continuously reduce engine power first. Even so, activate traction control, overheating the brakes. Or they might want to show off by EBCMs today operate very fast, so there “roasting” the tires or to experience driving without traction control as they would on a may only be a few milliseconds between racetrack. The TCS will automatically default back to On during the next ignition switch each action. cycle. In most cases, if the TCS is deactivated, the ABS will still be active.

▶▶ Electronic

Stability System with Other Driver Safety Systems

With the integration of the vehicle’s systems onto a computer network, the speed at which sensors can cause ECMs to react to the present conditions are increasing every day. The purpose of stability systems is to allow the driver to maintain control of the vehicle at all times, regardless of what situation the vehicle is in. With the driver in control, the potential for a collision or accident is lessened. Although it cannot eliminate the possibility of a collision, it can lessen the chances. This section will introduce some systems that can help with keeping the driver in control and minimizing the possibility of a collision or accident.

Collision Avoidance Systems Collision avoidance systems (CASs) are an integrated structure that alerts the driver and may provide further assistance to help the driver avoid a collision. Using radar, GPS, cameras, and other sensors, the collision avoidance computer (CAC) determines whether the vehicle is getting close to another object where there may be an impact (FIGURE 12-7). When it is determined that a collision is eminent, the CAC will

12-3 Connect an ESC to other driver safety systems.

Collision Warning with Brake Support

Brake Su pport Collissio on

Warrning

Collision Warning Using radar to detect moving vehicles ahead. the system warns the driver of a collision risk with an alarm and warning light.

Brake Support If the risk of a collision increases despite the warning, the brake support is activated, Brake support enables harder, quicker decleration to help drivers stop or reduce speed and lessen the impact of a collision.

FIGURE 12-7  The CAS uses multiple inputs to calculate whether the vehicle will impact a given obstacle. Each system is different, so verifying the capability of the vehicle is the first step in diagnosing it.

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Chapter 12  Electronic Stability Control Systems

alert the driver. On some ­models, it will actuate the braking system through the ABS ­module and direct the steering module to steer the vehicle clear of the hindrance. The system is susceptible to all forms of precipitation, which means rain and snow can affect the accuracy of the system. Inclement weather should always be taken seriously because it can cause the system to malfunction. This is a gateway to autonomous vehicle operation. The calculations being performed to determine how to avoid collisions are complex, and that complexity requires the technician to be current on the latest technology. The National Highway Safety Administration (NHTSA) is making the inclusion of a CAS as standard equipment on all vehicles sold in the United States, starting in the year 2022. The use of the CAS will increase to the point that it will be on every vehicle produced for the U.S. market, which means every technician will need to understand the operation of the system.

Lane Departure System and Lane Keeping Assist Systems To keep the driver aware of where their vehicle is at any given moment the vehicle manufactures have developed a lane departure and lane keeping assist systems (LKAS). These systems use a camera that is usually mounted on the front of the vehicle, a lot of the time at the top of the windshield. As the driver drives down the road, the camera recognizes the lines on the road and when the vehicle crosses those lines, it alerts the diver to pay attention to maintain their lane. The alert could be a visual light coming on that the driver can see without taking their eyes off of the road, or it could be a vibrating seat (­ FIGURE 12-8). The whole point of this system is to make the driver pay attention to where the vehicle is without taking the ­driver’s attention away from driving. In a lane keeping assist system, the vehicle uses the steering system to help guide the vehicle away from the road line markings (FIGURE 12-9). This system is close to autonomous driving system since it takes over for the diver to correct the possibility of a vehicle collision or the vehicle running off of the road. The integration of both of these systems increases the possibility of the driver keeping control of the vehicle and keeping the vehicle inside the lane that they are currently supposed to be in.

Adaptive Cruise Control

FIGURE 12-8  Lane departure helps keep the driver aware at all times of the position of the vehicle on the road. If the diver does deviate from the lane, it will alert the diver to correct this action.

Adaptive cruise control (ACC) is a version of cruise control that maintains the pace of the vehicle that is in front of the vehicle ahead of the driver. Using radar and other sensors, the vehicle calculates the distance that the vehicle in front of it is in relation to its own position and accelerates or decelerates itself to maintain that distance (FIGURE 12-10). In concert with the EBCM, powertrain

Depends on vehicle speed and angle of CORRECTION divergence

WARNIG

FIGURE 12-9  Lane keeping assist turns the vehicle’s steering wheel electrically to recenter the

vehicle between the road lane markings.



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DISTRONIC PLUS

300

200

100

Oft

R N P D

FIGURE 12-10  The adaptive cruise

control is a feature that keeps the vehicle a set distance away from the vehicle in front of it. This allows the diver to focus on keeping the vehicle on the road.

control module (PCM), ABS, and various other modules, the ACC allows for complete speed control of the vehicle. The ACC is adjustable for the comfort of the driver, so the driver can adjust the gap between their vehicle and the one in front of them. This customizable feature helps with putting the driver at ease by allowing them to participate in customizing the cruise control settings.

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The ESC system is used to maintain control of the ­vehicle. The ESC system uses the ABS and various other sensors to calculate what is happening in the vehicle. By using TCS, the vehicle can maintain controlled ­operation. The ESC system works in concert with the crash ­avoidance, lane departure, and adaptive cruise control (ACC) systems. Collision avoidance systems (CASs) use radar to sense obstacles in front of the vehicle. Lane departure and lane keeping assist systems use radar and cameras to verify that the vehicle is still in its lane. Alerting the driver is the purpose of the collision ­avoidance and lane departure systems. ACC systems are used to maintain the distance between the vehicle and the vehicle in front of it.

Key Terms adaptive cruise control (ACC)  A system that allows for maintaining the distance between the vehicle and the vehicle in front of it. boost valves  Valves that direct hydraulic pressure from the accumulator to the ABS solenoid valves so that individual wheel brake units can be applied independently.

collision avoidance systems (CASs)  Systems that use radar and other sensors to alert the driver to a situation that could potentially be a collision. It helps the driver to avoid a collision. electronic stability control (ESC)  An integral part of maintaining control of a potentially out-of-control automobile by using the ABS and other systems to allow the driver to regain control of the vehicle. lane departure  A system that alerts the driver when the vehicle wanders out of the lane of traffic that it is currently supposed to be in. lane keeping assist systems (LKAS)  A system that redirects the vehicle back to the center of the lane if it wanders out of the lane of traffic. lateral acceleration sensor  A sensor used to measure the lateral acceleration acting on the car, to help calculate its actual position. roll-rate sensor  A sensor that tells the computer the rate of roll and the amount of roll that the vehicle is experiencing. steering angle sensor  A sensor that tells the computer what the driver’s directional intent is. traction control system (TCS)  A system that uses the ABS, PCM, and (electronic stability control) ESC to help the driver maintain control of the vehicle throughout a loss-of-traction event. yaw sensor  A sensor that measures the amount of directional rotation of the vehicle on its vertical axis.

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Chapter 12  Electronic Stability Control Systems

Review Questions 1. The ESC system is used to __________. a. control the stability of the vehicle b. control the engine temperature of the vehicle c. control the charging system d. control the type of fluid in the braking system 2. The traction control system helps maintain _________. a. the driver’s ability to engage in high-speed driving b. control of the vehicle c. equal tire wear d. transmission slippage 3. The yaw sensor measures how much the vehicle is _________. a. driving b. turning c. crashing d. braking 4. A vehicle comes in with a vehicle steering angle sensor fault. The technician must diagnosis why the traction ­control/ESC light is on. What could be the potential reason that those systems are not in operation? a. A bad cruise control switch. b. A bad steering angle sensor. c. A bad ABS module. d. Nothing, because this is normal operation. 5. The traction control system operates because of wheel lockup. True or false? a. True. b. False. 6. The purpose of the traction control system (TCS) is so that the driver can maintain control of the vehicle at all times under all conditions. When the ABS has a fault, how does that affect the TCS? a. It does not affect it at all. b. It causes the system to become more crucial. c. It disables the system. d. It increases the effectiveness of the TCS. 7. Adaptive cruise control (ACC) uses what features to operate effectively? a. PCM control. b. ABS control. c. Radar-sensing control. d. All of the above. 8. When a lane keeping assist system senses that the vehicle is moving in the wrong direction, what does the vehicle do when it operates the steering? a. It maintains course. b. It directs the steering toward the center of the lane. c. It allows the driver to maintain control. d. It stops the vehicle. 9. The features of the ESC system require communication and sometimes control of other systems in the vehicle. If a communication wire gets shorted or becomes open, will the ESC work correctly? a. Yes. b. No.

c. Part of the system will operate. d. It will switch to Wi-Fi mode. 0. Using accumulator pressure, the TCS uses boost valves 1 to_____________. a. actuate the braking components on the affected wheel b. operate the abs c. boost the speed of the wheel d. maintain the state of the wheel

ASE Technician A/Technician B Style Questions 1. Technician A says that a collision avoidance system (CAS) alerts the driver to a potential collision. Technician B says that the stability of the vehicle is not very important as long as the braking system works. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 2. Technician A says that in a lane departure warning system, the PCM steers the vehicle toward the center of the lane. Technician B says that ACC maintains the distance between the front of the vehicle and the one that is in front of it when the cruise is engaged. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 3. Technician A says that a vehicle with collision avoidance is not affected by rain or snow. Technician B says that lane keeping assist systems (LKAS) operate the steering gear to recenter the vehicle to the middle of the lane. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 4. Technician A says that a vehicle with ABS has a TCS. Technician B says that a lateral acceleration sensor monitors how the vehicle is moving, to determine whether it is in a spin. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 5. Technician A says that an electronic braking system has sensors that monitor wheel speed. Technician B says that understeer is generally easier to recover from than oversteer. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 6. Technician A says that traction control can reduce the power output of the engine to increase traction. Technician B says



that electronic stability control (ESC) increases the risk of rollover. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 7. Technician A says that a yaw sensor tells the computer the vehicle’s actual direction. Technician B says that raising a vehicle’s curb height has no effect on the ESC system. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 8. Technician A says that the TCS uses the ABS information to make decisions on how to apply the brakes in an event. Technician B says that a vehicle without an ESC will fail. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

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9. Technician A says that using a scan tool on a modern automobile with a TCS is a must to diagnosis an issue. Technician B says that some vehicles with ACC have an option to allow for the driver to input how far they want the vehicle to follow behind the one in front of them. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B 10. Technician A says that a network problem in the vehicle can cause the ACC, TCS, ESC, and the CAS to fail. Technician B says that proper diagnosis of any of the ESCs requires the technician to understand how the system operates. Who is correct? a. Technician A b. Technician B c. Both A and B d. Neither A nor B

SECTION 1 A APPENDIX 2017 NATEF AUTOMOBILE ACCREDITATION TASK LIST CORRELATION GUIDE Task List

Chapter

BRAKES General: Brake Systems Diagnosis Identify and interpret brake system concerns; determine needed action.

4

Research vehicle service information including fluid type, vehicle service history, service precautions, and technical service bulletins.

2

Describe procedure for performing a road test to check brake system operation including an antilock brake system (ABS).

4

Install wheel and torque lug nuts. Identify brake system components and configuration.

4, 5 4

Hydraulic System Diagnosis and Repair Diagnose pressure concerns in the brake system using hydraulic principles (Pascal’s Law).

3

Measure brake pedal height, travel, and free play (as applicable); determine needed action.

3, 8

Describe proper brake pedal height, travel, and feel.

8

Check master cylinder for internal/external leaks and proper operation; determine needed action.

3

Remove, bench bleed, and reinstall master cylinder.

3

Diagnose poor stopping, pulling, or dragging concerns caused by malfunctions in the hydraulic system; determine needed action.

4

Inspect brake lines, flexible hoses, and fittings for leaks, dents, kinks, rust, cracks, bulging, wear; and loose fittings/ supports; determine needed action.

3

Replace brake lines, hoses, fittings, and supports.

3

Fabricate brake lines using proper material and flaring procedures (double flare and ISO types).

3

Select, handle, store, and fill brake fluids to proper level; use proper fluid type per manufacturer specification.

3

Inspect, test, and/or replace components of brake warning light system.

3

Identify components of hydraulic brake warning light system.

3

Bleed and/or flush brake system.

3

Test brake fluid for contamination.

3

Drum Brake Diagnosis and Repair Diagnose poor stopping, noise, vibration, pulling, grabbing, dragging, or pedal pulsation concerns; determine needed action.

3

Remove, clean, and inspect brake drum; measure brake drum diameter; determine serviceability.

5

Refinish brake drum and measure final drum diameter; compare with specification.

6

Remove, clean, inspect, and/or replace brake shoes, springs, pins, clips, levers, adjusters/self-adjusters, other related brake hardware, and backing support plates; lubricate and reassemble.

5

Inspect wheel cylinders for leaks and proper operation; remove and replace as needed.

5

Pre-adjust brake shoes and parking brake; install brake drums or drum/hub assemblies and wheel bearings; perform final checks and adjustments.

5

Disc Brake Diagnosis and Repair Diagnose poor stopping, noise, vibration, pulling, grabbing, dragging, or pulsation concerns; determine needed action.

4

Remove and clean caliper assembly; inspect for leaks, damage, and wear; determine needed action.

4 (continued)

306

APPENDIX A

Task List

Chapter

Inspect caliper mounting and slides/pins for proper operation, wear, and damage; determine needed action.

4

Remove, inspect, and/or replace brake pads and retaining hardware; determine needed action.

4

Lubricate and reinstall caliper, brake pads, and related hardware; seat brake pads; inspect for leaks.

4

Clean and inspect rotor and mounting surface; measure rotor thickness, thickness variation, and lateral runout; determine needed action.

4

Remove and reinstall/replace rotor.

4

Refinish rotor on vehicle; measure final rotor thickness and compare with specification.

6

Refinish rotor off vehicle; measure final rotor thickness and compare with specification.

6

Retract and re-adjust caliper piston on an integrated parking brake system.

4

Check brake pad wear indicator; determine needed action.

4

Describe importance of operating vehicle to burnish/break-in replacement brake pads according to manufacturer’s recommendations.

4

Check brake pedal travel with and without engine running to verify proper power booster operation.

8

Identify components of the brake power assist system (vacuum and hydraulic); check vacuum supply (manifold or auxiliary pump) to vacuum-type power booster.

8

Inspect vacuum-type power booster unit for leaks; inspect the check-valve for proper operation; determine needed action.

8

Inspect and test hydraulically assisted power brake system for leaks and proper operation; determine needed action.

8

Measure and adjust master cylinder pushrod length.

3

Related Systems (i.e., Wheel Bearings, Parking Brakes, Electrical) Diagnosis and Repair Diagnose wheel bearing noises, wheel shimmy, and vibration concerns; determine needed action.

9

Remove, clean, inspect, repack, and install wheel bearings; replace seals; install hub and adjust bearings.

9

Check parking brake system and components for wear, binding, and corrosion; clean, lubricate, adjust, and/or replace as needed.

7

Check parking brake operation and parking brake indicator light system operation; determine needed action.

7

Check operation of brake stop light system.

3

Replace wheel bearing and race.

9

Remove, reinstall, and/or replace sealed wheel bearing assembly.

9

Inspect and replace wheel studs.

4

Electronic Brake Control Systems: Antilock Brake (ABS), Traction Control (TCS), and Electronic Stability Control (ESC) Systems Diagnosis and Repair Identify and inspect electronic brake control system components (ABS, TCS, ESC); determine needed action.

11

Identify traction control/vehicle stability control system components.

12

Describe the operation of a regenerative braking system.

10

Diagnose poor stopping, wheel lock-up, abnormal pedal feel, unwanted application, and noise concerns associated with the electronic brake control system; determine needed action.

11

Diagnose electronic brake control system electronic control(s) and components by retrieving diagnostic trouble codes, and/or using recommended test equipment; determine needed action.

11

Depressurize high-pressure components of an electronic brake control system.

11

Bleed the electronic brake control system hydraulic circuits.

11

Test, diagnose, and service electronic brake control system speed sensors (digital and analog), toothed ring (tone wheel), and circuits using a graphing multimeter (GMM)/digital storage oscilloscope (DSO) (includes output signal, resistance, shorts to voltage/ground, and frequency data).

11

Diagnose electronic brake control system braking concerns caused by vehicle modifications (tire size, curb height, final drive ratio, etc.).

11

GLOSSARY 3 Cs  A term used to describe the repair documentation process of 1st documenting the customer concern, 2nd documenting the cause of the problem, and 3rd documenting the correction. ABS code retrieval key  A stamped sheet metal key used to access the ABS blink codes on earlier General Motors vehicles. ABS master cylinder  A cylinder that creates hydraulic pressure for each of the two hydraulic brake circuits. ABS pressure tester  A device that allows the technician to test accumulator and HCU pressure issues. ABS proportioning valve depressor  A device that depresses the proportioning valve on some ABSs while bleeding the brakes. acceleration  An increase in a vehicle’s speed. accumulator  A component that stores brake fluid under pressure. adaptive cruise control (ACC)  A system that allows for maintaining the distance between the vehicle and the vehicle in front of it. adjusting nut  The nut used to adjust the end play or preload of a wheel bearing. aerate  The tendency to create air bubbles in a fluid. aftermarket  A company other than the original manufacturer that produces equipment or provides services. air gap  The space or clearance between two components, such as the space between the tone wheel and the pickup coil in a wheel speed sensor. air-operated braking system, also called air brakes  A braking system that uses compressed air operating on large-diameter diaphragms to provide force to the braking assembly. anchor pin  A component of the backing plate that takes all of the braking force from the brake shoes. antifriction bearing  Wheel bearing assemblies that use surfaces that are in rolling contact with each other to greatly reduce friction, compared to surfaces in sliding contact. antilock brake system (ABS)  A safety measure for the braking system that uses a computer to monitor the speed of each wheel and control the hydraulic pressure to each wheel to prevent wheel lockup. automatic brake self-adjuster  A system on drum brakes that automatically adjusts the brakes to maintain a specified amount of running clearance between the shoes and drum. backing plate  A plate bolted to the steering or suspension components that supports the wheel cylinder(s), brake shoes, and hardware. ball bearings  The rolling components of a wheel bearing, consisting of hardened balls that roll in matching grooves in the inner and outer races.

band brake  A braking system that uses a metal band lined with friction material to clamp around the outside of a wheel or drum. bearing cage  The component in a wheel bearing that maintains the proper spacing between the roller bearings or ball bearings. bearing packer  A tool that forces grease into the spaces between the bearing rollers. bearing races  Hardened metal surfaces that roller or ball bearings fit into when a bearing is properly assembled. bendable tangs  Small tabs on the brake pad backing plate that are crimped onto the caliper, creating a secure fit and reducing noise. bleeder screw  A hollow screw that allows air and brake fluid to be bled out of a hydraulic brake system when it is loosened and seals the brake fluid in when it has been tightened. bleeding  The process of removing air from a hydraulic braking system. blink codes  Codes used to communicate DTCs. They are given by the EBCM as a series of blinks illuminated by the ABS warning lamp. body control module (BCM)  A computer that controls the electrical system in the body of the vehicle. bonded linings  More commonly found on light-duty vehicles, brake linings that are essentially glued to the brake pad backing plate. boost valves  Valves that direct hydraulic pressure from the accumulator to the ABS solenoid valves so that individual wheel brake units can be applied independently. brake booster  A vacuum or hydraulically operated device that increases the driver’s braking effort. brake drum  A short, wide, hollow cylinder that is capped on one end and bolted to a vehicle’s wheel. It has an inner friction surface that the brake shoe is forced against. brake fade  The reduction in stopping power caused by a change in the brake system, such as overheating, water, or overheated brake fluid. brake fluid  Hydraulic fluid that transfers forces under pressure through the hydraulic lines to the wheel braking units. brake hose  A flexible section of the brake lines between the body and suspension that allows for steering and suspension movement. brake lathe  A tool used to refinish the drum surface by removing a small amount of metal and returning it to a concentric, nondirectional finish.

308 GLOSSARY

brake lines  Lines made of seamless, double-walled steel that are able to transmit over 1,000 psi (6,895 kPa) of hydraulic pressure through the hydraulic brake system. brake lining thickness gauge  A tool used to measure the thickness of the brake lining. brake pad shims and guides  Small pieces of metal that cushion the brake pad and absorb some of the vibration, helping to cut down on unwanted noise. brake pedal emulator  A brake pedal assembly used in electronically controlled braking systems to send the driver’s braking intention to the computer; it mimics the feel of a standard brake pedal. brake shoe  A steel shoe and brake lining friction material that apply force to the brake drum during braking. brake shoe adjustment gauge  An adjustable tool used to pre-adjust the brake shoes to the diameter of the brake drum. brake spoon  A tool used to adjust the brake lining–to-drum clearance when the drum is installed on the vehicle. brake spring pliers  A tool used for removing and installing brake return springs. brake switch  An on/off switch mounted at the brake pedal that informs the EBCM of whether the driver is applying the brakes. brake wash station  A piece of equipment designed to safely clean and contain brake dust from drum and disc brake components. brake-by-wire system  A braking system that uses no mechanical connection between the brake pedal and each brake unit. The system uses electrically actuated motors or a separate hydraulic system to apply brake force. brakes  A system made up of hydraulic and mechanical components designed to slow down or stop a vehicle. caliper  A hydraulic device that uses pressure from the master cylinder to apply the brake pads against the rotor. caliper dust boot seal driver set  A set of drivers used to install metal-backed caliper dust boot seals. caliper piston pliers  A tool used to grip caliper pistons while removing them. caliper piston retracting tool  A tool used to retract caliper ­pistons on integrated parking brake systems. CANbus circuit  A two-wire communication network that transmits status and command signals between control modules in a vehicle. castellated nut  An adjusting nut with slots cut into the top such that it resembles a castle. It is used with a cotter pin to prevent the nut from loosening. cause  Part of the 3Cs, documenting the cause of the problem. This documentation will go on the repair order, invoice, and service history. C-clamp  A tool used to push pistons back into the caliper bore on non-integrated parking brakes.

channel  The number of wheel speed sensor circuits and hydraulic circuits that the EBCM monitors and controls. coefficient of friction  The amount of friction between two moving surfaces in contact with each other. collision avoidance systems (CASs)  Systems that use radar and other sensors to alert the driver to a situation that could potentially be a collision. It helps the driver to avoid a collision. compensating port  A port that connects the brake-fluid reservoir to the master cylinder bore when the piston is fully retracted, allowing for expansion and contraction of the brake fluid. concern  Part of the 3Cs, documenting the original concern that the customer came into the shop with. This documentation will go on the repair order, invoice, and service history. conservation of energy  A physical law that states that energy cannot be created or destroyed. correction  Part of the 3Cs, documenting the repair that solved the vehicle fault. This documentation will go on the repair order, invoice, and service history. cotter pin  A single-use soft metal pin that can be bent into shape and is used to retain bearing adjusting nuts. cylinder bore  The inside diameter of a cylinder. cylindrical roller bearing assembly  A type of wheel bearing with races and rollers that are cylindrical in shape and roll between inner and outer races, which are parallel to each other. deceleration  The process of decreasing a vehicle’s speed. dial indicator  A tool used to measure the lateral runout of the rotor. digital multimeter (DMM)  A handheld meter used to measure volts, ohms, and amps in electrical circuits. disc brake pads  Brake pads that consist of a friction material bonded or riveted to a steel backing plate, which are designed to wear out over time. disc brake rotor micrometer  A specially designed micrometer used to measure the thickness of a rotor. disc brakes  A type of brake system that forces stationary brake pads against the outside of a rotating brake rotor. double-row ball bearing assembly  A single ball bearing assembly using two rows of ball bearings riding in two channels in the races. drawing-in method  A method for replacing wheel studs that uses the lug nut to draw the wheel stud into the hub or flange. driveline parking brake  A driveline parking brake system is one that has a drum or disc brake assembly on the drive shaft or the pinion yolk on the rear end, where the driver can operate the brake to hold the vehicle stationary. drum brake micrometer  A tool used for measuring the inside diameter of the brake drum. drum brakes  A type of brake system that forces brake shoes against the inside of a brake drum.

GLOSSARY 309

drum-style parking brake  A mechanically operated drum brake that can be set while the vehicle is not moving, to serve as a parking brake. duo-servo drum brake system  A system that uses servo action in both the forward direction and the reverse direction. EBCM or electronic control unit (ECU)  An onboard computer that is programmed to monitor sensor data and send output control signals to electronic solenoid valves, which modify brake pressure to individual wheel brake units. edge code  A two-digit code printed on the edge of a friction lining, where the code describes its coefficient of friction. electric braking system  A braking system used to provide braking to trailers; the drum brakes are electrically activated in the trailer when the driver applies the brakes on the tow vehicle. electric parking brake  An electric parking brake is controlled by a switch, a module, and actuators that apply and release the brake based on control from the module. electrohydraulic brake unit  An electric hydraulic motor that creates pressure so that the braking system can act independently of the input from the brake pedal. electrohydraulic braking (EHB)  A hydraulic braking system that uses an electrically driven hydraulic pump to pressurize fluid for use in the master cylinder. electronic control module (ECM)  A computer that receives signals from input sensors, compares that information with preloaded software, and sends an appropriate command ­signal to output devices. It is used to manage the antilock brake ­system (ABS). electronic stability control (ESC)  An integral part of maintaining control of a potentially out-of-control automobile by using the ABS and other systems to allow the driver to regain control of the vehicle. electronic stability program (ESP)  A program run in an ECM type of component, which has been developed to operate under specific conditions. This type of program helps the operator to maintain control over the vehicle. exhaust brake  A brake system that restricts the flow of exhaust gases through the engine by closing a butterfly valve located in the exhaust manifold. Restricting the exhaust flow causes the engine speed to slow down, slowing the vehicle. fault codes  An alphanumeric code system used to identify potential problems in a vehicle system. fill plug  Usually a threaded plug that can be removed to allow the level of a fluid to be checked and filled. This could also be a rubber snap fit plug. fixed caliper  A type of brake caliper bolted firmly to the steering knuckle or axle housing, having at least one piston on each side of the rotor. force  The measurement of the amount of pressure created by a mass accelerating. The equation for force is F = m × a. freeplay  Is the amount of movement before the actuation of the master cylinder.

freeze frame data  Refers to snapshots that are automatically stored in a vehicle’s power train control module (PCM) when a fault occurs (only available on model year 1996 and newer). friction bearing  A bearing that uses sliding motion between components, such as a clutch pilot bushing. friction  The resistance created by surfaces in contact. Kinetic friction is resistance to motion when one surface moves over another. Static friction is resistance to motion between two ­surfaces that are not moving. fulcrum  The point around which a lever rotates and that ­supports the lever and the load. garter spring  A coiled spring that is fitted to the inside of the ­sealing lip of many seals, used to hold the lip in contact with the shaft. gear lube  A type of lubricant used primarily to lubricate transmission and differential gears but also used to lubricate some wheel bearings. grease  A lubricating liquid thickened to make it suitable for use with many wheel bearings. grease seal  A component that is designed to keep grease from leaking out and contaminants from leaking in. guide pins  Pins that allow the caliper to move in and out as the brakes operate and as the brake pads wear. Hall effect  An electrical effect where electrons tend to flow on one side of a special material when exposed to a magnetic field, causing a difference in voltage across the special material. When the magnetic field is removed, the electrons flow normally and there is no difference in voltage across the special material. This effect can be used to determine the position or speed of an object. heat fade  Brake fade caused by the buildup of heat in braking surfaces, which get so hot that they cannot create any additional heat, leading to a loss of friction. high-pressure accumulators  A storage container designed to contain high-pressure liquids such as brake fluid. hold-down spring tool  A tool used for removing and installing hold-down springs. hold-down springs  Springs that hold the brake shoes against the backing plate. hydraulic control unit (HCU) or modulator  A device that contains electric solenoid valves controlled by the EBCM to modify hydraulic pressure in each hydraulic circuit. hydraulic fade  Brake fade caused by boiling brake fluid. This causes a spongy brake pedal. hydraulic press method  A method for replacing wheel studs that uses a press to force the wheel stud into the flange until it bottoms out. hydroboost  A power-assist system that uses the power steering pump to help with the power assist of the hydraulic braking system. hygroscopic  A substance that attracts and absorbs moisture (e.g., brake fluid). independent rear suspension (IRS)  A type of suspension system where each rear wheel is capable of moving independently of the other.

310 GLOSSARY

inertia  The resistance of an object to change the state it is in. inlet port  A port that connects the reservoir with the space around the piston and between the piston cups in a brake m ­ aster cylinder. inner race  The inside component of a wheel bearing that has a smooth, hardened surface for rollers or balls to ride on. input force  The force applied to the input piston, measured in either pounds or kilograms. integral ABSs  Systems that combine all the following components into one larger unit—the master cylinder, HCU, and power booster are all mounted on the firewall. interference fit  A condition in which two parts are held together by friction because the outside diameter of the inner component is slightly larger than the inside diameter of the outer component. intermittent faults  A fault or customer concern that you ­cannot detect all of the time and only occurs sometimes. International Standards Organization (ISO) flare, also called a bubble flare  A method for joining brake lines. It is created by flaring the line slightly out and then back in, leaving the line bubbled near the end. inverted double flare  A method for joining brake lines that forms a secure, leak-proof connection. jake brake, also called a compression brake  A brake system that consists of an extra exhaust valve on a diesel engine, which releases compressed gases from the combustion chamber at the top of the compression stroke. keyed lock washer  The washer that fits between the adjusting nut and the locknut. The face of the washer is drilled with a series of holes that mate to a short pin from the adjusting nut, locking it to the spindle. keyed washer  The washer that fits between the adjusting nut and the wheel bearing and that has the center hole keyed to fit a slot on the spindle or axle tube. kinetic energy  The energy of an object in motion. It doubles with weight and increases by the square of the speed. lane departure  A system that alerts the driver when the vehicle wanders out of the lane of traffic that it is currently supposed to be in. lane keeping assist systems (LKAS)  A system that redirects the vehicle back to the center of the lane if it wanders out of the lane of traffic. lateral acceleration sensor  A sensor used to measure the lateral acceleration acting on the car, to help calculate its actual position. lateral runout, also called warpage  The side-to-side movement of the rotor surfaces as the rotor turns. leading shoes  Brake shoes that are installed so that they are applied in the same direction as the forward rotation of the drum and thus are self-energizing. leading/trailing shoe drum brake system  A type of brake shoe arrangement where one shoe is positioned in a leading manner and the other shoe in a trailing manner.

lever  A tool that allows the user to move a large load over a small distance at one end by applying a small force over a greater distance from the other end. lithium soap  A thickening agent for grease to give it the proper consistency. load transfer  Weight transfer from one set of wheels to the other set of wheels during braking, acceleration, or cornering. lock cage  The stamped sheet metal cap that fits over the bearing adjustment nut and is secured by a cotter pin going through it and the spindle/axle. locknut  The nut that holds the adjusting nut from turning, which is usually tightened much more tightly than the adjusting nut. low-drag caliper  A caliper designed to maintain a larger brake pad–to-rotor clearance by retracting the pistons farther than normal. low-pressure accumulators  A storage container for brake fluid coming from the release valves, which is under relatively low pressure. lug  A flange that is shaped to assist with aligning objects on other objects. machining  A process in which the brake rotor or drum is cut by a steel cutter to remove the upper layers of metal to restore the trueness of the surface that wears on the shoe or pad. magneto-resistive sensor  A type of wheel speed sensor that uses an effect, similar to what a Hall-effect sensor uses, to create its signal. master cylinder  Converts the brake pedal force into hydraulic pressure, which is then transmitted via brake lines and hoses to one or more pistons at each wheel brake unit. mechanical disadvantage  A situation in which the load distance on a lever is greater than the effort distance, which means the effort required to move the load is greater than the load itself. metering valve  A valve used on vehicles equipped with older rear drum/front disc brakes to delay application of the front disc brakes until the rear drum brakes are applied. It is located in line with the front disc brakes. molybdenum thickening agent  A compound used in some greases to give it the needed consistency. National Highway Traffic Safety Administration (NHTSA)  A U.S. federal agency that is in charge of making sure manufactures comply with safety protocols when developing their vehicles. National Lubricating Grease Institute (NLGI)  An organization that grades the thickness of automotive and industrial grease. Newton’s first law of motion  A physical law that states that an object will stay at rest or uniform speed unless it is acted upon by an outside force. non-integral ABSs  Systems that separate the master cylinder from the HCU so that each individual component can be ­serviced. This is the more prevalent than the combined unit.

GLOSSARY 311

off-car brake lathe  A tool used to machine (refinish) drums and rotors after they have been removed from the vehicle. on-car brake lathe  A tool used to machine (refinish) rotors while they are still attached to the vehicle. original equipment manufacturer (OEM)  The company that manufactured the vehicle. oscilloscope  A handheld electronic tool used to display electrical waveforms relative to voltage signals over divisions of time, such as wheel speed sensor patterns. outer race  The outside component of a wheel bearing that has a smooth, hardened surface for rollers or balls to ride on. outlet port  A port that links the cylinder to the brake lines. output force  The force that equals the working pressure multiplied by the surface area of the output piston, expressed as pounds, newtons, or kilograms. parallel regeneration system  The drive system is separate from the regeneration system, which means that it is variable in the amount of power it can recoup. parallelism, also called thickness variation  A situation where both surfaces of the rotor are parallel to less than 0.001" of each other, so that brake pulsations do not occur. parking brake  A brake system used for holding the vehicle when it is stationary. parking brake cable  A mechanism used to transmit force from the parking brake actuating lever to the brake unit. parking brake cable pliers  A tool used to install parking brake cables. parking brake cable removal tool  A tool used to compress the spring-steel fingers of the parking brake cable so that the cable can be removed from the backing plate. parking brake mechanism  A mechanism that operates the brake shoes or pads to hold the vehicle stationary when the parking brake is applied. Pascal’s law  The law of physics that states that pressure applied to a fluid in one part of a closed system will be transmitted equally to all other areas of the system. phenolic resin  A very dense material, used to create some brake pistons, that is very resistant to corrosion and heat transfer. pickup assembly  A component with a wire coil wrapped around a ferrous metal core; it is used to generate an electrical signal when a magnetic field passes through it. poppet valve  A valve that controls the flow of brake fluid at usually preset pressures. power booster  A device that boosts driver brake pedal force on the master cylinder. preload  A condition where the wheel bearing components are forced together under pressure and therefore have no end play. pressure-differential valve  A valve that monitors any pressure difference between the two separate hydraulic brake circuits. It usually contains a switch to turn on the brake warning light when there is a pressure difference.

primary cup  A seal that holds pressure in the master cylinder when force is applied to the piston. primary piston  A brake piston in the master cylinder moved directly by the pushrod or the power booster. It generates hydraulic pressure to move the secondary piston. proportioning valves  Valves mostly on older vehicles equipped with rear drum brakes, used to reduce rear-wheel hydraulic brake pressure under hard braking or light loads. They are located in line with the rear brakes. pushrod  (braking system) A mechanism used to transmit force from the brake pedal to the master cylinder. quick take-up master cylinders  Cylinders used on disc brake systems that are equipped with low-drag brake calipers to quickly move the brake pads into contact with the brake rotors. quick take-up valve  A valve used to release excess pressure from the larger piston in a quick take-up master cylinder once the brake pads have contacted the brake rotors. recuperation  A process by which brake fluid moves from the reservoir past the edges of the seal into the chamber in front of the piston. This prevents air from being drawn into the hydraulic system caused by low pressure when the brake pedal is released quickly. repair order  The document that is given to the repair technician that details the customer concern and any needed information. residual-pressure valve (residual-check valve)  In drum brake systems, a valve that maintains pressure in the wheel cylinders slightly above atmospheric pressure so that air does not enter the system through the seals in the wheel cylinders. return springs  Springs that retract the brake shoes to their released position. riveted linings  Brake linings riveted to the brake pad backing plate with metal rivets and used on heavier-duty or high-­performance vehicles. roller bearings  The rolling components of a wheel bearing, consisting of hardened cylindrical or tapered rollers. roll-rate sensor  A sensor that tells the computer the rate of roll and the amount of roll that the vehicle is experiencing. rotational force  The force created by the rotating wheel when the brakes are applied; it causes the brake components to twist the brake support, and ultimately the vehicle, in the direction of wheel rotation. rotor  The main rotating part of a disc brake system. running clearance  The amount of space between wheel bearing components while in operation. scan tool  A handheld electronic tool used for accessing ABS codes and live data from the EBCM. Some have bidirectional abilities used to command certain outputs, such as activating a solenoid during a diagnostic routine or when bleeding the brakes. They are also used to clear codes after repairs have been completed.

312 GLOSSARY

scratcher  A thin, spring-steel wear indicator that is fixed to the backing plate of the brake pad. It emits a high-pitched squeal when the brakes linings have become too thin. scrub brakes  A brake system that uses leverage to force a friction block against one or more wheels. sealed bearings  Wheel bearings that are assembled by the manufacturer with the proper lubrication and sealed for life. They normally cannot be disassembled. secondary cup  A seal that prevents loss of fluid from the rear of each piston in the master cylinder. secondary piston  A piston that is moved by hydraulic pressure generated by the primary piston in the master cylinder. self-energizing  The property of drum brakes that assists the driver in applying the brakes. When brake shoes come into contact with the moving drum, the friction tends to wedge the shoes against the drum, thus increasing the braking force. series regeneration system  In this system, the braking system works directly in-line with the propulsion system, which means the amount of regeneration is directly related to the amount of braking input from the driver. service advisor  The person at a repair facility that is in charge of communicating with the customer. service brake  A brake system that is operated while the vehicle is moving, in order to slow down or stop the vehicle. service history  A complete listing of all the servicing and repairs that have been performed on that vehicle. serviceable bearings  Wheel bearings that can be disassembled, cleaned, inspected, packed, reinstalled, and adjusted. servo action  A drum brake design where one brake shoe, when activated, applies an increased activating force to the other brake shoe, in proportion to the initial activating force. It further enhances the self-energizing feature of some drum brakes. single-piston master cylinder  A master cylinder with a single piston that creates hydraulic pressure for all wheel units. If there is a leak in the system, there is a loss of pressure for all wheel units. sliding or floating caliper  A type of brake caliper that has ­piston(s) only on the inboard side of the rotor. The caliper is free to slide or float, thus pulling the outboard brake pad into the rotor when braking force is applied. solid rotor  A type of brake rotor made of solid metal, not ventilated. specialty springs  Springs used to return links and levers on the parking brake system or the self-adjuster mechanism. springs and clips  Various devices that hold the brake shoes in place or return them to their proper place. square-cut O-ring  An O-ring with a square cross section that is used to seal the pistons in disc brake calipers. steering angle sensor  A sensor that tells the computer what the driver’s directional intent is. strategy-based diagnostic process  A systematic process used to diagnose faults in a vehicle.

tandem diaphragm booster  A brake booster that uses two diaphragms inside of it so that the amount of pressure created will equal that of a very large single-diaphragm booster. tandem master cylinder  A master cylinder that has two pistons that operate separate braking circuits so that if a leak develops in one circuit, the other circuit can still operate. tapered roller bearing  A type of wheel bearing with races and rollers that are tapered in such a manner that all of the tapered angles meet at a common point, which allows them to roll freely and yet control thrust. technical service bulletin (TSB)  Service notifications and procedures sent out by the manufacturers to dealer groups alerting technicians about common issues with a particular vehicle or group of vehicles. tone wheel  The part of the wheel speed sensor that has ribs and valleys used to create an electrical signal inside the pickup assembly. top-hat parking brake  A drum brake that is located inside a disc brake rotor in order to act as a parking brake. traction control system (TCS)  A system that uses the ABS, PCM, and (electronic stability control) ESC to help the driver maintain control of the vehicle throughout a loss-of-traction event. trailing shoes  Brake shoes installed so that they are applied in the opposite direction of the forward rotation of the brake drum. They are not self-energizing and are less efficient at developing braking force. transmission-mounted parking brake  A drum brake that is mounted on the drive shaft, just after the transmission, to serve as a parking brake. twin leading shoe drum brake system  Brake shoe arrangement in which both brake shoes are self-energizing in the forward direction. unitized wheel bearing hub  An assembly consisting of the hub, wheel bearing(s), and possibly the wheel flange, which is preassembled and ready to be installed on a vehicle. vacuum pump  An engine-driven or electric pump that generates vacuum so that vacuum-run components will be able to operate on a vehicle. variable reluctance sensors  Magnetic induction sensors that produce voltage as a metal-toothed wheel breaks the magnetic field from the sensors’ permanent magnet. vehicle speed sensor  The component that creates an electrical signal based on the speed of the vehicle, which is sent to the EBCM. ventilated rotor  A type of brake rotor with passages between the rotor surfaces that are used to improve heat transfer to the atmosphere. viscosity  The measurement of the thickness of a liquid. water fade  Brake fade caused by water-soaked brake linings. weight transfer  Weight moving from one set of wheels to the other set of wheels during braking, acceleration, or cornering.

GLOSSARY 313

wheel bearing  A component that allows the wheels to rotate freely while supporting the weight of the vehicle, made up of an inner race, outer race, rollers, and a cage. wheel cylinder  A hydraulic cylinder with one or two pistons, seals, dust boots, and a bleeder screw that pushes the brake shoes into contact with the brake drum to slow down or stop the vehicle. wheel cylinder piston clamp  A tool that prevents the pistons from being pushed out of the wheel cylinders while the brake shoes are being replaced.

wheel speed sensor  A device that monitors wheel speed and sends that signal to the EBCM. wheel studs  Threaded fasteners that are pressed into the wheel hub flange and used to bolt the wheel onto the vehicle. working pressure  The pressure within a hydraulic system while the system is being operated. yaw sensor  A sensor that measures the amount of directional rotation of the vehicle on its vertical axis.

INDEX Note: The letters ‘f ’ and ‘t’ following locators refer to figures and tables, respectively.

A ABS electrical components brake light switch, 277, 277f brake pedal position sensor, 277f EBCM circuit, 278f electronic brake control module, 277–279 ABS hydraulic components channels, 272f master cylinder hydraulic control unit operation, 271–272 hydraulic control units types, 272–273 non-integral and integral, 271f portless, 271f purpose and operation, 270–271 position valve, 273f ACC. See Adaptive cruise control (ACC) acceleration, 28 active listening skills, 4, 4f adaptive cruise control (ACC), 300 adjustable brake pedal system, 56, 56f air-operated braking systems, 34 anchor pin, 143 antilock brake bleeding EBC system, 281–282 depressurizing high-pressure, 284, 285 EBC systems, 281 EBC system’s hydraulic circuits, 282–284 electronic brake control system, removing and installation, 284 and electronic brake control systems, 279 tools, 279–280, 280f vehicle modifications, 280–281 antilock brake system components, 268–269 ABS primary components, 268 HCU, 268f oscilloscope pattern, 269f typical ABS, 268f wheel speed sensor and tone wheel, 269f antilock brake system operation HCU solenoid valve, 270f principles, 269–270 antilock braking system, 92 automatic brake self-adjuster, 138

B backing plate, 138 ball bearings, 228 bearing cage, 228 bearing races, 181

bleeding brake systems ISO flare method, 73 manual, 74–75 pressure, 77 vacuum, 76 blink codes, 279 bonded linings, 98 boost valves, 298 brake drum, 138 brake drum repair measuring brake drums, inspection, 156 removing, cleaning, inspecting, and ­measuring, 156–158 tools, 154–155, 155f brake fade, 31 brake hoses, 58 brake lines flaring, 71–72 inspection, 68 replacing, 69–70 brake lines, brake hoses, and associated hardware inspection, 69 brake lines, hoses, fittings, and supports replacement, 70 brake lining thickness gauges, 108 brake pedal, 33f brake pedal emulator, 37 brake pedal free play measuring, 78 brake pedal height measuring, 78 brake pedal inspection, 78 brake pedal travel measuring, 79 brake rotor deformation lateral runout, 180–181, 180f measuring equipment, 181f multipass vs. single pass, 187 parallelism/thickness variation, 180 refinishing hubless-style rotor on vehicle, 185–187 refinishing rotors off vehicle, 181 refinishing rotors on the vehicle, 181–185 thickness variation, 180f vehicle runout, 181f brake rotors refinishing and drums measuring, 178–179 overview of, 176 rotors, 176–177 service information, 178f specifications on rotors, 177–179, 177f brake shoe adjustment gauge, 154 brake shoes, 138 brake lining coefficient of friction, 146f examples of, 148f hold-down springs, 149f

and lining construction, 146 primary and secondary, 146, 146f rest position, springs, 148f riveted and bonded friction materials, 147–148 brake shoes dissembling, cleaning, inspecting, and reassembling a non-servo brake, 164–166 duo-servo brake removing, cleaning, inspecting, and reassembling, 159–162 and hardware, 159–166 installing wheels, 170–171 preadjusting brakes and installing drums and wheel bearings, 168–170 removing, inspecting, and installing wheel cylinders, 167–168 and torquing lug nuts, 170–171 brake spoon, 154 brake spring pliers, 154 brake switch, 268 brake-line flare types International Standards Organization (ISO), 57 inverted double, 57 flexible, 57f and hose use, 56 hoses issues, 59f materials, 59 sealing washers and fittings, 60 inverted double-flared line and matching fitting, 58f ISO flared line, 58f matching fitting, 58f materials, 57 steel, 57f brake-warning-light system checking, 82–84 circuit, 80f diagnosing, 80–81 fuse with test light, 81f light illumination, 79 non-CANbus system, 82–84 warning, 80f braking principles ABS, 25f disc brakes, 25f emergency case, 24f fundamentals rotational force, 25–26 service and parking, 24–25

316 Index braking principles (Continued) hydraulic forces, 25f modern vehicle, 25f overview, 24 parking, 24f rotational force, 25–26, 26f slow down/stopping, 24f weight transfer, 26f braking system application acceleration, 28, 28f deceleration, 28, 28f factors affecting, 26–27 kinetic energy, 27, 28f tires performance, 27f tires with differing loads, 27f braking system applications air, 34–35, 35f by-wire, 37 canister, 35f compression, 36 electric, 36–37 electrohydraulic, 37 exhaust, 35–36 hydraulic, 34, 34f pneumatic cylinder, 36f power booster, 35f regenerative, 37–38 relay valve, air reservoir, 36f braking systems application, 26–28, 34–38 energy transformation, 29–30 fade, vehicle stopping, 31–32 fundamentals, 24–26 heat transfer, 30–31 rotational force, 33–34

C caliper dust boot seal driver set, 108 caliper piston pliers, 108 caliper piston retracting tool, 108 calipers disassembling, 115–116 inspection, 111–113 mountings, slides and pins, 114 pad components, 116–119 reassembling, 116–119 removing, 111–113 retracting and readjusting pistons, parking brakes, 119 CASs. See Collision avoidance systems (CASs) C-clamp, 108 coefficient of friction, 29 collision avoidance systems (CASs), 299 compensating port, 47 components in braking system circuit leakage, 49f compensating port, 49f DOT ratings for brake fluid, 47t hydraulic pressure, 47f

master cylinder ABS, 49–50, 50f brake warning light, 51f quick take-up, 49 reservoirs and float switches, 50–52, 51f single-piston, 47–48, 48f tandem, 48–49, 48f, 49f quick take-up master cylinder, 50f single-piston master cylinder, 47f cylinder bore, 143 cylindrical roller bearing assembly, 229

D deceleration, 28 diagnosing power assist system brake pedal, 217 brake pedal free play, 219–220 brake pedal height measuring, 217 brake pedal travel measuring, 218 brake-assist issue, power steering system, 221f broken vacuum line, 222f engine compartment, 221f free play measuring, 218f hydroboost system, 222f hydroboost, vehicle power steering ­system, 222f–224f lack in braking system, 221 power booster testing, 219 and service, 217 vacuum power booster, 219–220 vacuum-type power booster, 220–221 diagnostic trouble codes (DTCs), 8 dial indicator, 108 disc brake caliper operation corroded caliper piston bore, 98f fixed and sliding/floating calipers, 95f fixed calipers, multiple pistons, 95f floating caliper and guide pins, 96f heat transfer, 98f low-drag caliper, 97f O-rings, 96–98, 97f sliding, 96f sliding/floating caliper application, 96f square-cut O-ring, 97f disc brake pads antinoise measuring, 101–102 bonded and riveted brake pads, 99f brake lining coefficient of friction, 99t brake lining edge code (FF), 100f coefficients of friction, 100 and friction materials, 98–101 lining grooves and contouring, 101f optimum brake composition, 100 pad bendable tangs, 101f pad locating lugs, 99f rotor wear, 99f shims and guides, 101f spring-loaded brake pad retainers, 101f disc brake rotor micrometer, 108

disc brake rotors ceramic composite, 106–107, 107f composite, 105, 105f, 106f directional ventilated, 106f dust shield, 105f inspection, 119, 121–122 measuring, 119, 121–122 removing and reinstalling rotors, 123–126 rotor thickness and heat capacity, 107f slotted and drilled rotor, 106f solid and ventilated, 105 standard solid, 105f ventilated, 105f disc brake safety, 92 disc brake system caliper operation, 95–98 calipers removing and inspection, 111–119 component application, 92–94 overview, 92 pads and friction material, 98–102 rotor measuring and inspection, 119–126 rotors, 105–107 service, 107–111 wear indicators, 102–104 wheel stud evaluation and installation, 126–132 disc brake system component application advantages, 94 disadvantages, 94 hub and hubless rotors, 94f operation, 93–94 pedal force into hydraulic pressure, 93f primary components, 92–93 sliding caliper mounting methods, 94f disk brake service diagnosing, 107–108 issues, 110t–111t tools, 108–110, 110f divided hydraulic systems, 60 double-acting wheel cylinder, 141 double-row ball bearing assembly, 231 drawing-in method, lug studs, 127 driveline parking brake, 201 driver safety systems adaptive cruise control (ACC), 300 collision avoidance systems (CASs), 299–300 with electronic stability system, 299 lane departure system, 300 lane keeping assist systems, 300 drum brake micrometer, 154 drum brake system types components, 138f duo-servo, 141–142, 141f hydraulic pressure, rotating drum, 139f leading/trailing shoe, 140–141, 141f operation, 138–140 self-energizing, 140f self-energizing and servo action, 139

Index shoe drum brake, forward direction, 141f twin leading shoe, 140 drum brake systems brake shoes and hardware, removal and inspection, 159–171 brake shoes and lining construction, 146–148 components, 142–143 operation, 152–153 parking, 152 repairing, 154–158 self-adjusters, 149–151 springs and hardware, 148–149 types, 138–142 wheel cylinders, 143–146 drum brakes operation dust, 153f noises, 153 parking brake assembly, 152f drum components backing plate, 142–143, 143f brake, 142f hubless, 142f hub-style, 142f labyrinth seal, 143f types, 142 drum parking brake systems, 152 drums measuring, 188–189 refinishing, 190–193 DTCs. See Diagnostic trouble codes (DTCs) duo-servo drum brake systems, 141

E EBC. See Electronic brake control (EBC) systems ECM. See Electronic control module (ECM) electric braking system, 36 electric parking brake, 200–201 electrohydraulic brake unit, 257 electronic brake control (EBC) systems, 266 electronic brake controls ABS, 267f operations, 266–267 oversteer condition, 267f TCS system, 267f understeer condition, 267f electronic control module (ECM), 200 electronic stability control (ESC) system driver safety systems, 299–301 operation, 296–298 traction control system operation, 298–299 electronic stability program (ESP), 201 energy transformation disc brake, 31f drum brake, 31f friction and friction brakes, 29–30 heat transfering, 30–31 kinetic energy, deceleration, 29f

ESC. See Electronic stability control (ESC) system ESP. See Electronic stability program (ESP) exhaust brake, 36

F fabricating brake lines, double-flare method, 71–72 fault codes, 279 fixed calipers, 95 floating calipers, 95 flushing brake systems, 77–78 freeze frame data, 8 fulcrum, 33

G garter spring, 232 gear lube, 233

H Hall-effect styles, 274 HCU boost valves, 298f heat fade, 31 high-pressure accumulators, 273 hold-down springs, 148 hoses inspection, 68 replacing, 69–70 hubless-style drum, 142 hub-style drum, 142 hub-style rotor, 93 hybrid systems, ICE battery charging, 259, 259f brake master cylinder differences, 258 electronics to managing, 258–259, 258f parallel regeneration systems, 257–258 series hybrid drive layout, 257f series regeneration systems, 256–257 hybrid vehicle braking systems diagnosing strategy, 259–261 hybrid systems, ICE, 256–259 regenerative, 256 hydraulic brake systems brake lines, 68 brake-line and hose use, 56–60 components, 46–52 controls, 60–68 driver, 55–56 fluid types and characteristics, 46 hardware, 68 hoses, 68 input force, 45 master cylinder service, 52–55 mechanical force, 45f output force, 45, 45f Pascal’s law(s), 44 pressure and force, 44–45 schematic view, 44f stop-light operation, 84–86

317

warning-light system, 79–84 working pressure, 45, 45f hydraulic braking system control combination valve, 65–66, 66f diagnosing, 67 divided types, 60–61, 60f metering valve testing, 67 metering valves front brakes, 64f operation, 64–65 pressure-differential valve operation, 65 pressure-differential valve leak, 66f pressure-differential valve testing, 68 proportioning valves adjustable, 62 closed position, 61f electronic brake, 63, 63f front brake circuit failure, 62f load-sensitive, 62–63 open position, 61f pressure-sensitive proportioning valve operation, 61–62, 61f hydraulic fade, 32 hydraulic power-assist operation electrohydraulic braking system, 216–217 force, power piston, 214f hydroboost operational stages, 213–215 hydroboost system, 213f inspection and testing, 215f steering gear box, 214f hydraulic press method, lug studs, 127 hydroboost, 212

I ICE. See Internal combustion engine (ICE) independent rear suspension, 93 intermittent faults, 7 internal combustion engine (ICE), 257 International Standards Organization (ISO) flare, 57 isolation valves, 266

K keyed lock washer, 240 kinetic energy, 27, 256

L lane keeping assist systems (LKAS), 300 lateral acceleration sensor, 296 lateral runout, 180 law of conservation of energy, 29 leading/trailing shoe drum brake system, 140 lithium soap, 235 LKAS. See Lane keeping assist systems (LKAS) load transfer, 61 low-drag calipers, 96 low-pressure accumulators, 272

318 Index

M magneto-resistive styles, 274 master cylinder service bench bleeding, 52–53 pushrod length, 54 master cylinder, 34 molybdenum thickening agents, 235

N National Lubricating Grease Institute (NLGI), 235 Newton’s first law of motion, 28 NLGI. See National Lubricating Grease Institute (NLGI)

O OEM. See Original equipment manufacturer (OEM) off-car/bench brake lathe, 108 on-car brake lathe, 108 original equipment manufacturer (OEM), 2

P parallel regeneration system, 257 parallelism, 180 parking brake, 24 parking brake cable pliers, 108 parking brake cable removal tool, 154 parking brake mechanism, 138 parking brakes diagnosing and service, 201–202 inspecting and maintaining, 203 overview of, 198 scan tool, electric parking brake system, 202f types cable-actuated parking brake system, 198–199 conventional drum brake feature, 198–199 driveline parking brakes, 201f electric over hydraulic, 201f electric parking brake, 200–201 integrated mechanical parking brake calipers, 199–200 integrated parking brake caliper, 200f integrated parking brake operation, 200f top-hat rotor and drum parking brake assembly, 200f top-hat-design parking brake, 200 Pascal’s law, 44 PCM. See Power train control module (PCM) phenolic resin, 98 pickup assembly, 273 poppet valve, 61 power train control module (PCM), 8

power-assist systems diagnosing and service, 217–224 hydraulic, 212–217 vacuum booster operation, 208–212 pressure-differential valve, 65

Q quick take-up master cylinders, 49 quick take-up valve, 49

R recuperation, brake pedal, 48 regenerative braking system diagnosing strategy, 259 disabling HEV battery pack, 260–261 repair documentation ordering, 18, 18f parts, 17–18 three Cs cause, 17 concern, 17 correction, 17 residual-pressure valve, 48 return springs, 148 riveted linings, 98 roller bearings, 228 roll-rate sensor, 296 rotational force, 25, 33

S sealed bearings, 228 self-adjusters, brake shoe automatic star wheel brake adjuster, 151f duo-servo-style brake, 150f ratchet-style adjuster, 151f star wheel assembly, 150f types, 150 series regeneration system, 256 service brake, 24 serviceable bearings, 228 serviceable wheel bearings adjusting and repacking, 240–241 adjustment, 240 bearing packer, 244 cotter pin-style wheel bearing, 240f installing, 245–247, 247–248 locknut-style wheel bearing locking, 240f non-serviceable, 240 packer, 241f packing grease hand, 243 races replacing, 248 removing and reinstalling, 249–250 removing, cleaning, and inspecting, 241–242 sealed, 240 tools, 239 wheel bearing hub style, 250–251

single leading shoe, 139 single-acting wheel cylinders, 140 single-piston master cylinders, 47 solenoid valves, 266 solid rotors, 105 specialty springs, 149 springs and hardware drum brake system hold-down, 148–149 return, 148 specialty, 149 square-cut O-ring, 96 steering angle sensor, 296 steering wheel position sensor, 267 stop-light operation brake-light-switch operation, 84f checking, 85–86 CHMSL mounted vehicle, 84f diagnosing, 84–85 strategy-based diagnostic process aftermarket source, 9f customer concern, 6–8 customer impressions, 5–6 documenting repair, 16–18 focused testing, 10–13 process, 5–16 vehicle service history, 2–5 need for, 5 repair performance customer approval, 15 job tools, 14 pay attention to details, 15 prior updates, 15 service procedures, 14 time for, 15 time for repairing, 15 researching faults and gathering ­information, 8–10 researching service information, 7f safe test, 13f shop manual page, 14f technical service bulletin, 9f test records, 10f test simple expectations, 12f testing faults, 11 verifying, 15–16 visual inspection, 7f

T tandem diaphragm booster, 209 tandem master cylinders, 48 tapered roller bearings, 230 TCS. See Traction control system (TCS) technical service bulletin (TSB), 9, 259 tone wheel, 273 traction control system (TCS), 266 traction control system deactivating, 299f trailing shoes, 139 TSB. See Technical service bulletin (TSB) twin leading shoe drum brake system, 140

Index

U unitized wheel bearing hub, 232

V vacuum booster operation diaphragm, atmospheric pressure, 208f dual (tandem) diaphragm, 209 pushrod adjustment, 209–210 type power booster unit for leakage, 210–212 vacuum pump, 210 variable reluctance sensors, 269 vehicle speed sensor, 278 vehicle stopping, brake fade hydraulic fade, 32f temperature, by friction, 31f water fade, 32f ventilated rotors, 105 viscosity, 234

W water fade, 32 wear indicators on brake pads, 102 and brake pads inspection, 103 checking brake pads, 104 scratcher, 102f types, 102–103 warning lamp, 103f

weight transfering, 26 wheel bearing service braking components, 228–229 failures, 237–238 rear drive axles, 233–237 serviceable, 238–251 types, 229–232 wheel bearing types ABS sensors, 229f axle bearings, 234f axle seals, 232 ball bearings, 231f chart, 238f cylindrical roller, 229–230, 230f failures, 237 full floating axle, 233f grease seals, 232 NLGI rating system, 235t rear axle assembly, 234f removing and installing rear axle wheel bearings, 235–237 sealed, 231–232, 231f semi-floating axle, 233f tapered roller, 230–231 tapered roller bearings, 230f three-quarter floating axle, 233f typical seal, 232f unitized wheel bearing hub assembly, 232f vehicles wiring harness, 229f

319

wheel bearings affect braking components, 228, 228f overview, 228 sensors, 229 wheel cylinder piston clamp, 154 wheel cylinders backing plate mounted, 144f components, 144f cutaway, 144f types, 145–146, 145f wheel speed sensor operation Hall-effect assembly, 276f Hall-effect operation, 276f high-pressure accumulator, 274f sine valve, 274f types, 274–276 variable reluctance assembly, 275f wheel speed sensors magneto-resistive testing, 288–290 and tone wheel diagnosing, 286 variable reluctance testing, 286–288 wheel studs, 126 evaluation, 126–127 installation, 126–127 lug nuts, 130f replacing, 127–130 wheels, torquing lug nuts installing, 130–132

Y yaw sensor, 296