ABS Know How

Antilock Brakes, Traction,

and Stability Control

Chapter 59

Objectives

• Describe the reason for an antilock brake

system (ABS)

• Explain the theory of operation of ABS

• Describe the parts of two-, three-, and four-

wheel ABS

• Explain the differences between integral and

nonintegral ABS

Objectives (continued)

• Explain how ABS provides traction control and

stability enhancement

• Explain ABS and normal brake warning light

operation

• Describe how to bleed ABS brakes

• Describe service procedures for ABS brakes

Introduction

• Effectiveness of braking limited by tire grip to

road surface

• Skids could be avoided if driver could release

brake pressure just before wheel locks

• When wheel stops turning, friction generates

heat, causing tire to lose traction

• Slip rate of 50% means the wheel is rolling 50%

slower than freely rolling tire

• Maximum traction occurs at 10-20% slip

Antilock Brakes

• Wheel speed sensors measure rotational speed

of the wheel

• If the wheel locks, antilock brake controller

pulsates the pressure to that wheel– Pulsates 10 – 20 times per second

• ABS disabled below a certain speed

• When ABS senses failure, system reverts to

conventional-only braking

• Bump followed by rapid pulsing felt in pedal

Antilock Brake System Components

• ABS computer known as:– Electronic brake control module (EBCM)– Controller antilock brake (CAB)– Electronic brake and traction control module

(EBTCM)

• Wheel speed sensors usually permanent magnet generators– Positioned near tone ring (exciter ring)– Frequency changes with wheel rotation speed

• Hydraulic control valve assembly causes pressure to pulsate

Types of Antilock Brake Systems

• Integral ABS– Combine master cylinder, power brake booster,

ABS hydraulic circuitry in single assembly– Early systems used pump for pressure– Reservoir usually much larger– Some systems have pressure sensitive switch

• Nonintegral ABS– ABS unit separate from master cylinder, in series

with brake lines– Hydraulic pump circulates fluid– Two wheel or four wheel– One-, three-, or four-channel

Two-Wheel ABS

• Only works on rear wheels– Found on SUVs, light trucks– Designed to stop fully loaded truck

• Rear brakes modulated simultaneously– System needs only one sensor

• Four wheel ABS either three or four channel– Front wheels controlled separately

• Three channel – two sensors in front, one in back

• Four channel – sensor on each wheel

Antilock Brake System Operation

• During two-wheel ABS stop, isolation valve closes

• When action by isolation solenoid not sufficient, dump valve cycles open and closed rapidly

• Dump valve closes when pressure to rear brakes relieved, and wheels turning

• In some three and four channel systems, combination valve used

• Nonintegral systems may use motor pack instead of electric pressure pump

Antilock Brake System Operation

(continued)

• If malfunction occurs in ABS system, computer shuts system off

• When testing ABS you should feel the pedal pulsing

• ABS may limit wheel spin during acceleration– Computer matches traction with engine power– controller disables traction control if brakes

overheat

• Computer stabilizes vehicle in sudden evasive maneuver– Compensates for understeering and oversteering

Antilock Brake (ABS) Service

• Fewer than 1% of problems in brake system

come from the ABS

• Two warning lights:– Amber for ABS problems only

– Red for hydraulic system

• False modulation – system operates when it

shouldn’t– Brake drum linings incorrect coefficient of friction

– Incorrect brake drum return springs

– Two primary linings on one side, secondary on other side

ABS Brake Fluid Service

• Follow correct procedure for inspecting brake fluid level

• Some integral ABS systems must be bled of pressure before service

• Fluid replaced every 2 years, every time pads are replaced

• Majority of ABS problems result from wheel sensor failure– Harsh operating conditions– Demagnetized or polarized by physical impact

ABS Brake Fluid Service (continued)

• Testing a sensor– Spin a wheel while reading AC voltage output

– Follow diagnostic flow chart

• Wheel sensors are magnetic and attract metal

shavings– Result is erratic sensor reading

• Damage to the harness is most common wheel

sensor service problem– Changing resistance or configuration changes the

signal to computer

ABS Brake Fluid Service (continued)

• Gap affects voltage

produced by sensor– Check using brass

feeler gauge

– Looseness in wheel bearing affects air gap

• Look for damaged tone

ring teeth– Use a press when

replacing tone ring

ABS Brake Fluid Service (continued)

• Precautions common to all antilock systems– Do not fast charge battery with computer

connected

– Do not use charger on fast charge setting to jump start a vehicle with ABS

– Do not arc weld on frame with computer connected

– Do not install antenna near ABS controller

– Do not change tire size other than width

– Do not disconnect or reconnect electrical ABS parts while ignition is on

ABS Brake Fluid Service (continued)

• Integral ABS uses same inputs as nonintegral

• RWAL records one soft code at a time– If code not a hard fault, controller shuts off MIL

and codes remain in memory

• Delco VI ABS failure may result in low pedal– Follow diagnostic chart to repair problem

• Speed sensor buffers fail– Check inputs and outputs – Check resistance and AC output voltage of VSS– Failed ABS/TCS relay with bad electrical contacts– Incorrect brake lamp can cause ABS to shut off