antilock brakes, traction, and stability control

© 2012 Delmar, Cengage Learning

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 (cont'd.)• 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• Ability of brakes to do their job

– 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 ten to twenty percent

slip


Antilock Brakes• Wheel speed sensors and computer

– Monitor wheel speed– Wheel speed sensors measure rotational speed

of the wheel• Wheel locks: antilock brake controller pulsates the

pressure to that wheel– ABS is disabled below a certain speed– ABS senses failure: system reverts to

conventional-only braking– Pedal feel: bump followed by rapid pulsing


Antilock Brake System Components

• Include:– ABS computer

• Known as: electronic brake control module, controller antilock brake, or electronic brake and traction control module

– Sensor inputs– Pressure modulator valves– Self-test– Wheel speed sensors

• Variations and wiring– Hydraulic control valve assembly


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 is usually much larger• Some systems have pressure sensitive switch

• Nonintegral ABS– ABS unit is separate from master cylinder and is in

series with brake lines• Two or four wheel• One, three, or four-channel


Two-Wheel ABS• Only works on rear wheels

– Found on SUVs and light trucks– Designed to stop a fully loaded truck

• Rear brakes: modulated simultaneously– Centrally located, single sensor

• Four wheel ABS– Either three or four channel

• Four channel: sensor on each wheel– Front wheels controlled separately


Antilock Brake System Operation

• During two-wheel ABS stop: isolation valve closes

• Action by isolation solenoid not sufficient– Dump valve cycles open and closed rapidly

• Pressure to rear brakes is relieved and wheels are turning– Dump valve closes

• Three- and four-channel systems– Some use single combination valve


Antilock Brake System Operation (cont'd.)

• Nonintegral systems – May use motor pack

• Malfunction occurs in ABS system– Computer shuts system off

• When testing ABS – Pedal pulsing should be felt


Traction Control System• Traction control system (TCS) or acceleration

slip regulation (ASR)– ABS limits wheel spin during acceleration

• Computer matches traction with engine power• Controller disables traction control if brakes

overheat• Electronic stability control

– Computer stabilizes vehicle in sudden evasive maneuver

• Compensates for understeering and oversteering


Antilock Brake (ABS) Service• Fewer than one percent of problems in brake

system come from the ABS– Warning lights

• Amber: ABS problems only• Red: hydraulic system

• False modulation – System operates when it shouldn’t

• Drum brake shoes have incorrect coefficient of friction

• Incorrect brake drum return springs


ABS Brake Fluid Service• Follow correct procedure for inspecting brake

fluid level– Some integral ABS systems must depressurized

before checking• Some operate under extremely high pressure

– Fluid should be replaced every two years• Majority of ABS problems

– Result from wheel sensor failure• Harsh operating conditions or abuse• Demagnetized or polarized by physical impact


ABS Brake Fluid Service (cont'd.)

• Testing a sensor– Follow diagnostic flow chart

• Wheel sensors are magnetic – Attract metal shavings

• Result is erratic sensor signal• Damage to the harness

– Most common wheel sensor service problem• Changing resistance or configuration changes the

signal to computer



• Gap affects voltage produced by sensor– Check using brass

feeler gauge– Looseness in wheel

bearing affects air gap• Look for damaged

teeth– Use a press when

replacing tone ring



• 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



• Integral ABS uses same inputs as nonintegral• RWAL records one soft code at a time• Delco VI ABS failure may result in low pedal• Speed sensor buffers fail quite often

– Check inputs and outputs – Check resistance and AC output voltage of VSS– Check for failed ABS/TCS relay with bad

electrical contacts– Incorrect brake lamp can cause ABS to shut off

antilock brakes, traction, and stability control

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