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 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
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.) • 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 (cont'd.) • 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