Antilock Brakes Traction and Stability Control Chapter 59

Objectives • Describe the reason for an antilock brake system (ABS) • Explain theory of operation of ABS • Describe the parts of two-, three-, and fourwheel ABS • Explain the differences between integral and nonintegral ABS © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

ABS Brake Fluid Service (cont'd. ) • 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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning

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 © 2012 Delmar, Cengage Learning