Antilock Brakes, Traction, and Stability Control

1. Antilock Brakes, Traction,and Stability Control Chapter 59

2. 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

3. 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

4. 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

6. 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

7. 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

10. 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

12. 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

13. 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

14. 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

16. 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

18. 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

19. 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

21. 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

23. 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

24. 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

25. 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