Publisher The Goodheart-Willcox Co., Inc. Tinley Park, Illinois

1. PublisherThe Goodheart-Willcox Co., Inc.Tinley Park, Illinois PowerPoint for Modern Automotive Technology by Russell Krick

2. Chapter 73 Anti-Lock Brakes, Traction Control, and Stability Control

3. Contents • Anti-lock brake systems (ABS) • Traction and stability control systems • ABS service • Traction and stability control system service • Final system check

4. Anti-Lock Brake Systems (ABS) An ABS uses wheel speed sensors, a computer (ECM), and a modulatorunit to prevent skidding during hard braking

5. Anti-Lock Brake System

6. ABS Operation • For maximum braking and control, extreme tire slip must be avoided • When the tire slips, its friction with the road surface drops, and stopping distance increases • ABS modulates braking effort at the controlled wheels to reduce the stopping distance and increase directional stability when braking

7. ABS Operation

8. Basic ABS Components

9. ABS Components

10. Wheel Speed Sensors • Produce ac signals that correspond to wheel and tire speed • These signals are fed to the anti-lock brake system control module • As the tire rotates, moving teeth on the sensor rotor induce current in the sensor coil

11. Wheel Speed Sensor

12. Sensor Signal Frequency • A rapid decrease in a sensor’s signal frequency indicates that a tire is starting to lose traction and lock up • The control module then sends an electrical signal to the hydraulic modulator to pulsate hydraulic pressure to the affected wheel cylinder, modulating braking effort

13. Sensor Signal Frequency

14. ABS Control Module Uses wheel speed sensor inputs to control the electro-hydraulic modulator

15. Electro-Hydraulic Modulator • Regulates the fluid pressure applied to each wheel brake assembly during ABS operation • Controlled by the ABS control module • Pressure modulation refers to the rapid cycling of pressure to the brake assemblies, preventing wheel lockup • occurs 15 to 20 times per second

16. Electro-Hydraulic Modulator

17. Modulator Components • Fluid reservoir • holds an extra supply of brake fluid • Solenoid valve block • coil-operated valves that control brake fluid flow to the wheel brake cylinders • Accumulator • chamber for storing fluid under high pressure

18. Modulator Components • Hydraulic pump and motor • motor-driven pump that provides brake fluid pressure for the system • Pressure switch • monitors system pressure and controls the operation of the motor-driven pump • Master cylinder-booster assembly • conventional master cylinder with power assist for normal braking

19. ABS Hydraulics Here, the brakes are applied, and pressure is being dumped from one wheel to prevent lockup

20. ABS Circuits

21. ABS Modes of Operation • Three modes of ABS operation are used: • isolation mode • dump mode • reapply mode

22. Isolation Mode • The control module detects that a tire is starting to slip • The module energizes a modulator solenoid to isolate that hydraulic circuit from the other brake lines • The fluid is trapped in the wheel brake at the currently applied pressure

23. Dump Mode • The control module detects that the tire is continuing to slip and slow down • The module energizes a solenoid to release the fluid pressure from the wheel brake assembly to reduce braking effort • The tire speed will tend to increase

24. Dump Mode

25. Reapply Mode • The control module detects that the tire speed has increased, so more braking is required • The module de-energizes the dump solenoid and isolation solenoid to allow fluid pressure to build in the brake circuit again, restoring braking effort

26. Reapply Mode

27. ABS Warning Light • Mounted in the dash • Used to alert the driver to an ABS malfunction • Turns on during engine cranking and ABS self-check at startup to verify operation

28. Integrated ABS • Combines the power booster, master cylinder, and modulator units into one assembly • More common on early systems • More expensive and difficult to repair

29. Nonintegrated ABS • Major units are separated • Uses a conventional master cylinder and booster • The electro-hydraulic modulator is a separate unit • More common on recent ABS units

30. ABS Channels • Separate hydraulic circuits that feed one or more wheel cylinders or calipers • One-channel ABS • operates only the rear wheel brakes together • Two-channel ABS • two separate hydraulic circuits are controlled by the modulator • rear-wheel ABS with separate control for each wheel brake

31. ABS Channel • Three-channel ABS • three separate hydraulic circuits are controlled by the modulator • one channel for each front wheel brake • one channel for both rear wheel brakes • Four-channel ABS • four separate hydraulic circuits are controlled by the modulator • provides a separate hydraulic circuit for each wheel brake

32. ABS Operation(Normal Braking) • Master cylinder pressure flows normally to the brake assembly • No pump or solenoid operation occurs

33. ABS Operation(Normal Braking)

34. ABS Operation(Pressure Reduction) • The ABS starts working to avoid a skid • Solenoids close the inlet valve and open the outlet valve • Pressure is released to the reservoir • The caliper pressure is reduced, allowing that wheel to turn

35. ABS Operation(Pressure Reduction)

36. ABS Operation(Pressure Increase) • The ABS increases caliper pressure to increase stopping action • Solenoids open the inlet valve and close the outlet valve • High-pressure fluid is sent to the back pressure chamber, and the piston slides to increase pressure to the brake caliper

37. ABS Operation(Pressure Increase)

38. ABS Operation(Failure Mode) • The ABS control unit monitors the pressure in the high-pressure passage • When the pressure drops, the control unit turns on the ABS warning light • The sleeve moves and hydraulically locks the back pressure chamber • The master cylinder is connected to the caliper for normal braking

39. ABS Operation(Failure Mode)

40. Traction and Stability Control Systems

41. Traction Control Systems • Designed to prevent the vehicle’s wheels from spinning and losing traction under hard acceleration • Most systems work with the anti-lock brake system to cycle hydraulic pressure to the wheel spinning the fastest

42. Traction Control Systems • The control module is capable of applying only one wheel brake at a time • Some systems also reduce the engine’s power output to reduce slipping • The indicator light is illuminated anytime the traction control system is activated • warns the driver that the tires are losing traction

43. Stability Control Systems • Advanced systems that reduce tire spin upon acceleration and prevent tire skid when cornering too quickly • Use more input signals from various sensors to provide greater control under severe cornering, braking, and acceleration conditions

44. Understeer • If understeer is detected, the control module will apply braking force to the rear wheel on the opposite side of the vehicle • this brings the front of the vehicle back under control for making the turn

45. Understeer

46. Oversteer • If oversteer is detected, the control module will apply braking force to the outside front wheel • this prevents the rear of the vehicle from sliding sideways in a turn

47. Oversteer

48. Stability Control System Inputs • Wheel speed sensors • detect individual wheel speeds • Steering angle sensor • measures how sharply the steering wheel is rotated • Lateral acceleration sensor • measures how much side force is generated by a turn

49. Stability Control System Inputs • Yaw sensor • measures the direction of the thrust generated by vehicle movement • Throttle position sensor • measures the demand for power • Brake pressure sensor • measures the amount of brake system hydraulic pressure generated by the driver

50. Stability Control System Inputs