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Chapter 73

Chapter 73. Anti-Lock Brakes, Traction Control, and Stability Control. Contents. Anti-lock brake systems (ABS) Traction and stability control systems ABS service Traction and stability control system service Final system check. Anti-Lock Brake Systems (ABS).

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Chapter 73

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

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

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

  4. Anti-Lock Brake System

  5. ABS Operation • For good drivers to get maximum braking and control, extreme tire slip must be avoided • On slick pavement 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

  6. With and Without ABS

  7. Without ABS • Without ABS cars tend to slide to the right because of higher tire adhesion on the road

  8. With ABS With ABS cars tend to travel further during hard braking

  9. Basic ABS Components

  10. ABS Components

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

  12. Wheel Speed Sensor Wheel sensors produce an ac signal that corresponds to wheel and tire speed Sensor Rotor moving teeth on the sensor rotor induces current in the wheel sensor coil

  13. Wheel Speed Sensor/Sensor Rotor • Wheel sensors produce an ac signal that corresponds to wheel and tire speed • The signal increases in frequency as the tire rotates faster • As the tire rotates, moving teeth on the sensor rotor induce current in the sensor coil

  14. Sensor Signal Frequency

  15. 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

  16. Electro-Hydraulic Modulator

  17. 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

  18. ABS CircuitWarning Light

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

  20. ABS Accumulator The accumulator in the ABS system stores high pressure fluid and Caution should be used when servicing!

  21. During Hard Braking • When the ABS system is in operation you will feel a pulsing in the brake pedal; this is caused by the solenoidvalves cycling pressure on and off. Some ABS systems can cycle up to 15 times per second.

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

  23. A:ABS control module • B: booster pump & motor • C: wheel speed sensor • D: sensor rotor • E: rotor

  24. 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

  25. 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

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

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

  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 • The ABS system is not functioning during normal driving • The ABS system only becomesactive during tire slippage

  33. Testing a Wheel Sensor Above: Using a hand-held scope and measuring the wheel sensor signal

  34. Traction and Stability Control Systems

  35. 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

  36. 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

  37. 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

  38. Understeer • When a vehicle has a tendency to turn less sharply than is intended • 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

  39. Understeer

  40. Oversteer • When a vehicle has a tendency to turn more sharply than is intended • 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

  41. Oversteer

  42. 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 drive • 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

  43. Stability Control System Inputs

  44. ABS Service • To service anti-lock brake systems properly, you should be familiar with how a normally operating system feels during operation • You should also be familiar with the exact type of ABS being repaired • Designs and procedures vary • Follow the service manual procedures

  45. ABS Inspection • Inspect for the following conditions: • ABS indicator light on • low or contaminated brake fluid • brake fluid leaks • brake pad lining and rotor wear • tires that will not rotate freely • loose or worn wheel bearings • different tire size or poor tire condition • wheel speed sensor damage

  46. Scanning ABS • Perform a diagnostic circuit check by starting the car and watching the ABS warning light • the lamp should illuminate and then go out after the system self-check is complete • If the ABS light stays on, use a scan tool to check for diagnostic trouble codes

  47. Scanning ABS Typical scan tool connections

  48. Scanning ABS • Connect a scan tool to the data link connector or separate ABS diagnostic connector • Check for a code history and review all trouble codes • When available, perform manual or automatic tests, which will verify system operation • pump, lamp, and solenoid operation

  49. Scanning ABS This tester can be placed on the seat to check operating values during a road test

  50. ABS Pinpoint Tests • Use pinpoint tests to determine the exact source of a problem • Use a multimeter or oscilloscope to check circuit components • Compare the voltage and resistance values to the service manual specifications

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