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Chapter 10

Chapter 10. Antilock Brakes: Traction and Stability Control. Introduction. Antilock brakes (ABS) prevent wheels from locking up during braking. A locked wheel (skidding) cannot be steered by the driver. Traction. 100 percent slip: There is no traction. 50 percent slip:

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Chapter 10

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  1. Chapter 10 Antilock Brakes: Traction and Stability Control

  2. Introduction • Antilock brakes (ABS) prevent wheels from locking up during braking. • A locked wheel (skidding) cannot be steered by the driver.

  3. Traction • 100 percent slip: • There is no traction. • 50 percent slip: • The wheel is rolling 50 percent slower than a free wheel on the same vehicle. • Maximum traction occurs at about 10–20 percent slip.

  4. Pedal Feel • When ABS is active, there may be a series of pedal pulsations felt by the driver. • The brake pedal may bump slightly as ABS action begins. • When ABS is not needed, pedal feel is just like a non-ABS vehicle.

  5. ABS Components • Electronic control unit • Wheel speed sensor • Lateral acceleration sensor (some vehicles)

  6. Electronic Control Unit Known by several names: • EBTCM. • CAB. • EBCM. • Controller.

  7. Wheel Sensors • Wheel speed sensors are used to determine if one wheel is slowing at a faster speed than the other wheels. • This means a lock-up of that wheel is starting to happen.

  8. Wheel Sensor Locations • Review Figures 10.4, 10.5, 10.7 and 10.8 in your text for illustrations of wheel sensor installation.

  9. Lateral Acceleration Sensor • Some vehicles use a lateral acceleration sensor to modify ABS action during a hard turn.

  10. Hydraulic Control Assembly • The hydraulic control unit takes electrical commands from the controller and turns them into hydraulic action in the brake system.

  11. Hydraulic Control Assembly (continued) • Integral ABS — usually incorporates the master cylinder, hydraulic booster, pressure pump, and brake controller in one unit. • Make sure manufacturer’s precautions are followed! • Stores brake fluid under high pressure!

  12. Non-Integral • Non-integral ABS — a base brake system with the addition of a hydraulic modulator and controller.

  13. Two-Wheel ABS • Some ABS systems control only the rear brakes. • Found mostly on light trucks. • Rear brake systems for trucks were difficult to design because of changes in loads carried.

  14. Four-Wheel ABS • In four-wheel ABS, all wheels are monitored. • Front brakes are controlled individually — rears are controlled in tandem or separately.

  15. One Channel • One-channel system: • Rear wheels controlled in tandem.

  16. Three and Four Channels • Three-channel systems: • Control front wheels independently— rear wheels as one. • Four-channel systems: • Control all four independently.

  17. ABS Operation • Pressure build or increase — normal braking. • When a wheel is about to lock: • Pressure is held and not allowed to increase. • If wheel continues to slow too quickly: • Pressure is released or reduced from that wheel.

  18. ABS Operation (continued) • When a wheel is turning at the proper speed: • Pressure is allowed to reapply. • This hold, release, and apply action can repeat 10 to 20 times per second as needed.

  19. Traction Control • Vehicles with traction control can apply the brakes to reduce wheel slip during acceleration. • Works with engine controls to limit the amount of torque applied to the drive wheels.

  20. Electronic Stability Control • This system works with both traction control and ABS. • The system can sense oversteer and understeer and apply corrective action.

  21. Antilock Brake Service • Most problems are related to base brake, not ABS. • Make sure the base brake system is in good repair before looking for ABS problems.

  22. Antilock Brake Service (continued) • Red warning light (on most vehicles) indicates a base brake problem. • Amber warning lamp indicates an ABS concern. • Both lamps could come on if a condition that affects both systems occurs.

  23. ABS Service • There are many variations to ABS units and service procedures. • Correct information for the vehicle is mandatory. • Be certain to verify your repair.

  24. ABS Brake Fluid Service • Follow procedure for checking ABS fluid level — some systems may only appear low. • Integral ABS systems have to be depressurized before service. • Some systems are pressurized to 2,700 pounds!

  25. ABS Brake Fluid Service (continued) • Brake fluid must be clean and moisture-free to avoid costly repairs. • Follow manufacturer’s maintenance recommendations for changing brake fluid. • A special procedure and/or tools may be required to bleed an ABS system. Check before you start!

  26. Wheel Speed Sensor Service • Wheel speed sensors are sensitive to: • Impact damage on the sensor or tone ring. • Poor connections. • Broken wiring due to contacting moving parts. • Metal shavings from excessive lining wear.

  27. Wheel Sensor Service Sensors can be checked for: • A/C output. • Waveform. • Resistance. • Air gap.

  28. General ABS Repair • Make sure base brakes are in good condition. • Make sure that all wheel/tire sizes are identical and factory-specified size. • Check for service bulletins. • Be sure to have service information/equipment needed for repair. • Verify your work!

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