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Discussion Starter #1 (Edited)
The ABS operates as follows:

  • When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the HCU (Hydraulic Control Unit) inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU and then through the outlet ports of the HCU to each wheel. The EBD ( Electronic Brake Distribution)operates as follows:
  • The EBD control system detects the front/rear wheel slip according to the signal of ABS wheel speed sensors. If the rear wheel slip increases beyond a preset limit in proportion to the front wheel, the ABS HU/CU optimally reduces the brake fluid to the rear wheels. Brake force can, therefore, be proportioned optimally according to vehicle load and road surface conditions. The EBD control changes the distribution rate of the brake fluid pressure to the rear wheel slip. If the ABS control conditions are satisfied, EBD control is stopped and ABS control takes priority.
  • If the ABS/EBD brake control module senses a wheel is about to lock, based on anti-lock brake sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit.
  • The anti-lock brake control module then looks at the anti-lock brake sensor signal from the affected wheel(s) again.
  • If that wheel(s) is still decelerating, it opens the closed solenoid valve for that circuit.
  • Once the affected wheel comes back up to speed, the anti-lock brake control module
    returns the valves to their normal condition, allowing fluid to flow to the affected brake.
  • The ABS/EBD control module monitors the electromechanical components of the system.
  • A malfunction in the ABS/EBD system will cause the ABS/EBD module to shut off or inhibit the system. However, normal power assisted braking remains.
  • Malfunctions are indicated by a yellow ABS warning indicator in the instrument cluster.
  • A malfunction in the EBD function will cause the ABS control module to inhibit the function of the rear brake force distribution. In the event that EBD control stops, the rear wheels may lock before the front wheels, causing the vehicle to skid.
  • Malfunctions are indicated by a red brake system warning light in the instrument cluster.
  • The anti-lock brake and EBD system is selfmonitoring. When the ignition switch is turned to the RUN position, the anti-lock brake and EBD control module will carry out a preliminary self-check on the anti-lock electrical system indicated by a three-second illumination of the yellow ABS warning indicator and the red brake warning indicator (if the parking brake is unapplied) in the instrument cluster.
  • During vehicle operation, including normal and anti-lock braking, the anti-lock brake
    control module monitors all electrical anti-lock functions and some hydraulic operations.
  • Each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h (12 mph), the anti-lock brake control module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the anti-lock brake control module.
  • Pedal pulsation coupled with noise while braking on loose gravel, bumps, wet or snowy roads is normal and indicates correct functioning of the vehicle's anti-lock brake
    control system.
Hydraulic Control Unit

The HCU consists of the following components:

  • brake pressure control valve block
  • pump motor
New brake pressure control valve block and pump motor are installed as an assembly.

Anti-Lock Brake Control Module

The anti-lock brake control module is mounted to the HCU. It is an on-board diagnostic, non-repairable unit consisting of two microprocessors and the necessary circuitry for their operation. The antilock brake control module monitors system operation during normal driving as well as during anti-lock braking. Anti-lock brake module operation is as follows:

  • Under normal driving conditions, the microprocessor produces short test pulses to
    the solenoid valves that check the electrical system without any mechanical reaction.
  • Impending wheel lock conditions trigger signals from the anti-lock brake control
    module that open and close the appropriate solenoid valves. This results in moderate
    pulsations in the brake pedal.
  • The anti-lock brake module used in 4x4 application includes a G-sensor. It detects vehicle movement during a brake lockup event that is transferred to other wheels through the powertrain. During normal braking, the brake pedal feel is identical to a standard brake system. Most faults that occur in the anti-lock brake system will be stored as a diagnostic trouble code (DTC) in the keep-alive memory of the antilock brake control module. The DTCs can be retrieved by following the on-board diagnostic procedures.
Anti-Lock Brake Sensor

NOTE: Any time an anti-lock brake sensor is removed, thoroughly clean the mounting
surfaces. On front anti-lock brake sensors, apply High-Temperature 4x4 Front Axle and Wheel Bearing Grease.

The anti-lock brake system uses four "active" sensors and four sensor indicators to detect the speed of each wheel. The teeth on the sensor indicator rotate past the stationary sensor at wheel speed. As the teeth pass the sensor, a digital input signal is generated. The control module uses the input to compute the speed of each wheel.
 
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