Antilock Brake System

DESCRIPTION
The anti-lock brake system (ABS) is a four-wheel system which prevents wheel lock-up by automatically modulating the brake pressure during an emergency stop. By preventing the wheels from locking, it enables the driver to maintain steering control and to stop in the shortest possible distance under most conditions.

OPERATION - OVERVIEW
During normal braking, the ABS and non-ABS brake pedal feel will be the same. During ABS operation, a pulsation can be felt in the brake pedal, accompanied by a fall and then rise in brake pedal height and a clicking sound.

OPERATION - ABS CYCLE
ABS operates as follows:
- When the brake pedal is applied, fluid is forced from the brake master cylinder outlet ports to the Hydraulic Control Unit inlet ports.
- This pressure is transmitted through four normally open solenoid valves contained inside the brake pressure control valve block, then through the outlet ports of the brake pressure control valve block to each wheel.
- If the anti-lock brake control module senses that a wheel is about to lock, based on 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 sensor signal from the affected wheel again.
- If that wheel is still decelerating, it opens the normally closed solenoid valve for that circuit. This dumps any pressure that is trapped between the normally open valve and the brake back to the accumulator.
- Once the affected wheel comes back up to speed, the anti-lock brake control module returns the valves to their normal condition allowing fluid flow to the affected brake.

OPERATION - ABS CONTROL UNIT
- The anti-lock brake control module monitors the electro-mechanical components of the system.
- Malfunction of the anti-lock brake system will cause the anti-lock brake control module to shut off or inhibit the system. However, normal power-assisted braking remains.
- The 4-wheel anti-lock brake system is self-monitoring. When the ignition switch is turned to "RUN", the anti-lock brake control module will perform a preliminary self-check on the anti-lock electrical system indicated by a one-to-two second illumination of the amber ABS indicator.
- 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 (13 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.
- In most malfunctions of the anti-lock brake/traction control system, the amber ABS will be illuminated.
- Most malfunctions are recorded as a diagnostic trouble code (DTC) in the anti-lock brake control module memory and assist in pinpointing the component needing service.

COMPONENTS
- Power brake booster
- Brake master cylinder
- Necessary brake tubes and hoses
- ABS hydraulic control valves and electronic control unit
- Brake pressure control valve block
- Pump motor
- Anti-lock brake control module
- Front and rear brake anti-lock sensors

NOTE: The ABS-only anti-lock brake control module, brake pressure control valve block and pump motor are physically different than the ABS with traction control anti-lock brake control module, brake pressure control valve block and pump motor.