Antilock Brake System

DESCRIPTION
The ABS (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. The brake pedal force required to engage the ABS function may vary with the road surface conditions. A dry surface requires a higher force, while a slippery surface requires much less force.

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 ABS 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 ABS 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 ABS control module returns the valves to their normal condition allowing fluid flow to the affected brake.
- The primary (rear) circuit of the brake master cylinder feeds the right front and left rear brakes. The secondary (front) circuit of the brake master cylinder feeds the left front and right rear brakes.

OPERATION - ABS CONTROL UNIT
- The ABS control module monitors the electro-mechanical components of the system.
- Malfunction of the ABS will cause the ABS control module to shut off or inhibit the system. However, normal power-assisted braking remains.
- The 4-wheel ABS is self-monitoring. When the ignition switch is turned to "RUN", the ABS control module will perform a preliminary self-check on the ABS electrical system indicated by a one-to-two second illumination of the amber ABS indicator.
- During vehicle operation, including normal and ABS braking, the ABS control module monitors all electrical ABS functions and some hydraulic operations.
- Each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h (13 mph), the ABS 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 ABS control module.
- In most malfunctions of the ABS/traction control system, the amber ABS will be illuminated.
- Most malfunctions are recorded as a diagnostic trouble code (DTC) in the ABS 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 unit (HCU).
- brake pressure control valve block.
- pump motor.
- ABS control module.
- front and rear brake ABS sensors.
- -ABS pedal sensor switch

OUTPUTS
The ABS control module controls the operation of the following:
- ABS power relay.
- ABS motor relay.
- ABS indicator.
- ABS valve assembly.

BRAKE FLUID LEVEL SWITCH
There are two brake fluid level switches in use. One in in the master cylinder reservoir the other is in the hydraulic control unit reservoir. The switch on the master cylinder does not affect ABS operation. The Brake fluid level switch on the hydraulic control unit reservoir does control ABS operation.