Four Wheel Anti-Lock Brake System (4WABS)

PURPOSE
The 4-wheel anti-lock brake system (4WABS) prevents wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver can improve steering control during hard braking and stop the vehicle in the shortest possible distance under most conditions.

OPERATION
The 4WABS system controls both front and rear brakes separately. The brake pedal force required to engage the 4WABS function may vary with the road surface conditions. A dry surface requires a higher force, while a slippery surface requires much less force.

During the 4WABS operation, the driver will sense a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal height. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are similar to that of a conventional power brake system.

It is an on-board diagnostic, non-repairable unit consisting of two microprocessors and the necessary circuitry for their operation. These microprocessors are programmed identically. The anti-lock brake control module monitors system operation during normal driving as well as during anti-lock braking.

Under normal driving conditions, the microprocessors produce short test pulses to check the electrical system without any mechanical reaction. The anti-lock brake control module continuously monitors the speed of each wheel (1007) to determine if any wheel is beginning to lock. When a wheel locking tendency is detected, the anti-lock brake control module signals the appropriate solenoid valve in the anti-lock hydraulic control unit to open or close as well as the pump motor to turn on and recycle brake fluid back to the brake master cylinder (2140). This results in moderate pulsations of the brake pedal (2455) and pump noise which may be heard in the passenger compartment. During normal braking, the brake pedal feel will be identical to a standard brake system.

Most concerns which occur to the anti-lock brake system will be stored as a coded number in the Non-volatile RAM (NRAM) of the anti-lock brake control module. This means that once a code is stored, it will be retained by the anti-lock brake control module even with the key in the OFF position. The codes can be retrieved by following the on-board diagnostic procedures. Refer to Diagnosis and Testing.

SUB-SYSTEMS
^ Anti-lock hydraulic control unit (2C215)
^ Anti-lock brake control module
^ Two front wheel speed sensors
^ Speed sensor indicators
^ One rear axle speed sensor
^ G-switch
^ Main relay
^ Pump motor relay