Rear Anti-Lock Brake System (RABS)

Component Location (Part 1 Of 2):

Component Location (Part 2 Of 2):

RABS II CONSISTS OF THE FOLLOWING COMPONENTS
- Anti-lock electronic control module centrally located within the instrument panel, mounted to a brace behind the instrument panel finish panel.
- Rear ABS (RABS) valve located approximately five inches rearward of the No. 1 front crossmember on the inboard side of the left frame side rail.
- Speed sensor and speed sensor ring located in the rear axle housing.
- Yellow REAR ABS warning light in the instrument cluster.
- RABS diagnostic connector located on the forward end of the power distribution box in the engine compartment.
- Diode/resistor element located in the power distribution box in the engine compartment.
- Sensor test connector with cap located under the hood slightly behind the washer bottle attached to the power distribution box.

OPERATION
The Rear ABS (RABS II) continually monitors rear wheel speed with a speed sensor mounted on the rear axle housing while the brakes are applied. When the teeth on the speed sensor ring, mounted on the ring gear, pass the sensor pole piece, an AC voltage is induced in the sensor circuit with a frequency proportional to the average rear wheel speed. In the event of an impending lockup condition during braking, the RABS II system will signal the RABS valve to close off to relieve hydraulic pressure to the rear brakes. This inhibits rear wheel lockup.

When The Brake Pedal Is Applied
The anti-lock electronic control module senses the drop in rear wheel speed. If the rate of deceleration is too great, indicating that wheel lockup is going to occur, the anti-lock electronic control module activates the Rear Anti-lock Brake System (RABS) valve causing the isolation valve to close. With the isolation valve closed, the rear wheel cylinders are isolated from the brake master cylinder and the rear brake pressure cannot increase. If the rate of deceleration is still too great, the anti-lock electronic control module will energize the dump solenoid (within the RABS valve) with a series of rapid pulses to bleed off rear wheel cylinder fluid into an accumulator built into the RABS valve. This will reduce the rear wheel cylinder pressure and allow the rear wheels to spin back up to vehicle speed. Continuing under control of the anti-lock electronic control module, the dump and isolation solenoids will be pulsed in a manner that will keep the rear wheels rotating while still maintaining high levels of deceleration during braking.

When The Brake Pedal Is Released
At the end of the stop, when the operator releases the brake pedal, the isolation valve de-energizes and any fluid in the accumulator is returned through the RABS valve input tube and up to the brake master cylinder. Normal brake operation is resumed.