Rear Anti-Lock Brake System (RABS)

PURPOSE
The Rear Antilock Brake System (RABS) is used to release brake hydraulic pressure to the rear wheels to prevent rear wheel lock-up.

OPERATION
The RABS continuously monitors rear wheel speed with a RABS sensor mounted in the rear axle housing. When the teeth on a RABS sensor ring, mounted on the ring gear, pass the RABS sensor pole piece, an AC voltage is induced in the sensor circuit with a frequency and amplitude proportional to the average rear wheel speed. In the event of an impending lockup condition during braking, at vehicle speeds above approximately 5 mph, the antilock electronic control module modulates hydraulic pressure to the rear wheel cylinders. This inhibits rear wheel lockup.

When the brake pedal is applied, the antilock 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 antilock electronic control module activates the 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 antilock electronic control module will energize the dump solenoid 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 antilock electronic control module control, 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.

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 to the brake master cylinder. Normal brake operation is resumed.

CONSTRUCTION
The RABS II consists of the following components:
- Antilock electronic control module.
- RABS valve (dual solenoid electro-hydraulic valve) located on the left frame side rail just behind the No. 1 crossmember.
- RABS sensor and RABS sensor ring located in the rear axle housing.
- Yellow REAR ABS warning light in the instrument cluster.
- RABS II diagnostic connector.
- Diode/resistor element located under the power distribution box.
- RABS sensor test connector.