Rear Anti-Lock Brake System (RABS)

Rear Anti-Lock Brake System (RABS) Components:

Rear Anti-Lock Brake:

PURPOSE
The Rear Anti-lock Brake System (RABS) prevents rear wheel lockup by automatically modulating the brake pressure during an emergency stop. By not locking the wheels, the driver has improved steering control during hard braking and can stop the vehicle in the shortest possible distance under most conditions.

OPERATION
Wheel Speed
The Rear Anti-Lock Brake System (RABS) continuously monitors rear wheel speed with a sensor mounted in the differential case. A toothed wheel (Anti-lock Sensor Indicator) is mounted on the ring gear. When the teeth on indicator pass the RABS sensor pole piece, an AC voltage is induced in the sensor circuit with a frequency proportional to the average rear wheel speed.

ABS Braking
In the event of an impending lockup during braking, at vehicle speeds above approximately 8 km/h (5 mph), the anti-lock electronic control module senses the drop in rear wheel speed. If the rate of deceleration is excessive, the anti-lock 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 fluid pressure cannot increase. If the rate of deceleration is still excessive, the anti-lock electronic control module will energize the dump solenoid with a series of rapid pulses to bleed the rear wheel cylinder fluid into an accumulator built into the RABS valve. This reduces the rear wheel cylinder brake fluid pressure and allows the wheels to spin at vehicle speed.

The anti-lock electronic control module pulses the dump and isolation solenoids in a manner that will keep the wheels rotating while still providing high levels of deceleration during braking. Once stopped, the operator releases the brake pedal, the isolation valve de-energizes and any fluid in the accumulator returns to the brake master cylinder. Normal brake operation resumes.