Rear Anti-Lock Brake System (RABS)

PURPOSE
The Rear Anti-lock Brake System (RABS) continuously monitors rear wheel speed with a sensor mounted on the rear axle. When the teeth on a excitor ring (mounted on the ring gear) pass the sensor pole piece, an AC voltage with a frequency proportional to the average rear wheel speed is induced in the sensor circuit. In the event of an impending lockup condition during braking, the RABS modulates hydraulic pressure to the rear brakes. This inhibits rear wheel lockup.

SYSTEM OPERATION
When the brake pedal is applied, the module senses the drop in rear wheel speed. If the rate of deceleration is too great, indicating an impending wheel lockup, the RABS module activates the electro-hydraulic valve, causing the isolation valve to close. With the isolation valve closed, the rear wheel-cylinders are isolated from the master cylinder and the rear brake pressure cannot increase. If the rate of deceleration is still too great, the RABS 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 RABS module control, the dump and isolation solenoids will be pulsed to keep the rear wheels rotating while maintaining high levels of deceleration during braking.

At the end of the stop, when the operator releases the brake pedal, the isolation valve deenergizes and any fluid in the accumulator is returned to the master cylinder. Normal brake operation is then resumed.

NOTE: A RABS malfunction may also cause the E4OD or 4R70W transmission to malfunction.

SYSTEM SELF-TEST OPERATION
The RABS has self-test capabilities much like those in other electronic control systems. Two warning lamps are located in the instrument panel to alert the driver to a system malfunction. The red brake warning lamp indicates a low fluid level condition or parking brake "ON".

The yellow anti-lock indicator lights up for approximately 2 seconds when the ignition switch is initially moved to "ON" or "START" for circuit prove-out. The indicator also lights when the RABS module detects a malfunction in the system.

The RABS II module performs system tests and self tests during start-up and normal operation. The RABS II valve assembly, sensor, and fluid level circuits are monitored for proper operation. If a concern is found, the RABS II will be deactivated and the REAR ABS light will be illuminated. Most concerns will cause the light to stay illuminated until the ignition is turned "OFF", at which time the module will retain the concern in memory, also referred to as Keep Alive Memory (KAM). At amy time while the ignition is turned "ON", a diagnostic trouble code may be obtained. The code is retrieved by momentarily grounding the diagnostic pigtail and counting the flashes of the yellow REAR ABS light. A special code of 16 indicates that the RABS II system is operating normally.