Antilock Brakes / Traction Control Systems: Description and Operation

PURPOSE
If the an impending lockup condition occurs during braking, the Rear Anti-lock Brake System (RABS) modulates hydraulic pressure to the rear brakes to inhibit rear wheel lockup.

OPERATION
The RABS continuously monitors rear wheel speed with a sensor mounted on the rear axle. When the teeth on an excitor ring (mounted on the ring gear) pass the sensor pole piece, an AC voltage with 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 Module modulates hydraulic pressure to the rear brakes. This inhibits rear wheel lockup.
When the brake pedal is applied, the RABS 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 wheel 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 in a manner that will keep the rear wheel 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 master cylinder. Normal brake operation is resumed.

SYSTEM SELF-TEST

NOTES:
- A RABS malfunction will also cause the E4OD Transmission to malfunction.
- If the key is turned off before the diagnostic test is performed, the code will be lost. The light will remain out until another malfunction is detected.

The RABS system 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 self-test feature contains thirteen codes to indicate the area of the malfunction. When a malfunction is detected, the control module will shut down the System and the yellow REAR ANTI-LOCK Indicator warning lamp will come on. This will permit normal braking. A code will be set in the System memory that can be retrieved by momentarily grounding the diagnostic pigtail and counting the flashes of the yellow light. If more than one fault exists, only the first code recorded will be displayed. Additional codes will output only after the first fault is corrected. Reading and Clearing Diagnostic Trouble Codes