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 a frequency proportional to the average rear wheel speed is induced in the sensor circuit.
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 rear wheel lockup) the RABS Module activates the RABS Valve Assembly Isolation Solenoid, 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 reduces the rear wheel cylinder pressure and allows the rear wheels to spin back up to vehicle speed.
When the driver releases the brake pedal at the end of a stop, the isolation valve deenergizes, and any fluid in the accumulator is returned to the master cylinder. Normal brake operation then resumes.

SYSTEM SELF-TEST

NOTE: If the key is turned OFF before the diagnostic test is performed, the code will be lost, and the light will remain out until another malfunction is detected.

The Rear Anti-lock Brake System has selftest capabilities similar to those in other electronic control systems. Two warning lamps, located in the instrument panel, alert the driver to a System malfunction. The red Brake Warning Lamp indicates a low fluid level or that the parking brake is ON. The yellow Anti-lock Brake Indicator lights up for approximately two seconds when the Ignition Switch is first moved to ON or START for circuit prove out. The Indicator also lights up when the RABS Module detects a malfunction in the System.
The self-test feature contains thirteen codes that indicate the area of the malfunction. When a malfunction is detected, the RABS Control Module shuts down the System and the yellow Anti-lock Warning Lamp comes on. This permits normal braking. A code that can be retrieved by momentarily grounding the diagnostic pigtail and counting the flashes of the yellow brake warning light is then set in the system memory. If more than one fault exists, only the first code recorded will be displayed. Additional codes will be output only after the first fault is corrected. Testing and Inspection