Antilock Brakes / Traction Control Systems: Description and Operation

SYSTEM DESCRIPTION
The Rear Anti-lock Brake System (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. 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 RABS module senses the drop in rear wheel speed. If the rate of wheel deceleration is too great, indicating that wheel lockup is going to occur, 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 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 deenergizes, and any fluid in the accumulator is returned to the master cylinder. Normal brake operation is resumed.

SYSTEM SELF-TEST
The rear anti-lock brake 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 park brake "ON". The yellow anti-lock warning lamp lights for approximately 2 seconds when the ignition switch is initially moved to "ON" or "START" for circuit prove-out. The indicator will also light when the rear anti-lock brake 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 can be retrieved by momentarily grounding the diagnostic pigtail (BK/O) after it is disconnected from Keep-Alive Memory (KAM) power red wire, then grounding the diagnostic pigtail and counting the flashes of the yellow ABS lamp.

NOTE: To ensure the fault code is not lost from memory, the ignition key must be left in the "ON" position before the diagnostic lead is disconnected from KAM power.

If more than one fault exists, only the first code stored will be displayed. Additional codes will be output only after the first fault is corrected. Two new codes (12 and 16) are available for the 1993 model year:

- Code 12 indicates a fault with the base brake system.
- Code 16 indicates that the system is OK.