Antilock Brakes / Traction Control Systems: Description and Operation

NEW FEATURES FOR RABS II

Unlike early versions of RABS modules, the RABS II module remembers the diagnostic code even after the vehicle has been turned off. This feature is known as Keep Alive Memory (KAM).

NOTE: While the diagnostic test is being performed, the key must be turned to the "ON" position, otherwise the code will be lost.

If the RABS II self-test detects no faults, the RABS II module will flash out a (code 16). This is normal and indicates that the RABS II system is functioning correctly.

SYSTEM OPERATION
The rear anti-lock brake system 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. Should an impending lockup condition occur during braking, the RABS II modulates hydraulic pressure to the rear brakes, inhibiting rear wheel lockup.

When the brake pedal is applied, the (RABS II) module senses the drop in rear wheel speed. If the rate of deceleration is too great, indicating an impending wheel lockup, the (RABS II) 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 II) 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 II valve. This will reduce the rear wheel cylinder pressure and allow the rear wheels to spin back up to vehicle speed. Continuing under (RABS II) 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 de-energizes, and any fluid in the accumulator is returned to the master cylinder. Normal brake operation is resumed.

SYSTEM SELF TEST
The (RABS II) 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: a RED BRAKE warning lamp and a YELLOW REAR ABS warning lamp. The red BRAKE warning lamp indicates a low fluid level condition or parking brake "ON"; the yellow REAR ABS warning lamp will light only if the (RABS II) module detects a malfunction in the anti-lock system that will affect the operation of that system, or for approximately two seconds during ignition "ON" or engine cranking as a circuit prove-out.

The self-test feature contains thirteen codes that indicate areas of malfunction. When a malfunction is detected, the control module will shut down the system and the yellow REAR ABS 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.

NOTE: If there is more than one fault, only the last recognized fault will be retained and flashed. Additional codes will output only after the first fault is corrected.