Rear Antilock Brakes (RABS)

Principles of Operation

The anti-lock brake control module carries out system tests during start-up and normal operation. The RABS valve, rear anti-lock brake sensor, and fluid level circuits are monitored for proper operation. if a concern is found, the anti-lock brake control module deactivates and the yellow ABS warning indicator illuminates.


Most concerns cause the yellow ABS warning indicator to stay illuminated until the ignition is turned OFF, at which time the anti-lock brake control module retains the diagnostic trouble code (DTC) in memory (also referred to as keep alive memory or KAM). At any time while the ignition is turned on, a DTC may be obtained. DTC 16 indicates that the system is operating normally. However, those concerns associated with loss of power to the anti-lock brake control module cause the system to be deactivated and the yellow ABS warning indicator to be illuminated, but do not cause a DTC.


The brake pedal position (BPP) switch, when activated, signals the anti-lock brake control module that a braking condition is occurring.


The rear anti-lock brake system (RABS) continuously monitors rear wheel speed with a rear anti-lock brake sensor. When the teeth on the rear anti-lock brake sensor indicator, pass the RABS sensor pole piece, an AC voltage is induced in the sensor circuit with a frequency proportional to the average rear wheel speed. In the event of an impending lockup during braking, at vehicle speeds above approximately 8 km/h (5 mph), the anti-lock brake control module senses the drop in rear wheel speed. If the rate of deceleration is excessive, the anti-lock brake control module activates the RABS valve, causing the isolation valve to close. With the isolation valve closed, the rear wheel cylinders are isolated from the brake master cylinder and the rear brake fluid pressure cannot increase. if the rate of deceleration is still excessive, the anti-lock brake control module will energize the dump solenoid with a series of rapid pulses to bleed the rear brake caliper fluid into an accumulator built into the RABS valve. This reduces the brake fluid pressure to the rear brake caliper and allows the rear wheels to spin at vehicle speed. The anti-lock brake control module pulses the dump and isolation solenoids in a manner that will keep the rear wheels rotating while still maintaining rapid deceleration during braking.


Once stopped, the operator releases the brake pedal, the isolation valve de-energizes and any fluid in the accumulator returns to the brake master cylinder. Normal brake operation resumes.