Antilock Brakes / Traction Control Systems: Description and Operation

The rear anti-lock brake system constantly monitors rear wheel speed. In the event of an impending rear brake lockup, the system regulates hydraulic pressure to the rear brakes, preventing total lockup when the brakes are applied and thereby reducing the possibility of skidding.

The system consists of a yellow warning light on the instrument panel, an electro-hydraulic rear anti-lock brake system (RABS) valve mounted on the frame rail behind the crossmember, a speed sensor mounted on the rear drive axle, an exciter ring mounted on the differential case and a computer module. On 1987-90 F-Series and Bronco models, the computer module is located to the right of the brake pedal on the upper dash panel. On 1987-90 Bronco II and Ranger models, the computer module is located behind an access cover in back of the driver's side door pillar. On 90 Aerostar models, the computer module is located on the driver's side center console under the center of the instrument panel. On 1990 E-Series models, the computer module is located under the lefthand side of the instrument panel near the parking brake mechanism.
During normal vehicle operation, the computer module monitors rear wheel speed through the speed sensor. When the exciter ring teeth pass by the speed sensor probe, the sensor generates a frequency signal proportionate to wheel speed and relays this signal to the module accordingly.
When brake pedal is applied, the computer module immediately senses the drop in vehicle speed. If the rate of deceleration indicates that wheel lockup is going to occur, the module activates the electro-hydraulic brake valve, causing the internal isolation valve withing it to close, thereby holding fluid pressure to the rear wheel cylinders. If speed sensor input signals indicate that lockup is still going to occur, the module will then energize the dump solenoid, which will initiate a series of rapid pulses to bleed off wheel cylinder pressure to the accumulator. When the computer module determines that a lockup condition is no longer imminent, the solenoid de-energizes and brake operation is returned to normal.