See Figures 1, 2, 3 and 4
Found on 1992-93 Pick-ups, this system is a variant of ABS which acts only on the rear wheels. It is designed to allow maximum rear wheel braking on slick surfaces, thus preventing locked wheel skids and possible loss of control. On this system, the front wheels can be locked and receive no modulation from the system.
The system includes a pulse generator (speed sensor) in the rear differential case which measures rear axle speed, a deceleration or G-sensor generating a signal according to vehicle speed, a load sensing proportioning valve, a control unit or computer which interprets the signals and a modulator which controls the pressure of the brake fluid to the rear wheels.
Should any of the components fail, the control unit enters the fail-safe mode, illuminating the dash warning light and disabling the lock-up control. Normal braking is available.
A simple test of this system can be performed from the driver's seat. With the truck stationary, run the engine for at least 5 seconds. Turn the ignition key to LOCK, depress the brake pedal and while your foot is still on the brake, turn the key back to the ON not START ) position. If you hear a muffled clicking or tapping, the modulator solenoid valve is working and the system is operating fully.