See Figures 1 and 2
The rear wheel anti-lock system continually monitors rear wheel speed with a sensor mounted on the rear axle. When the teeth on an excitor ring, mounted on the differential ring gear, pass the 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 condition during braking, the anti-lock system modulates hydraulic pressure to the rear brakes inhibiting rear wheel lockup.
When the brake pedal is applied, a control module senses the drop in rear wheel speed. If the rate of deceleration is too great, indicating that wheel lockup is going to occur, the module activates the electro-hydraulic valve causing the isolation valve to close. With the isolation 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 module will energize the dump solenoid with a series of rapid pulses to bleed off rear cylinder fluid into an accumulator built into the electro-hydraulic valve. This will reduce the rear wheel cylinder pressure and allow the rear wheels to spin back to the vehicle speed. Continuing under 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 de-energizes and any fluid in the accumulator is returned to the master cylinder. Normal brake operation is resumed.