The Rear Anti-lock Brake System (R-ABS) found on Nissan pickup trucks and Pathfinders is designed to prevent or reduce locking of the rear wheels under heavy braking. Because of the vehicle's load carrying capacity, the handling and braking characteristics will change significantly as the weight increases. The R-ABS system senses impending lock-up at the rear wheels under all operating conditions and modulates brake line pressures to allow the wheels maximum braking without locking. This increases vehicle stability under hard braking.
When the brake pedal is applied, the R-ABS module senses the drop in rear wheel speed based on signals from the rear wheel speed sensor. If the rate of deceleration is too great (based on pre-programmed values), the module determines that wheel lock will occur.
The module energizes the isolation solenoid, an electro-hydraulic valve, to isolate the wheel cylinders from the master cylinder line pressure. With this valve actuated, line pressure is held constant and cannot increase. Additional pedal pressure has no effect. If the rate of deceleration is still seen to be too great, the module pulses the dump solenoid momentarily.
The dump solenoid releases fluid trapped between the isolation solenoid and the wheel, reducing line pressure. The excess fluid is temporarily routed to an accumulator where it is stored until the isolation valve is de-energized. Reducing the line pressure allows the wheel to roll more freely. The module sees the increase in wheel speed and releases the isolation solenoid.
When the isolation solenoid is de-energized, the pressure from the brake pedal is allowed to act directly on the brakes. The module senses the wheel speed reduction and the cycle repeats until the brake pedal is no longer applied or braking force does not threaten to lock the wheels. The module is capable of repeating the control cycle several times per second.