Operation

OPERATION

When the brakes are applied, hydraulic fluid is routed from the master cylinder's secondary circuit to the RWAL valve. From there hydraulic fluid is routed to the rear brakes. The Controller Antilock Brake (CAB) contains an Electronic Variable Brake Proportioning (EVBP) control algorithm, which proportions the applied braking force to the rear wheels during braking. The EVBP function of the RWAL system takes the place of a conventional hydraulic proportioning valve. The CAB monitors the rear wheel speed through the rear wheel speed sensor and calculates an estimated vehicle deceleration. When an established deceleration threshold is exceeded, an isolation valve is closed to hold the applied brake pressure to the rear brakes constant. Upon further increases in the estimated vehicle deceleration, the isolation valve is selectively opened to increase rear brake pressure in proportion to the front brake pressure. If impending rear wheel lock-up is sensed, the CAB signals the RWAL valve to modulate hydraulic brake pressure to the rear wheels to prevent lock-up.

NORMAL BRAKING
Since the RWAL valve also performs the EVBP or proportioning function, vehicle deceleration under normal braking may be sufficient to trigger the EVBP function of the RWAL system without full RWAL activity as would normally occur during an impending rear wheel lock-up. As previously mentioned, the isolation valve is selectively closed and opened to increase rear brake pressure in proportion to the front brake pressure under EVBP control. Slight brake pedal pulsations may be noticed as the isolation valve is opened.