Antilock Braking Mode

4WAL Hydraulic System:

The Antilock Brake System (ABS) will work together with the foundation brake system to monitor the wheel speed sensors and control the sequence of pressure changes until the vehicle has come to a complete stop or the brake pedal has been released.

The Four Wheel Antilock (4WAL) system operates through a four step process:
^ Pressure Isolate/Maintain
^ Pressure Decrease
^ Pressure Increase
^ Brake Release (Fluid Return)

ABS operation is characterized by the following sequence of events:
1. With the vehicle at speed, the driver depresses the brake pedal.
2. The stoplamp switch opens and the wheel speeds begin to decrease as the master cylinder pressure and brake pressure increase.
3. As the wheel speed continues to change with respect to true vehicle speed, the normally open isolation valve for the affected channel is closed to prevent the buildup of additional pressure at the wheel. The master cylinder pressure continues to increase as the driver presses the pedal, but the wheel pressure is now limited to ABS pressure.
4. When the controller determines that the wheel speed departure is significant, the normally closed dump valve is opened. This bleeds off some of the pressure at the wheel cylinder or caliper to allow the wheel to return to a speed closer to the true vehicle speed.
5. The dump valve is again closed and the isolation valve remains closed to allow the wheel speed to completely recover from the departure.
6. Once the wheel has recovered from the departure, the isolation valve is momentarily pulsed open to allow master cylinder pressure and pump pressure to reach the brakes. This controlled pressure rise continues until the wheel is at optimum brake output or until the brake pressure is brought up to master cylinder output pressure. The ABS allows the brake fluid to flow to the wheel, to build pressure and to try to force another departure, repeating steps 3-6.