Antilock Braking Mode

4WAL Hydraulic System:

DESCRIPTION
The antilock brake system 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.

OPERATION
The Four Wheel Antilock (4WAL) system operates through a four step process:
- Pressure Isolate/Maintain
- Pressure Decrease
- Pressure Increase
- Brake Release (fluid return)

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 and brake pressure increases.
3. As the wheel's speed continues to depart further from true vehicle speed, the normally-open isolation cartridge for the affected channel is closed to disallow 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 the Antilock Brake System (ABS) pressure.
4. When the controller determines that the wheel departure is significant, the normally-closed dump cartridge 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 cartridge is again closed and the isolation cartridge remains closed to allow the wheel speed to completely recover from the departure.
6. Once the wheel has recovered from the departure,the isolation cartridge 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.