Antilock Brake System for Early Subarus:

ABS System Operation:

The Antilock Brake System (ABS) system has a passive and an active mode. During the passive mode, the Antilock Brake System (ABS) system is not activated and normal power braking is available. The Antilock Brake System (ABS) is activated when the ECU computes a slip ratio at the preset value. Fluid pressure is decreased and restored to each wheel based on the acceleration/deceleration of each wheel to prevent wheel lockup. If the wheel is accelerating too quickly, the ECU signal causes the HCU to restore the fluid pressure to that wheel. Vice versa, the fluid pressure is reduced if the wheel is decelerating too quickly. The system will pump the fluid 2-3 times per second until wheel slip ceases.

No Current Position (Passive):

This is the passive mode of the Antilock Brake System (ABS). The magnetic valves are not activated, and the system operates as a normal power brake system. System pressure developed by the master cylinder when the brake pedal is depressed is routed to the F valve. The pressure causes the valve piston and stem to retract. Fluid pressure will reopen the check valve and allow the brake fluid to flow through the magnetic valves directly to the wheel cylinders. The HCU reservoir and the accumulator are not affected during the no-current position.

When the brake pedal is released, master cylinder pressure decreases, which allows wheel cylinder pressure to return via the return check valve. When the master cylinder pressure is less than 142 psi the F valve return spring opens the valve, and residual pressure is returned to the master cylinder.