Antilock Brake System for Early Subarus:
The ECU receives the wheelspeed sensor signals from the four sensors. It computes and compares the speed of each wheel. This results in the slip ratio between the wheels and the vehicle. The ECU sends a control signal to the hydraulic control unit (HCU) to prevent wheel lockup.
In a vehicle equipped with 4EAT, the Antilock Brake System (ABS) ECU also signals the TCU. This signal forces shift control during downshifts from 4th to 3rd. It cancels engine braking during ABS operation by deactivating the overrunning clutch. It also fixes the duty ratio of the MPT clutch solenoid at 95 percent On providing mostly FWD. The hydraulic control unit receives a signal from the ECU and in turn individually controls the fluid pressure to each wheel. The HCU is inactive until a predetermined slip ratio occurs. The HCU is located in the right front of the engine compartment.
The HCU is an electronically controlled, motorized plunger pump. There are four magnetic control valves, two fill valves, a motor relay and a valve relay. The relays may be replaced individually, but the HCU cannot be serviced. When the HCU is removed from the vehicle, do not bump or drop the HCU, turn it on its side or upside down, or allow dirt, etc. to enter the unit. Always apply rust-preventive wax to the bracket attaching bolts when installing a new HCU.
The G-sensor is a two stage mercury switch which detects the rate of deceleration of 4WD MT equipped Legacy vehicles. It is required due to the small wheel speed differential caused by the FT4WD system. The G sensor is located on the RF strut tower. The Antilock Brake System (ABS) warning lamp located on the instrument panel illuminates during vehicle start-up to check lamp operation, and during Antilock Brake System (ABS) malfunction.