Antilock Brake System for Early Subarus:

ABS-2E System Differences:

The ABS-2E system differs from the earlier ABS designs in four ways:

• Earlier designs used four solenoid valves. The ABS-2E hydraulic control unit (HCU) uses three solenoid valves and one mechanical valve.

• Its electronic control unit can store up to three trouble codes, rather than just one.

• The number of separate error conditions the ECU can recognize has been increased. That means there are more trouble codes available.

• There is a revised bleeding procedure.

ABS Operating Modes:

During antilock braking, the HCU operates one or more of the solenoid valves to control the hydraulic pressure acting on the brakes. Each solenoid valve can operate independently in any of three pressure modes. These are pressure-reduce, pressurehold and pressure-increase. These modes are related to the amount of current flowing through the solenoid valve, as determined by the ABS ECU. Note: The term “pressure-increase” may suggest that the HCU raises pressure above that achieved by pressing the brake pedal. This is not the case.

The Mechanical Valve:

In the ABS-2E hydraulic control unit, the fourth solenoid valve has been replaced by a mechanical valve containing a plunger piston. This mechanical valve controls the left rear hydraulic brake circuit.