SVX Power Steering Systems on Early Subarus Part 4:

Reaction Mechanism Components/Operation

The pinion assembly functions similar to previous Subaru systems with the addition of a reaction mechanism (located between the brackets).

The reaction mechanism components are:

• Reaction piston assembly,
• Pinion output shaft and torsion bar,
• Steel balls,
• Pinion input shaft with rotary valve.

During normal operation, such as straight ahead driving, the duty ratio is low and the pressure to the reaction piston is high. The piston holds the steel balls in the grooves of the input shaft reaction arm, allowing for less torsional twist of the torsion bar. This decreases the pressure flow to the system, which decreases the power assist to increase driver steering effort.

A high duty ratio during low vehicle speeds provides low pressure to the reaction piston. The piston allows the steel balls to move in the grooves of the input shaft reaction arm. This allows more torsional twist of the torsion bar, which increases the pressure flow to the system and increases power assist to decrease driver steering effort.

Fail-safe operation is dependent upon the type of failure, i.e., the reaction solenoid, sensor signal failure, or steering system ECU. The control unit discontinues the duty signal to the reaction solenoid. This results in high reaction chamber pressure and the power assist is fixed at the minimum level.