SVX Power Steering Systems on Early Subarus Part 4:
Vehicle Speed Sensitive Power Assisted Steering (Optional System)
(Optional System) Main components unique to this system are:
• Power steering control unit,
• Reaction (duty) solenoid valve,
• Vehicle speed sensor (speedometer system),
• Modified pinion assembly.
SVX Power Steering System Diagram
The steering system ECU receives input from the engine speed sensor through the MPFI ECU and the transmission vehicle speed sensor. The ignition switch provides power for the system, while the battery provides power for the memory. The diagnostic test mode connectors are also an input to the control unit.
The control unit sends signals to the reaction control solenoid valve located on the pinion assembly housing and to the steering system warning lamp located on the dash to the left of the steering wheel.
SVX Power Steering Solenoid Pinion Assembly
The reaction solenoid valve controls the amount of hydraulic power assist to the rack. The valve is mounted to the pinion housing. The valve is controlled by the steering system ECU output signal based upon vehicle and engine speed input to the ECU. The solenoid valve functions on a duty ratio to control the power assist.
A high duty ratio means the solenoid activation period is longer and low fluid pressure is supplied to the pinion reaction chamber. More assist pressure is supplied to the system, to reduce driver steering effort.
Conversely, a low duty ratio means a shorter solenoid activation period and high fluid pressure is provided to the pinion reaction chamber. Less assist pressure is supplied to the system, increasing the driver steering effort.
When there is zero duty ratio, such as a disconnected solenoid harness, maximum pressure is supplied to the pinion reaction chamber. This results in minimum assist pressure to the system and maximum driver steering effort.