Steering: Description and Operation

DESCRIPTION
The Variable Assist Power Steering (VAPS II) system uses the existing steering components, a power steering auxiliary actuator and the Generic Electronic Module (GEM) to improve steering characteristics. The VAPS II system begins operation as soon as the GEM sees a change in the ignition switch position from START to RUN. The GEM maintains VAPS II control as long as the ignition switch is in the RUN position. The level of assist provided by the VAPS II system depends on the vehicle speed. The faster the vehicle speed, the less assist provided by the VAPS II system. The vehicle speed is determined by the GEM, based upon the Vehicle Speed Sensor (VSS) signal received by the GEM.

COMPONENTS
The variable assist power steering system consists of the following components:
^ Generic Electronic Module (GEM)
^ power rack and pinion steering gear
^ power steering auxiliary actuator
^ rotary valve assembly (part of power steering short rack)
^ pressure and return hoses
^ high efficiency, power steering pump
^ power steering pump reservoir
^ power steering fluid cooler
^ necessary wires and connectors

OPERATION
The VAPS II system uses a rotary valve in the power rack and pinion steering gear, with two independent hydraulic circuits called the primary and secondary circuits.
^ During parking and low speed operation, the flow from the power steering pump is routed to the primary circuit by an electrically controlled power steering auxiliary actuator.
^ As vehicle speed increases, the power steering auxiliary actuator gradually opens, allowing an increasing amount of fluid to the secondary circuit. This secondary circuit requires more effort to reach a given level of assist.
^ The GEM receives inputs from a VSS and provides the variable current to adjust the opening of the power steering auxiliary actuator valve.
^ The GEM is programmed to perform a self-diagnostic check every 16 milliseconds.
^ If a concern is detected, the GEM deactivates its output signals.
^ The GEM is programmed to perform an on-board diagnostic procedure when activated by the service technician.