Description & Operation
Variable Assist Power Steering (VAPS) control system consists of a control module, high-efficiency power steering pump, rack and pinion, actuator valve assembly and interconnecting hoses.
VAPS system controls power steering fluid pressure through actuator valve assembly, mounted on rack and pinion. Actuator valve is a pressure-balanced variable orifice valve, controlled by a stepper motor-driven spool.
System uses a modified rotary valve in steering rack with 2 separate hydraulic circuits. During parking and low-speed operation, flow from pump is routed through primary circuit by actuator valve. As vehicle speed increases, actuator valve gradually opens, diverting an increasing amount of fluid into secondary circuit.
VAPS module receives inputs from Vehicle Speed Sensor (VSS) and signals stepper motor-driven spool to adjust opening of actuator valve. VAPS module is programmed to perform a self-diagnostic check every 16 milliseconds. If a malfunction is detected, control module deactivates its outputs. VAPS module is programmed to perform a service diagnostic procedure when activated by service technician. See Fig 1 .
Power steering pump is a 10-slipper pump with an integral reservoir. Spring-loaded slippers within cam and rotor create pumping action. See Figure . A flow control relief valve maintains pump volume and pressure. Pump design incorporates a swivel pressure fitting. An identification tag attached to pump body indicates pump model. Use model code when ordering service parts.