Description & Operation
Variable Assist Power Steering III (VAPSIII) control system automatically adjusts power steering fluid pressure to provide light steering effort during low speed driving, and firm steering during high speed driving. VAPSIII system consists of Vehicle Dynamics Module (VDM), an actuator valve assembly, power steering pump, rack and pinion steering gear, hoses and an in-line fluid filter. See Fig 1 .
VAPSIII system controls power steering fluid flow through an actuator valve, mounted on steering gear. See Fig 2 . System uses a rotary valve in steering gear with 2 separate hydraulic circuits. Primary circuit provides greater power assist than secondary circuit. At low speed, fluid is routed through primary circuit by actuator valve. As speed increases, actuator valve gradually opens, diverting fluid to secondary circuit. VDM module has self-diagnostic capabilities. If a fault is detected, VDM module will revert to full steering assist.