Steering: Description and Operation

VAPS System:

VAPS Gear Assembly:

The Variable Assist Power Steering (VAPS) system consists of a microprocessor-based control module, a power rack and pinion gear, an actuator valve assembly, interconnecting hose assemblies and a high efficiency power steering pump, Fig. 2.

The VAPS system incorporates a modified rotary valve in the gear with two independent hydraulic circuits called the primary and secondary circuits. During parking and low vehicle speed operation, fluid flow from the pump is routed to the primary circuit by an electrically controlled actuator valve assembly. As vehicle speed increases, the actuator valve gradually opens, diverting the increased fluid volume to the secondary circuit.

The actuator valve assembly, Fig. 3, is a pressure balanced variable orifice valve, controlled by a stepper motor driven linear spool. The VAPS module receives inputs from the vehicle speed sensor and transmits signals to the stepper motor driven spool to adjust orifice opening of the actuator valve.

The VAPS module is programmed to perform a self-diagnostic check every 16 milliseconds. If a malfunction is detected, the module microprocessor deactivates its outputs allowing control assist power steering operation.

The VAPS module is programmed to perform a service diagnostic procedure when activated by a service technician.

The rotary design control valve directs fluid flow using relative rotational motion of the input shaft and valve sleeve. When the steering wheel is turned, resistance of the wheels and the weight of the vehicle will cause a torsion bar to deflect. The deflection action changes the position of the valve spool and sleeve ports, directing pressurized fluid to the appropriate end of the the power cylinder. The pressure differential acts on the piston and helps move the rack to assist in the turning effort. The piston is attached directly to the rack and the housing functions as the power cylinder. The fluid in the opposite end of the power cylinder is forced to the control valve and back into the pump reservoir. When no steering effort is applied, the valve is forced back to a centered position by the torsion bar. When this occurs, pressure is equalized on both sides of the piston and the front wheels return to a straight-ahead position.