Power Steering

DESCRIPTION
The electronic variable orifice (EVO) system is designed to vary flow from the power steering pump as a function of vehicle speed and steering wheel rate of rotation, giving a form of variable assist. The system provides:
^ Full power steering assist at low vehicle speed for light parking effort.
^ Minimum assist at high speed for good road feel and directional stability.
^ Full assist during evasive maneuvers.

COMPONENTS
^ EVO power steering control valve actuator is attached to the CIII power steering pump.
^ A filter is built into the power steering pump reservoir. Check for contamination and clean or replace the reservoir assembly as required.
^ Shock absorber electronic steering sensor is attached to lower steering column.
^ Vehicle speed sensor (VSS) is attached to transmission.
^ Test connector is located in luggage compartment behind RH side quarter-panel carpet.
^ Control module is located between the dash panel and instrument panel on the air conditioning plenum on the passenger side behind the glove compartment.

MECHANICAL OPERATION
The steering gear is designed with the one-piece piston, worm and sector shaft in one housing and the rotary valve sleeve assembly in an attached housing. This makes possible internal fluid passages between the rotary valve sleeve and power cylinder, eliminating all external lines and hoses except the power steering pressure hose and power steering return hose between the power steering pump and steering gear. The power cylinder piston is an integral part of the steering gear housing.
^ The piston is double acting.
^ Fluid pressure may be applied to either side of the piston.
^ The steering gear ratio is 14:1.
^ The steering gear allows 3.3 turns from steering stop to steering stop.

The control valve is a unique rotary design that uses relative rotational position of the power steering gear input shaft and control and valve sleeve to direct fluid flow. In a neutral (straight-ahead) position, where power assist is not required:
^ The power steering gear input shaft and control valve and sleeve are held in a central position by the torsion bar.
^ The fluid flows from the inlet port through the valve to the outlet port and back to the power steering pump.
^ No area of the steering gear is under high pressure.
^ The rotary design control valve uses relative rotational motion of the power steering gear input shaft and control to the valve sleeve in order to direct fluid flow.

The valve and housing cylinder are always full of power steering fluid which dampens road shock that otherwise would be transmitted to the driver through the steering wheel.

HYDRAULIC OPERATION
The steering gear uses an integral valving that directs pump flow and controls pressure as required to reduce steering effort during operation. The steering gear contains:
^ A rotary hydraulic fluid control valve integrated to the power steering gear input shaft and control.
^ A boost cylinder integrated with the steering gear sector shaft.

Power steering assist is provided as follows:
^ One end of the power steering gear input shaft and control is attached to the steering column tube and steering wheel.
^ The other end, providing the link to the wheel side of the steering system, is connected to the worm through the torsion bar.
^ As a steering maneuver is initiated, the rotation force applied through the steering wheel/steering column to the power steering gear input shaft and control is transmitted to the worm where it meets the resisting forces of the road wheels.
^ When the steering forces are great enough, the rotational deflection (twisting) of the torsion bar occurs.
^ This deflection changes the relative position of the power steering gear input shaft and control and control sleeve to cause a throttling of the hydraulic flow and directs the high-pressure fluid to the appropriate end of the piston in the steering gear housing.
^ The difference in pressure forces on the piston helps move the steering gear sector shaft, thus assisting in the turning effort.
^ The oil in the opposite end of the steering gear housing is forced out through the return outlet of the control valve and back to the power steering pump reservoir.
^ With higher steering efforts, increased valve displacement gives the driver proportionately higher assisting pressure.
^ The driver will have a smooth hydraulic assist at all times.
^ hen the driver stops applying steering effort, the following actions occur:
^ The valve sleeve and power steering gear input shaft and control are returned to a centered position by the torsion bar.
^ This equalizes pressure on both sides of the piston.
^ The steering geometry will tend to return the front wheels to the straight-ahead position.

NOTE: A hissing noise is a normal characteristic of rotary valve steering gear and in no way affects steering. A replacement valve will also exhibit slight noise, so it is not always a cure for the condition. Any metal-to-metal contacts at the flexible steering shaft coupling (due to column misalignment) will transmit a severe valve hissing noise into the passenger compartment that should be corrected.

^ The only service that can be performed on the control valve assembly is the replacement of the four Teflon seals on the control valve sleeve.