DESCRIPTION & OPERATION
Electronic Variable Orifice (EVO) system is designed to regulate power steering effort according to vehicle speed and steering wheel rotation rate. System provides full assist at low vehicle speed for light parking effort and minimum assist at high vehicle speed for improved handling and road feel. Full power steering assist returns during evasive maneuvers.
During periods of system malfunction, system will provide full power steering assist at all times. Diagnostic codes are accessible through service diagnostic connector, inside glove box. EVO system consists of vehicle speed and steering wheel rotation sensors, control module, actuator valve and interconnecting wiring. (Scheme 16)
Steering wheel rotation sensor and vehicle speed sensor feed information to EVO control module, which electronically regulates EVO actuator valve assembly on power steering pump.
Steering wheel rotation sensor is an optical or photo-cell sensor. Sensor mounts on steering column and establishes steering wheel rotation rate. Steering wheel rotation sensor, located at lower end of steering column, may be removed with column in vehicle. Steering column must be removed to remove sensor ring.
Vehicle speed sensor is mounted on transmission and sends electrical signal to EVO control module. Current produced by sensor increases and decreases linearly, according to vehicle speed. Current is constant at 80 MPH or faster.
EVO actuator assembly generates a differential pressure, according to flow and current input, to control spool valve in C-II power steering pump. Electronic variable orifice is inside actuator valve.
EVO control module contains a microprocessor, which analyzes input from vehicle speed and steering wheel rotation sensors.
On Thunderbird LX and Cougar LS with anti-lock brakes, the control module is located in the module tray in the trunk, behind driver's side of the back seat. On Cougar LS without anti-lock brakes, the module is mounted on a bracket below left rear speaker opening, immediately below package tray.
C-II pump is a 10-slipper pump with an integral reservoir. Spring-loaded slippers within cam and rotor create pumping action. A flow control relief valve maintains pump volume and pressure.
Pump design incorporates a swiveled pressure fitting. An identification tag attached to reservoir body indicates pump model. Use model code when ordering service parts, as some differences in internal components do exist.
All vehicles use a one-piece power steering gear unit. Gear and valve housing are combined into an aluminum die casting. Quick-connect fittings are used for pressure and return lines. Fittings are designed to swivel and should not be overtightened.
A hydraulic-mechanical unit with an integral piston and rack provides power-assisted steering control. Internal valving directs pump flow and controls pressure. Unit contains a rotary fluid control valve and a boost cylinder integral with gear rack.
