Steering System - Variable Assist: Operation
The steering wheel rotation sensor and vehicle speed sensor feed information to the EVO control module, which electronically regulates the EVO actuator valve assembly on the power steering pump.
The steering wheel rotation sensor is an optical or photo-cell sensor. The sensor mounts on the steering column and establishes the rate of steering wheel rotation. The steering wheel rotation sensor, located at lower end of the steering column, may be removed with the column in or out of the vehicle. The sensor and sensor ring are removed separately.
The vehicle speed sensor is mounted on the transmission and sends electrical signal to the control module. The current produced by the sensor increases and decreases linearly, according to vehicle speed. The current is constant at 80 MPH or faster.
The EVO actuator assembly generates a differential pressure, according to flow and current input, to control spool valve in C-II power steering pump. The electronic variable orifice is inside the actuator valve.
The 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. On Thunderbird Super Coupe and Cougar XR-7, the module is integrated within the Automatic Ride Control System.