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. EVO system consists of speed and steering sensors, control module, actuator and interconnecting wiring. See Figure .

Steering wheel rotation sensor and Vehicle Speed Sensor (VSS) 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 located at lower end of steering column. Sensor mounts on steering column and establishes rate of steering wheel rotation. Sensor and sensor ring are removed separately.

VSS is mounted on transmission and sends electrical signals to 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 power steering pump. EVO is inside actuator valve. EVO control module contains a microprocessor, which analyzes input signals from VSS and steering wheel rotation sensor. On models with air suspension, EVO module and air suspension module are integrated.