Description & Operation

The major components of the power steering system are the steering wheel, steering column and shaft assembly, intermediate shaft, power steering gear, tie rods, steering knuckles, power steering pump and hoses, Electronic Variable Orifice (EVO) actuator, Vehicle Speed Sensor (VSS), and power steering pressure switch.

When steering wheel is turned, steering gear input shaft/pinion gear teeth mesh with horizontal rack teeth to move rack left or right. Rack is connected to steering knuckles by inner tie rods and tie rod ends. Power steering pump supplies pressurized fluid to the steering gear input shaft. Shaft contains a spool valve that directs fluid to either side of the rack piston, depending on turning direction. Fluid pressure reduces the effort required to turn the steering wheel.

The EVO system varies the effort required to turn the steering wheel based on vehicle speed. As vehicle speed increases, EVO system decreases the amount of pump fluid pressure applied to the steering gear. EVO system is controlled by Powertrain Control Module (PCM), located behind left end of instrument panel. PCM receives signals from Vehicle Speed Sensor (VSS) on transaxle. Based on VSS signals, PCM cycles EVO actuator on and off to control power steering pump pressure at the pump outlet hose fitting. See Figure .

When EVO actuator is off (de-energized), full pump pressure is allowed through pressure line outlet to steering gear. When EVO actuator is on (energized), pump fluid by-passes pressure line outlet and is routed back into pump, decreasing pump pressure to the steering gear. EVO actuator cycling occurs at vehicle speed of 12 MPH or greater. EVO system has self-diagnostic ability that is part of PCM self-diagnostics. See SELF-DIAGNOSTIC SYSTEM  under TESTING.