Description & Operation

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 by the driver to turn the steering wheel.

Power steering system is also equipped with variable effort steering (VES) feature, also called Electronic Variable Orifice (EVO) steering. The EVO system varies the effort required by the driver to turn the steering wheel based on vehicle speed. This increased effort provides firmer steering (road) feel and improved directional stability. As vehicle speed increases, EVO system decreases the amount of pump fluid pressure applied to the steering gear. Reduced pump pressure increases steering effort, which improves steering feel.

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 speeds 12 MPH or greater. The PCM cycles EVO actuator more frequently as vehicle speed increases, resulting in firmer steering (increased road feel). EVO system has self-diagnostic ability that is part of PCM self-diagnostics. See SELF-DIAGNOSTIC SYSTEM  under TESTING.