Description & Operation: Notes
The Electronic Variable Orifice (EVO) power steering system consists of the following:
- Power steering pump, outlet and return hoses, steering box and linkage.
- Power steering fluid flow actuator, mounted on pump.
- Power Steering Control Module (PSCM), mounted under instrument panel (behind instrument panel, to right of radio in Astro and Safari).
- Steering Wheel Position (SWP) sensor, mounted in lower bearing of steering column.
- Vehicle Speed Sensor (VSS), mounted on transmission or transfer case (4X4).
- Powertrain Control Module (PCM) (diesel only), mounted in center of instrument panel (Express and Savana), or right side of instrument panel, above blower motor (Escalade, Sierra, Suburban, Tahoe, Yukon, and "C" & "K" Series Pickup).
- Vehicle Control Module (VCM) (gasoline only), mounted in left side of engine compartment.
The system increases or decreases the amount of fluid leaving the power steering pump. Outlet hose has a check valve to prevent kickback on some models. When vehicle is stopped or moving at low speeds, system allows full hydraulic fluid flow for maximum assist. As vehicle speed increases, a variable orifice closes at pump, reducing fluid flow to improve road feel and directional stability. Flow ranges from 3.5 gallons per minute at idle to.7 gallon per minute at speed.
Steering Wheel Position (SWP) sensor detects rapid movement when defensive maneuvers are made. PSCM uses this sensor input to adjust the current to fluid flow actuator. If PSCM detects a fault, EVO system is disabled, allowing maximum power assist. When ignition is turned off and vehicle is restarted, PSCM resets and allows EVO operation until it detects another fault. PSCM has self-diagnostics and stores diagnostic trouble codes. See DIAGNOSIS & TESTING . Codes will clear after 100 ignition cycles without fault.