The electronic variable orifice (EVO) actuator is a 10 ohm (nominal) linear solenoid. The PCM controls the current to the high and low side of the actuator so that current control is retained in the presence of any output fault condition.

The control of the actuator is accomplished by pulse width modulating (PWM) voltage to the actuator and monitoring the actual current flow. The PCM varies the PWM signal in order to make the actual current equal to the desired current.

The steering system on the DOHC 1.9L engine utilizes variable power assist. The system supplies maximum assist at idle to near zero at highway speeds. Vehicle speed is the sole input in determining the amount of assist to the rack and pinion. Full assist (EVO solenoid commanded Off) is given below 16 km/h (10 mph) and no assist (EVO solenoid commanded On, 100% duty cycle) is given above 113 km/h (70 mph). Variable assist is given in between these speed ranges. If the PCM loses the vehicle speed input, a vehicle speed will be calculated based on engine rpm and engine load. This calculated value will be used to determine the amount of assist needed.

The powertrain control module (PCM) controls an electronic variable orifice (EVO) solenoid, which varies the power steering pressure flow from the power steering pump. The PCM controls the high side of the EVO solenoid by pulse width modulating (PWM) ignition voltage. The low side of the solenoid is tied directly to a low resistance resistor. which is tied directly to aground in the PCM. If a fault occurred in the EVO solenoid circuit, e.g. open or short to ground, the default assist mode is full assist.