Power Steering

GENERAL INFORMATION
This vehicle offers two optional power steering systems for its vehicles: electronic variable orifice (EVO) and constant flow design. Vehicles with these engines have the constant flow design. Both power steering systems are of the rack and pinion design, the major components being the steering wheel, steering column and shaft assembly, [1][2]intermediate shaft, power steering gear, tie rods, steering knuckles, and the power steering pump and hoses. Vehicles equipped with the EVO design also have an EVO actuator.

The front wheels are steered by a power rack and pinion steering system in the following manner: The steering wheel is turned by the driver, causing the steering shaft in the steering column to turn. The steering column turns the [1][2]intermediate shaft (the intermediate shaft is a jointed shaft that connects the steering column to the input shaft of the steering gear), which finally turns the input shaft of the steering gear. The input shaft of the steering gear controls a rotary spool valve in the power steering gear. Pressurized power steering fluid is supplied to the spool valve by the engine mounted, belt-driven power steering pump. This fluid exits the pump (through the pressure hose) to the gear and is directed through the spool valve to one of two paths, depending on which direction the wheel is being turned. If the wheel is turned clockwise (right turn), the spool valve will direct the fluid to the right side of the steering gear integral rack piston, attached directly to the rack. Fluid pressure acting on the right side of the rack pinion will assist the rack in turning the wheels to the right. The opposite occurs when the steering wheel is turned counterclockwise (left turn).

Unlike the constant flow power steering system, the EVO feature adjusts the amount of steering assist to the speed of the vehicle. Vehicle speed is monitored by a vehicle speed sensor (VSS). This information is read electronically by the powertrain control module (PCM), which commands the EVO actuator to control the power steering pump output flow. At low speeds, where high levels of assist are desired, the EVO actuator is opened more, providing more flow to the steering gear and greater assist. At higher speeds, the actuator is closed more, providing less pump flow and therefore less steering assist and improved road feel. All of the additional flow of the pump not needed by the gear is allowed to return to the reservoir.

ELECTRONIC VARIABLE ORIFICE (EVO) ACTUATOR
The 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.