Power Steering System Description
The power steering system consists of three components: the power steering pump, the power steering fluid reservoir and the the power steering rack and pinion gear. The power steering pump is a vane-type pump providing hydraulic pressure for the system and is powered by the engine. It draws on the power steering fluid reservoir, which in turn is connected to the power steering gear. A pressure-relief valve inside the flow control valve limits the pump pressure. The power steering rack and pinion gear has a rotary control valve which directs hydraulic fluid coming from the power steering pump to one side or the other side of the rack piston. The integral rack piston is attached to the rack. The rack piston converts hydraulic pressure to a linear force which moves the rack to the left or the right. The force is then transmitted through the inner and the outer tie rods to the steering knuckles, which turn the wheels.
Speed Sensitive Power Steering System Description
The speed sensitive power steering (SSPS) system varies the driver effort required to steer as the vehicle speed changes. At low speeds, the system provides maximum power assist for easy turning and parking maneuvers. At higher speeds, the steering power is reduced to provide the driver with firmer steering and directional stability. The SSPS system accomplishes this by reducing the amount of power steering fluid flow from the power steering pump to the power steering gear as the vehicle speed increases. When the vehicle is stationary, the SSPS system provides maximum fluid flow to the steering gear. As the vehicle speed increases, the fluid flow to the steering gear is decreased.
The SSPS control module processes the vehicle speed information from the engine control module (ECM) and uses the steering wheel rotation sensor to provide a control signal to the electronic variable orifice (EVO) actuator located on the power steering pump.
Electronic Variable Orifice (EVO) Actuator
The electronic variable orifice (EVO) actuator is located on the power steering pump and contains a solenoid operated pintle valve. Fluid leaving the pump passes through an orifice in the actuator tip. When the EVO actuator is powered by the SSPS control module, the pintle moves into the orifice and reduces the power steering fluid flow. As the vehicle speed increases, current from the SSPS control module increases, and the pintle blocks more and more of the orifice.
Steering Wheel Rotation Sensor
The steering wheel rotation sensor is located at the end of the steering column housing and is used to send a signal to the controller when abrupt or evasive steering maneuvers are needed.
Power Steering Pressure Hose
SSPS vehicles have a specific pressure hose assembly which includes an in-line check valve in the rack and pinion assembly. This reduces the amount of steering wheel " kick" when driving over irregular road surfaces while operating at speeds with reduced flow rate and pressure.