Speed Sensitive Power Steering System Description
Speed Sensitive Power Steering System DescriptionThe 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.
Control Module
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 solenoid valve located on the power steering gear assembly.
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.
Power Rack and Pinion
Except for differences in valve machining, the design of the SSPS power rack and pinion assembly is the same as for the a non-SSPS system. The steering wheel movement is transferred to the pinion via the intermediate shaft. The pinion moves the rack left or right through meshing the pinion and the rack teeth. The force is then transmitted through the tie rods and steering knuckle to steer the wheels.
The power rack and pinion steering system has a rotary control valve which directs the hydraulic fluid from the power steering pump to one side or the other side of the rack piston. The piston is attached to the rack and uses hydraulic pressure to move the rack left or right. The rotary control valve regulates the degree of assist by responding to the driver's torque input. If hydraulic assist is not available, manual control is maintained. However, under this condition, more steering effort is required.
Power Steering Pump
The standard vane-type pump, which provides hydraulic pressure for the system, incorporates a special discharge fitting to hold the SSPS actuator.
System Operation
System operation originates with input from the vehicle speed sensor via the ECM to the SSPS control module. The SSPS control module sends a signal to the SSPS actuator to vary the rate of fluid flow output by the power steering pump.
Circuit Operation
The SSPS system uses inputs from the speed sensor and steering wheel rotation sensor to the SSPS controller to determine the desired amount of power steering assist. The SSPS control module constantly compares the amount of current flowing through the SSPS actuator to the desired current it has calculated. The SSPS actuator has a pints that moves in and out of an orifice, regulating power steering fluid flow. The SSPS control module can change the amount of current flowing through the SSPS actuator by varying the output duty cycle. If the SSPS actuator is disconnected, the pints will be pulled out of the orifice for maximum power steering assist.
The SSPS control module has the ability to detect faults in the steering wheel rotation sensor, the SSPS actuator, or the circuitry to those components. Any default detected will cause the power steering assist to remain at maximum with the pintle in its normally retracted position for all speeds.
When the system is operating normally, increasing the vehicle speed will decrease power steering assist, allowing the driver to have improved road feel and directional stability. When sudden steering wheel turns are made, as in evasive maneuvers, they are detected by the SSPS control module through the steering wheel rotation sensor. When detected, the SSPS control module reduces current to the SSPS actuator, allowing greater power steering assist.