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 speed 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 Speed Sensor
The steering wheel speed 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 EVO actuator.
System Operation
System operation originates with input from the vehicle speed sensor via the engine control module 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 speed 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 EVO actuator to the desired current it has calculated. The EVO actuator has a pintle 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 EVO actuator by varying the output duty cycle. If the EVO actuator is disconnected, the pintle 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 speed sensor, the EVO 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 speed sensor. When detected, the SSPS control module reduces current to the EVO actuator, allowing greater power steering assist.
SSPS (Speed Sensitive Power Steering) is to control power steering effort properly according to varying conditions for both convenience and safety.
At low speed or standstill, power steering effort becomes light to provide easy steering. On the contrary, at high speed, power steering effort becomes heavy to provide stable steering. SSPS offered to this vehicle is EVO (Electric Variable Orifice) type, which controls steering effort by modulating pressurized fluid supply to the steering unit from the power steering pump.
Steering wheel speed sensor is installed at the bottom of the steering column.
This sensor detects steering wheel rotation which is used to calculate the angle speed of the steering wheel by SSPS module.
If the angle speed was big enough for intervention into normal steering force control during high vehicle speed, which means a driver made an evasive steering maneuver to avoid an accident, SSPS control module would supply enough steering support.
The sensor consists of a variable resistor.
Checking procedure is as follows;
1. Remove SSPS connector.
2. Turn the ignition ON.
3. Try to measure 5 V across A and C terminals. If you fail, it might be A, C wiring open or short, or SSPS module malfunction.
4. Check if B (+) and B (-) are supplied to terminal "8" and "10" respectively.
The flow control actuator is installed on the outlet of the conventional vein type of the power steering pump. The actuator modulates fluid flow to the steering gear from the pump by current control from the SSPS control module.
At low speed, small current is supplied to the actuator. Then the actuator modulates flow high and steering effort becomes light.
At high speed, On the contrary, big current to the actuator. Low fluid flow and heavy steering effort.
If there happens an actuator malfunction, by default its pintle valve opens the orifice 100% to supply light steering effort.
The maximum pumping pressure with SSPS option is 74.4 - 81.6 kg/cm2.
If you try to measure voltage at the actuator terminal with the connector removed, you can measure 6.5 V.
[Vehicle Speed - Actuator Current Relation]
SOLENOID RESISTANCE 13 Ohms
The control module is installed on the right side of the audio and can be accessible after the glove box is removed.
It receives signals from the vehicle speed sensor and the steering wheel speed sensor and processes them to decide proper fluid flow through the actuator.
It also has a self-diagnosis. If it detects any malfunction, it starts default mode, which is basically actuator full open for light steering effort.
