Power Steering Pump Speed Sensor: Description and Operation

Power Steering Speed Sensor





The power steering speed sensor is a trochoid-rotor, hydraulic pump combined with a relief valve and a one- way valve. It is driven by the speedometer gear shaft which in turn is driven by a helical gear on the differential.
The power steering speed sensor turns only when the car is moving, controlling the gain control valve.





The constant pressure is generated by the pressure control valve. This pressure is used as a reference pressure for the response to the car's speed. By introducing this pressure to the power steering speed sensor through the sensor orifice, the pressure downstream of the orifice is changed according to the speed of the car.





With the engine running at idle in a parked car, fluid flow through the sensor rotors is blocked because the rotors are not turning. Therefore the gain control valve moves to the left. On the gain control valve, the orifice resistance is high on pump side, while it is low on the reservoir side, with the result that pressure in the reaction chamber is lowered and steering assist is high.





As the car is driven, the rotors start turning and the fluid returns to the reservoir, reducing the fluid pressure at the gain control valve. Therefore, the gain control valve begins to move to the right. The orifice resistance on the pump and reservoir sides is appropriately balanced, with the result that the reaction chamber is in the medium range and the steering resistance is moderate.





When the car is moving at high speed, the sensor reduces the pressure further and the gain control valve moves further to the right. The orifice pressure on the pump side is low and the pressure on the reservoir side is high, the fluid pressure in the reaction chamber is also high giving the steering wheel less assist.


One-way Valve (In Power Steering Speed Sensor)





When the car is moving at high speed, negative pressure develops at the sensor inlet because the power steering speed sensor is pumping faster than the fluid can be supplied. To compensate for this, the outlet and inlet ports are connected internally by a passage containing a one-way valve that lets output fluid recirculate to the inlet port to equalize pressure.


Relief Valve (In Power Steering Speed Sensor)





When the car is moving in reverse, the power steering speed sensor also turns backward and pumps fluid in the opposite direction. To avoid building up pressure in the reaction chambers that would increase steering effort while driving in reverse, the inlet and outlet-ports are connected by a second internal passage containing a relief valve that allows the fluid to recirculate.