Power Steering Pump Speed Sensor: Description and Operation

Speed Sensor




The 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.





It turns only when the car is moving, controlling the cut-off valve by regulating fluid pressure in the control unit according to the speed of the car.
With the engine running in a parked car, fluid flow through the sensor rotors is blocked because the rotors are not turning.





As the car is driven away, the rotors start turning and pump fluid back to the reservoir, reducing pressure at the cut-off valve. The cut-off valve begins cycling, staying open for longer and longer intervals as the car accelerates and the sensor reduces the pressure further. This allows pressure in the reaction chambers to rise, restricting control valve movement more and more, and gradually reducing the assist as speed increases.

One-way Valve (In Speed Sensor)




When the car is moving at high speed, negative pressure develops at the sensor inlet because the 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 Speed Sensor)




When the car is moving in reverse, the 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.