Power Steering Pump: Description and Operation

DESCRIPTION






On all vehicles equipped with power steering, the hydraulic pressure for operation of the power steering gear is provided by a belt driven power steering pump. It is mounted on the back side of the engine, above the front suspension crossmember.
Early production vehicles are equipped with variable-assist power steering pumps. Later production vehicles are equipped with standard power steering pumps only.






Vehicles that are equipped with variable-assist power steering use a droop flow style power steering pump. The droop flow power steering pump is a constant displacement vane type, but has a variable flow rate. This provides the speed-sensitive variable-assist power steering. The variable-assist power steering pump can be easily identified by the type of power steering fluid pressure hose fitting connection. While standard pumps use a standard tube fitting, variable-assist power steering pumps use a banjo style fitting.
The power steering pump uses a remote mounted reservoir for storage of the power steering fluid.
Because of unique shaft bearings, flow control levels or pump displacements, power steering pumps may be used only on specific vehicle applications. Be sure that the power steering pump is only replaced with a pump that is the correct replacement for that specific application.

OPERATION
Hydraulic pressure is provided for operation of the power steering gear by the belt driven power steering pump. The power steering pump is connected to the steering gear by a power steering fluid pressure hose, return hose, power steering fluid cooler and remote power steering fluid reservoir.
Rectangular pumping vanes in the shaft driven rotor move power steering fluid from the intake to the cam ring pressure cavities of the power steering pump. As the rotor begins to turn, centrifugal force throws the vanes against the inside surface of the cam ring to pick up residual fluid. This fluid is then forced into the high pressure area. As more fluid is picked up by the vanes, the additional fluid is forced into the cavities of the thrust plate through two crossover holes in the cam ring and pressure plate. The crossover holes empty into the high pressure area between the pressure plate and the housing end cover.
As the high pressure area is filled, fluid flows under the vanes in the rotor slots, forcing the vanes to follow the inside surface of the cam ring. As the vanes reach the restricted area of the cam ring, fluid is forced out from between the vanes. When excess fluid flow is generated during high-speed operation, a regulated amount of fluid returns to the pump intake side through a flow control valve. The flow control valve reduces the power required to drive the pump and holds down temperature build-up.
The speed-sensitive variable-assist power steering which is built into the early power steering pump provides full power steering assist at engine idle. As engine speed is increased, power assist is gradually reduced by varying the pump flow rate to provide a firm, responsive feel to the steering system at higher vehicle speeds.
When steering conditions exceed maximum pressure requirements, such as when the wheels are turned against the stops, the pressure built up in the steering gear exerts pressure on the spring end of the flow control valve inside the pump. The high pressure lifts the relief valve ball from its seat and allows fluid to flow through a trigger orifice located in the outlet fitting. This reduces pressure on the spring end of the flow control valve which then opens and allows the fluid to return to the intake side of the pump. This action limits maximum pressure output of the power steering pump.
Under normal power steering pump operating conditions, pressure requirements of the pump are below maximum, causing the pressure relief valve to remain closed.
In the event of a power steering pump drive belt failure, manual steering control of the vehicle can still be maintained without pump assistance. However, under these conditions, steering effort will be significantly increased.