S90 L6-2.9L VIN 96 B6304S (1998)
Power Steering Pump: Description and Operation
Power Steering Pump Control Valve
NORMAL POSITION
There is a control valve in the pump housing which regulates the oil quatity and maximum pressure.
When the pump starts the spring (7) holds the valve (5) to the left (see illustration). The oil delivered by the pump passes through the pressure
channel (3) via the throttle valve (2) into the pressure pipe (1) and then to the steering gear.
The space to the right of the control valve is connected to the pressure pipe (1) by the connecting channel (8) and has the same pressure.
The throttle valve (2) makes the pressure on the left hand side of the control valve piston higher than in the pressure pipe and the space to the right
of the piston. When the spring pressure is overcome the piston is pushed to the right.
When rotation is sufficiently high in relation to the counter pressure the piston has been moved so much that the surplus oil can pass back to the
pump intake side, see illustration. Because the pump should deliver a smaller amount than the maximum capacity this valve setting can be
considered normal.
If the pump outlet flow is stopped, by the steering movement being blocked for example, the pressure rises in the pipe (1) and the pressure
difference on the ends of the control valve is equalized.
The spring then pushes the valve back to the left, the connection with the return channel is closed and the pressure rises again.
MAXIMUM PRESSURE
Under maximum pressure the spring pressure on the safety valve (6), that is the inner part of the control valve, is overcome and oil can pass out to
return channels (4), see illustration
The pressure on the right side of the piston decreases and the control valve is pushed to the right and opens the connection with the return channel.
When the pressure has sunk to a normal value the safety valve shuts and the control valve returns to the normal position.
