300Tdi Discovery
STEERING
7
DESCRIPTION AND OPERATION
Pump and regulator valve operation
The pump which is belt driven from the engine is an
eccentric roller type and also houses the pressure
regulator and flow control valve. The pressure is
controlled by a spring loaded ball valve 3 which is
housed inside the flow control valve piston 4.
No demand for assistance High flow through box -
Low pressure
With no demand for assistance as illustrated in
RR3568M the rotary valve in the steering box acts as
a pressure relief valve, allowing fluid A to flow freely
through the steering box and back to the reservoir and
pump inlet B.
No demand for assistance
1. Reservoir
2. Pump
3. Pressure control ball valve and spring
4. Flow control valve and spring
5. Press fit plug (ball bearing)
6. Restrictor
The ball plug item 5 is pressed into the valve 4
during manufacture and determines the opening
pressure of pressure relief valve 3.
No flow, through box - High pressure
When the steering is turned, the rotary valve
effectively stops all fluid flow through the steering box,
thus causing an increase in pressure A. This increase
in pressure is felt in the flow control valve spring
chamber where, at a pre-determined pressure the
relief valve 3 will open and allow the pressure to
escape. The fall in pressure in the flow control spring
chamber, allows the flow control valve to move to the
right, which in turn allows pump output A to escape
directly into the pump inlet B, as seen in RR3569M.
Assistance demanded
As soon as the steering wheel is released after
making a turn, the system reverts to the condition
seen in RR3568M and the road wheels are returned
to the straight ahead position by the mechanical
steering geometry.
In the event of any hydraulic failure steering control
though heavy, will be maintained through the
mechanical components in the steering box.
