Discovery 2. Manual — part 169

STEERING

DESCRIPTION AND OPERATION

57-11

Rotary control valve in demand mode

1 Worm gear
2 Torsion bar
3 Valve sleeve
4 Pin

5 Input shaft and valve rotor
6 Piston/rack
7 Coarse spline
8 Spline (torque shaft to worm gear)

When the steering wheel and input shaft is turned steering resistance transmitted to the worm causes the torsion bar
to be wound up and the valve ports in the valve rotor and valve sleeve to be aligned for a right or left turn. The
alignment of the valve ports directs fluid pressure 'A' from the PAS pump to one side of the piston/rack . The other
side of the piston/rack is now connected to return 'B' (due the valves port alignment) and displaced fluid returns to the
reservoir. The pressure difference in the cylinder on each side of the piston gives the power assistance to move the
rack and so turn the steering.

The greater the resistance of the road wheels to the steering rotary movement, the greater torque acting on the torsion
bar and input shaft causing greater changes of alignment of the ports in the valve. As the change of alignment
becomes greater, the fluid pressure passing to the applicable side of the piston/rack increases.

Only when the steering wheel stops turning and the torsion bar has unwound, will the valve rotor return to the neutral
position. In the neutral position the fluid circulates through the ports in the valve rotor and valve sleeve and back to
the reservoir where it is cooled.

STEERING

57-12

DESCRIPTION AND OPERATION

PAS pump - V8

1 Bolt 2 off
2 Nut
3 Stud
4 Auxiliary housing
5 PAS pump

6 Outlet
7 Inlet
8 Bolt 3 off
9 Drive pulley

The PAS pump is located on the auxiliary housing and is attached by two bolts, the bolts go through flanged bushes
in the auxiliary housing. A stud passes through the PAS pump and through a flanged bush in the auxiliary housing,
the auxiliary housing and PAS pump are secured by a nut. As the two bolts and nut are tightened the bushes move
slightly and the flange of each bush clamps the PAS pump. A drive pulley is attached to the pump drive shaft with
three bolts, and is belt driven at a ratio of 1.4 crankshaft revolutions to 1 of the drive pulley. Fluid is drawn into the
PAS pump inlet from the reservoir through a flexible hose at low (suction) pressure. Fluid at high pressure from the
PAS pump outlet is supplied to the rotary control valve on the steering box.

The PAS pump is a roller vane type and has an internal pressure regulator and flow control valve. The roller vanes
can move in slots in the pumps rotor and are moved outwards by centrifugal force as the pump rotates. The pump
rotor rotates in the pump housing, the internal shape of the housing forms a 'cam' shape. Due to the 'cam' shape the
volume of the housing decreases between the inlet and outlet ports.

As the pump rotor rotates towards the pump inlet the volume between the roller vanes and the pump housing
increases, this action causes a depression in the chamber between the pump roller vanes and the housing. As the
rotation continues the chamber is opened to the pump inlet, and the depression in the chamber causes fluid to be
drawn in. The roller vanes continue past the inlet port, closing off the inlet port and trapping the fluid in the chamber
between the rollers and the pump housing.

STEERING

DESCRIPTION AND OPERATION

57-13

The internal 'cam' shape of the pump housing causes the rollers to move closer together as the pump rotor rotates
towards the outlet port. The reduced volume of the chamber between the roller vanes causes the fluid to become
pressurised. When the chamber is opened to the outlet port of the pump the fluid escapes at high pressure. The roller
vanes continue turning and go past the outlet port, closing off the chamber between the two roller vanes.

As rotation continues the inlet sequence begins again. The inlet and pressurisation/outlet sequences continue as the
pump rotates, and is repeated between each two roller vanes. The pump is a positive displacement type and the
potential pump output increases with engine (drive pulley) speed. The pressure relief and flow control valve regulates
flow/pressure by diverting fluid back to the pump inlet through internal recirculation passages in the pump body.

STEERING

57-14

DESCRIPTION AND OPERATION

PAS pump - diesel

1 Drive pulley
2 Mounting bracket
3 Bolt 2 off
4 Bolt
5 PAS pump
6 Coupling

7 Auxiliary housing
8 Outlet
9 Inlet adaptor pipe and seal

10 Bolt 2 off
11 Bolt 2 off
12 Bolt 3 off

The PAS pump is located on the auxiliary housing, two bolts go through the PAS pump, mounting bracket and into
the auxiliary housing. One bolt also attaches the mounting bracket to the PAS pump, two bolts secure the mounting
bracket to the auxiliary housing. An inlet adaptor pipe with a seal is secured to the bottom of the pump with two bolts.
A drive pulley is attached to the pump drive shaft with three bolts, and is belt driven at a ratio of 1.35 crankshaft
revolutions to 1 of the drive pulley. A coupling on the rear of the PAS pump rotates the water pump impeller located
in the auxiliary housing. Fluid is supplied to the PAS pump inlet from the reservoir through a flexible hose at low
(suction) pressure. Fluid at high pressure from the PAS pump outlet is supplied to the rotary control valve on the
steering box.

The PAS pump is a roller vane type and has an internal pressure regulator and flow control valve. The roller vanes
can move in slots in the pumps rotor and are moved outwards by centrifugal force as the pump rotates. The pump
rotor rotates in the pump housing, the internal shape of the housing forms a 'cam' shape. Due to the 'cam' shape the
volume of the housing decreases between the inlet and outlet ports.