m
ember. The quick-fit connector O-rings are not serviceable.
Special tool JD 182 is required to disconnect the quick-fit connector.
Hydraulic System
P
arts List
Hydraulic System
Op
erating Principle
The rot
ary motion of the steering wheel is converted, via the st
eering rack pinion, to the lateral motion of the rack. Steering
assistance is provided by pressurized fluid being directed against a piston operating within the rack cylinder. Rack
movement is achieved by diffe rential pressures being applied on either si de of the rack piston, caused by unequal
restrictions within the hydraulic control valve.
The main components of the hydraulic control valve are the rota ry distributor, control sleeve and hydraulic reaction piston.
It
em
Par
t Number
De
scription
1—F
luid reservoir
2—Pum
p
3—Pressure and fl
ow limiting valve
4—Supply line
5—R
eturn line
6—Steeri
ng rack
7—Pin
ion
8—O
perating cylinder and piston
9—Hydrau
lic control valve
10—Hydraulic reaction
piston
11—Control
sleeve
12—R
otary distributor
13—F
luid cooler
A
quick-fit connector with double O-ring se
als is incorporated in the return line and is clipped to the right-hand chassis side
The rotary distri
butor and control sleeve turn relative to ea
ch other, forming the unequal restrictions which create the
differential pressures to operat e the rack piston; the displacement of the di stributor and sleeve being controlled by the
elastic deformation of a torsion bar which is concentric with the pinion and valve. Refer to Positive Center-Feel Torsion Bar
in this sub-sectio n.
The hydraulic reaction piston moves axially, relative to th e rotary distributor, and is connected to the control sleeve by a
three-bearing helical screw. Pressure applied either side of the hydraulic reaction piston is translated into a rotational force
which increases steering effort. For detail s of Servotronic speed-sensitive steering control, refer to Servotronic Control in
this sub-section.
When the vehicle is travelling straight ah ead, the valve restrictions are balanced, thus providing equal pressures on either
side of the rack piston. When load is a pplied at the steering wheel, the two halves of the control valve (rotary distributor
and control sleeve) are displaced making the restrictions unequa l. The resulting differential pressures on either side of the
rack piston, assist the steering rack to mo ve to left or right. As the turning load is removed, the pressures equalize again
and the steering return s to the straight ahead position , aided by suspension geometry.
Serv
otronic Control
P a
rts List
The Servotronic system provides steering assistance which is pr oportional to vehicle speed. It comprises the steering control
module, and Servotronic valve forming part of the hydraulic control valve assembly.
The Servotronic valve is an electro-hydrauli c device with a variable orifice, which varies in size according to the position of a
needle valve. The needle valve is controlled by a solenoid in response to a variable current input from the steering control
Ite
m
Part
Number
Descr
iption
1—Vehi
cl
e speed input
2—Steeri
ng control
module
3—Servotronic valve
4—Hydraulic
control valve housing
5—PAS stee
rin
g pump
6—Batt
ery
