GMC 4000 Series Service Manual page 501

Sec. 9B
Page 490
POWER STEERING
open-center position of the valve reduces pump
lo sses to a minimum. Valve is in open-center posi­
tion at all tim es except when turning. The power
cylinder is always full of fluid, which acts as a
cushion to absorb shocks so that they are not trans­
ferred to the driver. This fluid lubricates all in­
ternal components of valve, making it unnecessary
to lubricate the valve.
With the steering wheel turned to the right, the
torsion bar is deflected, changing the relationship
of the spool grooves and valve body grooves with
each other. The right-turn grooves of the spool
are closed off from the return grooves and opened
to the pressure grooves. The left-turn grooves of
the spool are closed off from the pressure grooves
and opened to the return grooves. This causes the
fluid to flow into the appropriate half of the power
cylinder, overcoming the tire friction in that di­
rection. The fluid in the opposite end of the cylin­
der is simultaneously forced out through the valve
and back to the pump reservoir.
The greater the resistance to turning between
road bed and front wheels, the more the valve
spool is displaced and the higher the fluid pressure
is on the resisting side of the piston. Since the
amount of valve displacement and consequently the
amount of hydraulic pressure built in the cylinder
is dependent upon the resistance to turning, the
G M C S E R V I C E M A N U A L
Figure 3 — Power Steering Control Valve (Typical)
operator is assured of the proper amount of smooth
hydraulic a ssist at all tim es. The instant the oper­
ator stops applying steering effort to the steering
wheel, the valve is returned to its neutral position
by the torsion bar. The fluid pressure is equalized
on both sides of the piston when the torsion bar
returns to neutral position. The wheels return to
a straight-ahead position due to the steering geom­
etry of the vehicle.
the fluid flow in the valve is the same as when
making a right-turn but takes place in the opposite
direction. Parking pressure, the most difficult of
turning conditions, should range from 900 to 1000
psi on vane-type hydraulic pump models and from
1000 to 1100 psi on the slipper type hydraulic pump,
depending upon road bed conditions, weight of ve­
hicle, and pressure relief setting in the pump.
ing wheel effort will be approximately the same
as manual effort under the same conditions. The
control valve will give the driver a smooth trans­
ition through the driving range of wheel effort and
retain the "road-feerT necessary for effortless
driving.
nosing noise, and steering problems can be found
1 Valve Body
Top Cover
2
3 Valve Spool
4 Valve Spool Dampener
Upper Thrust Bearing
5
Spool Spring
6
7 Stub Shaft Seal
Seal Retaining Ring
8
9 Stub Shaft
Torsion Bar Pin
10
Torsion Bar Sleeve
11
Torsion Bar Seal
12
Seal Back-up Washer
13
14 Needle Bearing
Adjuster Plug Lock Nut
15
16 Adjuster Plug
17 O-ring Seal
18 Torsion Bar
19 Valve Body Ring
Back-up Seal
20
Valve Assembly
21
Torsion Bar Cap
22
23 Back-up Washer
24 Worm Bearing
25 Worm Oil Seal
When the steering wheel is turned to the left,
During normal straight-ahead driving, steer­
NOTE: Trouble Diagnosis Chart to aid in diag­
-9060
tpm