GMC 4000 Series Service Manual page 508

#2 and comparing it with test No. 1.
Test #2 is Performed With the Oil Circuit
Closed. Set engine idle at 450 rpm and turn the
shut-off valve of gauge to the closed position.
NOTE: Shut-off valve must be located between
the gauge and steering gear. Observe pump p res­
sure on gauge at idle and compare it with Test #1.
Diagnosis of the Two T ests. If the first test is
below specifications and the second test is equal
to specifications, or greater, the steering gear is
An axle mounted power cylinder acts as a
power a ssist to the mechanical steering, and is
bracketed to the front axle at one end, and to the
steering rie rod at the other end.
Stationary metal tubes and flexible hose a s­
sem blies carry hydraulic fluid to operate the power
cylinder piston for right or left turns.
When steering wheel is turned, the control
valve on steering gear housing, directs hydraulic
fluid, under pressure, from the hydraulic pump to
right or left side of power cylinder piston. This
produces movement of piston and attached steering
linkage. The force applied by the power cylinder
is the amount of thrust required for all steering
requirements.
When turn is completed, reduced effort in
steering wheel allows steering geometry of the
vehicle to return wheel to a neutral or straight­
ahead position. When returning to a neutral posi­
tion, oil on one side of the cylinder piston is forced
back to the hydraulic pump reservoir by oil on
other side of the piston, thus equalizing the oil
p ressure. This constant amount of oil in the cyl-
1 Outer Tube
2 Inner Tube
3 Pressure Relief Valve
4 Oil Seal______________
G M C S E R V I C E M A N U A L
POWER CYLINDER
POWER STEERING
11
Oil Seal
Extension Port
Retaining Ring
Retraction Port
Figure 8 — Pow er C ylinder
at fault. When the first test is below specifications
and the second test is not more than 50 psi greater,
the pump is at fault.
NOTE: If steering gear is found to be at fault,
use the troubleshooting procedures for the steer­
ing gear described under "Troubleshooting The
Power Steering Gear" previously in this manual.
NOTE: For additional troubleshooting inform­
ation, refer to "Quick Reference Troubleshooting
Chart" later in this section.
inder acts as a shock absorber or cushion to dis­
sipate road shocks to the operator.
POWER CYLINDER REPLACEMENT
REMOVAL
1. Clean dirt from around hose connections at
power cylinder ports.
2. Disconnect hoses from power cylinder,
catching hydraulic fluid in a clean container. Cover
hose fittings and ports in cylinder to keep dirt out.
3. Remove cotter pin and loosen adjuster
screw securing end of power cylinder to support
bracket mounting; then remove cylinder from
mounting ball.
4. Repeat procedures in Step 3 previously,
and remove opposite end of cylinder from tie rod
ball stud.
INSTALLATION
1. Position power cylinder on tie rod ball stud
and on support bracket mounting ball.
2. Turn adjuster screw at each end of power
Sec. 9B
Page 497
POWER STEERING
Assembly
9 Wiper
10 Guide Assembly
Rod
11 Piston
12 Piston
T-3346