GMC 4000 Series Service Manual page 500

3. Close shut-off valve and observe pressure
gauge. Pressure should be 1000 to 1100 psi.
NOTE: Do not close valve for more than a few
seconds, as this would increase fluid temperature
and cause excessive pump wear.
4. Open shut-off valve; then turn wheels to
extreme right and left with wheels on ground. At
extreme positions, the maximum pressure reading
should be within above lim its.
5. If pressure is greater than 1100 psi or less
than 1000 psi, replace the pressure relief valve
and repeat the test. If pressure is still not within
lim its specified, remove the pump and disassem ble.
PUMP DRIVE BELT
MAINTENANCE
The drive belt must be kept at proper tension.
A loose belt will reduce output of the hydraulic
pump, while a tight belt will cause eventual bear­
ing failure. A regular, periodic inspection is rec­
ommended to check condition of drive belt. Re­
place belt if frayed or badly worn.
NOTE: On a new vehicle or after having in­
stalled a new drive belt, check belt tension twice
in first 200 miles of operation.
ADJUSTMENT
1. Loosen hydraulic pump to mounting bracket
attaching bolts.
2. Position pump so that a reading of 120-130
pounds (new belt) or 90-100 pounds (used belt) is
obtained on a strand tension gauge.
3. Tighten pump to mounting bracket bolts
firm ly.
NOTE: Mounting brackets and bolt holes should
be inspected for cracks or elongated condition. If
defective, they should be repaired or replaced.
POWER STEERING GEAR
The 549-DV and 553-DV type power steering
gear assem blies are basically the same except for
size and mounting. Refer to "Model Application"
at end of this section for steering gear application
on each series vehicle.
CONTROL VALVE
The power steering system control valve is
mounted on top of the steering gear housing and is
activated by a torsion bar that tends to keep the
valve in neutral position. Should the torsion bar
break, the spool and valve body "lock up" into a
complete unit, and steering gear then operates as
a mechanical unit.
G M C S E R V I C E M A N U A L
to carry hydraulic fluid through the power steering
system . These tubes and hoses connect the steer­
ing gear to the power cylinder and hydraulic pump.
ted for leakage at regular intervals. Fittings must
be tightened to torque listed in "Specifications" at
end of this section. Make sure clips, clamps, and
unions supporting tubes and hoses are in place and
properly secured.
AN D CONTROL VALVE
tered, rotary-type, three-way valve. The spool is
held in the neutral position by means of the tor­
sion bar. The spool is attached by means of a stud
fastened to one end of the torsion bar and the valve
body to the other end. Twisting of the torsion bar
allows the spool to displace in relation to the valve
body, thereby operating the valve.
position, the fluid flows from pump through the
open-center valve, and back to pump reservoir
without traveling through the power cylinder. This
POWER STEERING
Figure 2 — Checking Pum p Hydraulic Pressure
(Slipper Type P um p ) (Typical)
TUBES, HOSES, A N D FITTINGS
Stationary tubes and flexible hoses are used
All tubes, hoses, and fittings should be inspec­
The valve shown in figure 3 is an open-cen-
CONTROL VALVE OPERATION
When the valve is in neutral or straight-ahead
Sec. 9B
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