GMC 4000 Series Service Manual page 507

Sec. 9B
Page 496
POWER STEERING
increased as when making a full cramped U-turn.
The noise is caused by the relief valve and is
normal. Momentary aeration of the oil is som e­
tim es noticeable under these conditions. Some
noise may also be present under standstill parking
condition, particularly when the wheel stops are
contacted. Power steering pump noise can be con­
fused with many other things such as transmission,
rear axle, generator, etc. If excessive noise is
present, remove the pump drive belt to make sure
the pump is at fault. If it is determined that the
pump is at fault proceed as follows:
1. Check Drive Belt Tightness. Adjust belt
tension, if necessary, as described previously
under "Pump Belt Tension Adjustment."
2. Check Oil Level. Refill, if necessary, as
described in LUBRICATION (SEC. 0) of this manual.
3. Check Hose. Make sure hose is not touch­
ing any other parts of the vehicle, especially the
sheet metal.
4.
C heck F or P r e s e n c e
will show up as bubbles or oil will appear milky.
Small amounts of air cause extremely noisy oper­
ation. If air is present in oil accomplish the fol­
lowing:
a. Tighten all fittings and bolts.
b. Check the entire system for source of air
leak. Air can leak into the system at many places.
Air leaks usually occur at joints in the system
where oil p asses through at high velocity - such
as hose connections or at the drive shaft seal.
NOTE: After each step in attempting to elim ­
inate air, the pump should be operated for a few
minutes at idle speed while occasionally turning
the steering wheel between extreme turns to allow
air to bleed out of oil.
5. After Eliminating Air From The Oil. In­
stall a pressure gauge in the pressure line between
the pump and steering gear. If, when racing the
engine to about 1000 rpm and without turning the
wheels, pressure exceeds 125 psi, the hose and/or
the steering gear are restricting the oil flow.
These parts should be examined to determine
cause of restriction.
6. If The P ressure is Less Than 125 Psi in
Step 5 Above, remove the pump and repair.
SOURCE OF LEAK
Top of reservoir .
At the pressure
fitting or studs
At shaft seal ....................
Leaks in metal parts . . .
G M C S E R V I C E M A N U A L
of Oil.
A ir in The A ir
CAUSE
Reservoir too full
Air in the oil . .
Not tightened su fficien tly.................
Cross threaded or defective seat on
fittings or hose end. Damaged seals.
Defective seal or damaged shaft
Damaged c a s t in g s ...............................
PUMP LEAKS
Whenever oil leakage occurs in the hydraulic
pump, all fittings and bolts should be cleaned and
tightened. If this does not stop leakage, wipe pump
assembly clean to determine where it is leaking.
Following are some of the possible causes of pump
leakage (see Table below):
POOR, OR NO ASSIST, OR
PUMP INOPERATIVE
When the pump is inoperative, providing little
or no assist in turning, first check the belt tension
and adjust it as necessary, as previously described.
If this does not correct the problem, check the oil
level, fill and bleed the system as described pre­
viously.
If there is no improvement, it must be estab­
lished whether the pump, steering gear, or hose
is the source of trouble. This may be determined
by two tests after placing a pressure gauge in the
pressure line between pump and steering gear.
Test #1 is Performed With the Oil Circuit
Open. Raise the temperature of oil to operating
range of 150 to 170 degrees measured with a ther­
mometer. This may be accomplished by turning
the wheels from wheel stop to wheel stop several
tim es. Do not hold steering wheel against wheel
stop for any extended period as this will overheat
the oil. As previously mentioned, the pressure re­
lief valve in the pump will not be able to handle
the excessive pressure created by contacting the
wheel stop for an extended period of time, there­
fore, the oil will overheat.
After heating the oil to at least 150°F., set
engine idle to 450 rpm; then turn the steering
wheel from one end to the other and read pressure
on gauge while holding the wheel momentarily
against the stops. The reading should be the max­
imum with engine idling at 450 rpm. If it is not,
trouble is in the hydraulic circuit, but it does not
indicate whether the pump or steering gear or both
are at fault.
This can be determined by performing Test
REMEDY
. . . Fill to proper level.
. . . Eliminate air as previously
described under "Pump Noise."
. . . Torque to 25-40 foot-pounds.
. . . Correct as necessary.
. . . . . Replace damaged parts.
. . . Replace damaged parts.