GMC 1976 ZEO 6083 Maintenance Manual page 287

6A- 2 2
ENGINE
G . NOISY WATER PUMP
NOTE :
Listen for a ratchety or grinding sound which increases with engine rpm.
In the early stages, the grinding noise may disappear at higher engine rpm.
Disconnect the fan belt, and run engine . If noise disappears, trouble most likely
is the water pump bearing. Bearing failure or start of failure can be detected by
grasping the water pump pulley with both hands and moving it in a sidewise
motion . If sloppiness is present, the bearing is unserviceable .
GENERAL MECHANICAL
Rough bearing. Shaft pulley loose.
Pump seal too hard . Impeller loose on shaft .
Too much end play in pump shaft .
Too much clearance between shaft and
bearings .
Impeller blades rubbing against pump
housing.
Impeller pin sheared off.
Impeller broken .
Brush squeal .
Bearings .
Faulty diode or stator .
H . NOISY GENERATOR
(Refer to Generator Diagnosis Charts)
GENERAL
I. NOISY FAN
J . NOISY FUEL PUMP
MECHANICAL
Loose mounts .
Belt too tight .
GENERAL MECHANICAL
Fan blades bent . Fan blades loose on clutch .
Fan out-of-balance when made . Fan blades strike shroud .
Fan shaft end play excessive.
NOTE : Diagnosis of fuel pumps suspected as noisy, requires that some form of
sounding device be used . Judgment by ear alone is not sufficient, otherwise a fuel
pump may be needlessly replaced in attempting to correct noise contributed by
some other component. Use of a stethoscope, a long screwdriver, or a sounding
rod is recommended to locate the area or component causing the noise. The
sounding rod can easily be made from a length of copper tubing 1/4 to 3/16 inch
in diameter. Dowel rods are also good .
If the noise has been isolated to the fuel pump, remove the pump and run
the engine with the fuel remaining in the carburetor bowl . If the noise level
does not change, the source of the noise is elsewhere and the original fuel
pump should be reinstalled .