Chevrolet Light Duty Truck 1973, Service Manual

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REAR S U S P E N S IO N A N D D R IV ELINE 4 -3 7
Side Bearings
produce a constant rough noise of a lower
pitch than pinion bearings. Side bearing noise may also
fluctuate in the above wheel bearing test.
NOTE: Bearing Diagnosis Charts appear
later in this section.
Gear Noise
There are two basic types of gear noise. The first type is
produced by broken, bent, or forcibly damaged gear
teeth and is usually quite audible over the entire speed
range and presents no particular problem in diagnosis.
For example, hypoid gear tooth scoring as seen in figure
79 generally results from the following: insufficient
lubricant improper breakin, improper lubricant, insuffi­
cient- gear backlash, improper ring and pinion gear
alignment, or loss of drive pinion nut torque. The
scoring will progressively lead to complete erosion of the
gear tooth, or gear tooth pitting and eventual fracture if
the initial scoring condition is not corrected. Another
cause of hypoid tooth fracture is extended overloading
of the gear set which will produce fatigue fracture, or
shock loading which will result in sudden failure.
Differential pinion and side gears rarely give trouble.
Common causes of differential failure are shock loading,
extended overloading, and seizure of the differential
pinions to the cross shaft resulting from excessive wheel
spin and consequent lubrication breakdown.
The second type of gear noise pertains to the mesh
pattern of the gear teeth. This form of abnormal gear
noise can be recognized as it produces a cycling pitch
(whine) and will be very pronounced in the speed range
at which it occurs, appearing under either "drive",
"float" or "coast" conditions. "Drive" is acceleration or
heavy pull. "Coast" is with a closed throttle and vehicle
in gear and "float" is using just enough throttle to keep
the car from driving the engine—the vehicle slows down
gradually but engine still pulls slightly. Gear noise tends
to peak in a narrow speed range or ranges, and will tend
to remain constant in pitch. Bearing noise will vary in
pitch with vehicle speeds. See figure 80.
LIGHT DUTY TRUCK SERVICE MANUAL