GMC 1986 P-Truck Service Manual page 390

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
generally results from the following: insufficient
lubricant, improper break-in, improper lubricant,
insufficient 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 then eventual
fracture if the initial scoring condition is not corrected
(figure 1). 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 because it
produces a cycling pitch (whine) and will be very
pronounced in the speed range at which it occurs,
appearing under either " drive," or " float" or " coast"
conditions. " Drive" is acceleration or heavy pull.
" Coast" is with a closed throttle and the vehicle in
gear and " float" is using just enough throttle to keep
the vehicle from driving the engine; the vehicle slows
down gradually but the 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.
Refer to figures 2 through 5 for bearing diagnosis.
REAR AXLE 4B-5