Chevrolet 1977 light duty truck Service Manual page 361

that noise is caused by rear axle assembly. The rear axle
should be tested on a smooth level road to avoid road
noise. It is not advisable to test rear axle for noise by
r u n n in g with rear wheels jacked up.
Noises in rear axle assembly may be caused by a
faulty propeller shaft, faulty rear wheel bearings, faulty
differential or pinion shaft
between two U-joints, or worn differential side gears and
pinions; noises may also be caused by mismatched,
im properly adjusted, or scored ring and pinion gear set.
Rear Wheel Bearing Noise-
bearing produces a vibration or growl which continues
with vehicle coasting and transmission in neutral. A
brinelled rear wheel bearing causes a knock or click
approxim ately every two revolutions of rear wheel, since
the bearing rollers do not travel at the same speed as the
re a r axle and wheel. With rear wheels jacked up, spin
re a r wheels by hand while listening at hubs for evidence
o f rough or brinelled wheel bearing.
Differential Side Gear and Pinion
tial side gears and pinions seldom cause noise since their
m ovem ent is relatively slight on straight ahead driving.
Noise produced by these gears will be most pronounced
on turns.
Pinion Bearing failures
because they rotate at higher speeds than differential
side bearings and axle
brinelled pinion bearings produce a continuous low
pitched w h irrin g or scraping noise starting at relatively
low speed.
Side Bearings produce a constant rough noise o f a
lower pitch th a n pinion bearings. Side bearing noise may
also fluctuate in the above wheel bearing test.
N O TE : Bearing Diagnosis Charts appe a r later in
this section.
Gear Noise
There are two basic types o f gear noise. The first
type is produced by broken, bent, or forcibly dam aged
gear teeth and is usually quite audible over the entire
speed range and presents no particular problem in
diagnosis.
F or example, hypoid gear tooth scoring as seen in
figure 4B-10 generally
insufficient lu b ric a n t im p ro p e r
lubricant, insufficient gear backlash, im proper ring and
bearings, misalignment
A rough rear wheel
Noise^Differen-
can be distinguished
shaft bearings. Rough
results from the following:
breakin, im p ro p e r
Fig. 4B-10--Two Causes of Gear Noise
pinion gear alignment, or loss of drive pinion nut torque.
The scoring will progressively lead to complete erosion
o f the gear tooth, or gear tooth pitting and eventual
fracture if the initial scoring condition is not corrected.
A nother cause o f hypoid tooth fracture is extended
overloading o f the gear set which will produce fatigue
or
fracture, or shock loading which will result in sudden
failure.
Differential pinion
trouble. C om m on causes o f differential failure are shock
loading, extended
differential pinions to the cross shaft resulting from
excessive wheel spin
breakdown.
The second type o f gear noise pertains to the mesh
pattern o f the gear teeth. This form o f a bnorm al 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, a ppearing under either "drive",
"float" or "coast" conditions. "D riv e " is acceleration or
heavy pull. "C o ast" 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. G e a r noise tends
to peak in a narrow speed range or ranges, and will tend
to rem ain constant in pitch. Bearing noise will vary in
pitch with vehicle speeds. See figure 4B-11.
and side gears rarely
overloading, and seizure
and con seq uen t lubrication
give
of the