Chevrolet 1977 light duty truck, Service Manual

Page: 361 / 861

th at 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 re a r 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 bearings, misalignm ent
between two U-joints, or w orn differential side gears and
pinions; noises may also be caused by mismatched,
im p ro p erly adjusted, or scored ring and pinion gear set.
Rear Wheel Bearing Noise- A rough re a r wheel
b e a rin g produces a v ibration or growl which continues
with vehicle coasting and transm ission in neutral. A
brinelled re a r wheel bearing causes a knock or click
app ro x im ately every two revolutions o f re a r wheel, since
the bearing rollers do not travel at the same speed as the
re a r axle and wheel. With re a r 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 Noise^Differen-
tial side gears and pinions seldom cause noise since their
m o v em en t is relatively slight on straight ahead driving.
Noise produced by these gears will be most pronounced
on turns.
Pinion Bearing failu res can be distin g u ish ed
because they rotate at h ig h er speeds than differential
side bearings and axle sh aft bearings. Rough or
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 m ay
also fluctuate in the above wheel bearing test.
N O T E : Bearing D iagnosis Charts a p p e a r later in
this section.
Gear Noise
T h e re are two basic types o f gear noise. The first
type is produced by broken, bent, or forcibly d a m a g e d
gear teeth an d is usually quite audible over the entire
speed range an d presents no particular problem in
diagnosis.
F o r example, hypoid g ear tooth scoring as seen in
figure 4B-10 generally results from the following:
insufficient lu b ric a n t i m p r o p e r break in , i m p r o p e r
lubricant, insufficient gear backlash, im p ro p er ring and
Fig. 4B-10--Two Causes of Gear Noise
pinion gear alignment, or loss o f drive pinion nut torque.
The scoring will progressively lead to complete erosion
o f the gear tooth, o r g ear 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
fracture, or shock lo ading which will result in sudden
failure.
Differential pinion an d side gears rarely give
trouble. C o m m o n causes o f differential failure are shock
loading, extended overloading, and seizure o f the
differential pinions to the cross shaft resulting from
excessive wheel
spin an d co n se q u e n t lu b ricatio n
breakdown.
The second type o f gear noise pertains to the mesh
pattern o f the gear teeth. This form o f a b n o rm a l 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 p p e a rin g under either "drive",
" float" or "c o a st" conditions. " D r iv e " is acceleration or
heavy pull. " C o a s t" is with a closed throttle an d vehicle
in gear and " flo a t" is using just enough throttle to keep
the car from driving the en g in e—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.