Chevrolet 1977 light duty truck, Service Manual

Page: 360 / 861

must be expected and can n o t be elim inated by
conventional repairs or adjustm ent. See figure 4B-9.
Acceptable noise can be defined as a slight noise
h e a rd only at a certain speed or u n d e r unusual o r remote
conditions. For example, this noise tends to reach a
" p e a k " at speeds from 40 to 60 miles p er hour
d ep en d in g on road and load conditions, or on gear ratio
and tire size. This slight noise is in no w ay indicative o f
trouble in the axle assembly.
Drive line noises m ay baffle even the best
d ia g n o s tic ia n . Vehicle noises c o m in g fro m tires,
transmission, propeller shaft, universal joints, and front
or re a r wheel bearings, are often mistaken for axle noise.
Such practices as: raising tire pressure to elim inate tire
noise (although this will not silence tread noise o f mud
an d snow tires), listening for the noise at varying speeds
and ro ad surfaces, on drive, float, and coast conditions
will aid in locating the source o f alleged axle noises.
Thus, every effort should be m ad e to isolate the noise to
a specific drive line com ponent instead o f m aking a
r a n d o m guess th at could be a costly waste o f time.
Elimination of External Noises
W h e n a rear axle is suspected o f being noisy, it is
advisable to m ake a thorough test to determ in e w hether
the noise originates in the tires, road surface, front wheel
bearings, engine, transmission, or rear axle assembly.
N oise which originates in other places cannot be
corrected by adjustm ent or replacem ent o f parts in the
re a r axle assembly.
Road N o is e -S o m e ro ad surfaces, such as brick or
rough-surfaced concrete, cause noise which m ay be
m istaken for tire or rear axle noise. D riving on a
different type o f road, such as smooth asphalt or dirt,
will quickly show w hether the road surface is the cause
o f noise. R oad noise usually is the same on drive or
coast.
Tire Noise- Tire noise may easily be mistaken for
re a r axle noise, even though the noisy tires m ay be
located on the fro n t wheels. Tires worn unevenly, or
having surfaces on non-skid divisions worn in saw-tooth
fashion, are usually noisy and m ay produce vibrations
which seem to originate elsewhere in the vehicle. This is
particularly true with low tire pressure.
Test for Tire N o ise -T ire noise changes with
different road surfaces, but rear axle noise does not.
T em porarily inflating all tires to approxim ately 50
pounds pressure,
for test purposes only will materially
alter noise caused by tires but will not affect noise caused
by the re a r axle. R e a r axle noise usually ceases when
coasting at speeds und er 30 miles p e r hour; however, tire
noise continues but with lower tone as vehicle speed is
reduced.
R e a r axle noise usually c h a n g e s w hen
com paring "p u ll" and "coast" but tire noise rem ains
about the same.
Engine and Transmission N o ise s-S o m e tim e s a
noise which seems to originate in the rear axle is actually
caused by the engine or transmission. To determine
which unit is actually causing the noise, observe
approxim ate car speeds and conditions u n d e r which the
noise is most pronounced; then stop vehicle in a quiet
place to avoid interferin g noises. W ith transmission in
neutral, run engine slowly up and down through engine
speeds corresponding to vehicle speed at which the noise
was most pronounced. If a sim ilar noise is produced with
vehicle standing, it is caused by the engine or
transmission and not the re a r axle.
Front Wheel Bearing N o ise - Loose or rough front
wheel bearings will cause noise which m ay be confused
with rear axle noises; however, fro n t wheel bearing noise
does not change w hen com p arin g "p u ll" and "coast".
Light application o f brake, while holding vehicle speed
steady, will often cause wheel bearing noise to diminish,
as this takes some weight off the bearing. F ro n t wheel
bearings m ay be easily checked for noise by jacking up
the wheels and spinning them, and also be shaking
wheels to determ ine if bearings are excessively loose.
Body Boom Noise or Vibration
Objectional " b o d y b o o m " noise or v ibration at 55-
65 m ph can be caused by an unbalanced propeller shaft.
Excessive looseness at the spline can contribute to this
unbalance.
O ther items th at may also contribute to the noise
problem are as follows:
1. U ndercoating or mud on the shaft, causing
unbalance.
2. S h aft or com panion flange balance weights
missing.
3. S h aft d am age, such as bending, dents, or nicks.
4. Tire-type roughness. Switch tires from a known
good car to determ in e tire fault.
If, after m aking a comprehensive check o f the
vehicle, all indications point to the rear axle, further
diagnostic steps are necessary to determ ine the axle
com ponents at fault. True axle noises generally fall into
two categories: gear noise and b earin g noise.
Rear Axle Noises
SOME t/O/SE/S
ACCEPTABLE
If a careful test o f vehicle shows that noise is not
caused by external items it is then reasonable to assume