Chevrolet 1977 light duty truck Service Manual page 360

must be expected and
conventional repairs or adjustment. See figure 4B-9.
Acceptable noise can be defined as a slight noise
heard only at a certain speed or under unusual or remote
conditions. For example, this noise tends to reach a
" p e a k " at speeds from
depend in g on road and load conditions, or on gear ratio
and tire size. This slight noise is in no way indicative of
trouble in the axle assembly.
Drive line noises
d ia g n o stic ia n . Vehicle
transmission, propeller shaft, universal joints, and front
or rear wheel bearings, are often mistaken for axle noise.
Such practices as: raising tire pressure to eliminate tire
noise (although this will not silence tread noise o f mud
and snow tires), listening for the noise at varying speeds
and road surfaces, on drive, float, and coast conditions
will aid in locating the source o f alleged axle noises.
Thus, every effort should be m ade to isolate the noise to
a specific drive line com ponent instead o f making a
r a n d o m guess that could be a costly waste o f time.
Elimination of External Noises
W h e n a rear axle is suspected of being noisy, it is
advisable to m ake a thorough test to determ ine whether
the noise originates in the tires, road surface, front wheel
bearings, engine, transmission, or rear axle assembly.
N oise which originates
corrected by adjustm ent or replacement o f parts in the
rear axle assembly.
Road N o ise -S o m e road surfaces, such as brick or
rough-surfaced concrete, cause noise which may be
mistaken for tire or rear axle noise. Driving 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
rear axle noise, even though the noisy tires m ay be
located on the front wheels. Tires worn unevenly, or
SOME t/O/SE/S
ACCEPTABLE
cann ot be eliminated
40 to 60 miles per hour
m ay baffle even the
noises co m ing from tires,
in other places cannot
having surfaces on non-skid divisions worn in saw-tooth
by
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
different road surfaces, but rear axle noise does not.
T emporarily inflating all tires to approximately 50
pounds pressure,
alter noise caused by tires but will not affect noise caused
by the rear axle. R ear axle noise usually ceases when
best
coasting at speeds under 30 miles p e r hour; however, tire
noise continues but with lower tone as vehicle speed is
reduced. R e a r axle
com paring "p ull" and "coast" but tire noise remains
about the same.
Engine and
noise which seems to originate in the rear axle is actually
caused by the engine or transmission. To determine
which unit is actually
approximate car speeds and conditions un der which the
noise is most pronounced; then stop vehicle in a quiet
place to avoid interfering 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 similar noise is produced with
vehicle standing,
transmission and not the rear axle.
Front Wheel Bearing N o ise-
be
wheel bearings will cause noise which m ay be confused
with rear axle noises; however, front wheel bearing noise
does not change w hen com paring "pu 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. Front wheel
bearings may 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 "bo dy boo m " noise or vibration at 55-
65 mph can be caused by an unbalanced propeller shaft.
Excessive looseness at the spline can contribute to this
unbalance.
Other items that may also contribute to the noise
problem are as follows:
1. U ndercoating or mud on the shaft, causing
unbalance.
2. S haft or com panion
missing.
3. S haft dam age, such as bending, dents, or nicks.
4. Tire-type roughness. Switch tires from a known
good car to determ ine tire fault.
If, after m aking a comprehensive check of 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 bearing noise.
Rear Axle Noises
If a careful test of vehicle shows that noise is not
caused by external items it is then reasonable to assume
N oise-T ire noise changes
will materially
for test purposes only
noise usually ch ang es
Transmission N oises-Som etim es
causing the noise,
it is caused by the
Loose or rough front
flange balance weights
with
when
a
observe
engine or