Chevrolet 1980 Light Duty Truck Service Manual page 334

4B-8 REAR AXLE
SOME NOISE / S \
ACCEPTABLE
Fig. 4B-9--Noise Level
conditions. For example, this noise tends to reach a "peak"
at speeds from 40 to 60 miles per hour (60 to 100 km/h)
depending 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 may baffle even the best diagnostician.
Vehicle noises coming from tires, 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 of 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 of alleged
axle noises. Thus, every effort should be made to isolate the
noise to a specific drive line component instead of making
a random guess that could be a costly waste of time.
Elimination of External Noises
When a rear axle is suspected of being noisy, it is
advisable to make a thorough test to determine whether the
noise originates in the tires, road surface, front wheel
bearings, engine, transmission, or rear axle assembly. Noise
which originates in other places cannot be corrected by
adjustment or replacement of parts in the rear axle
assembly.
Road Noise -Some 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 of road, such as smooth asphalt or dirt, will quickly
show whether the road surface is the cause of noise. Road
noise usually is the same on drive or coast.
-Tire noise may easily be mistaken for rear
Tire Noise
axle noise, even though the noisy tires may be located on
the front wheels. Tires worn unevenly, or having surfaces
on non-skid divisions worn in saw-tooth fashion, are usually
noisy and may produce vibrations which seem to originate
elsewhere in the vehicle. This is particularly true with low
tire pressure.
Test for Tire Noise
different road surfaces, but rear axle noise does not.
Temporarily inflating all tires to approximately 50 pounds
pressure, for test purposes only will materially alter
noise caused by tires but will not affect noise caused by the
\
Y<
-Tire noise changes with
rear axle. Rear axle noise usually ceases when coasting at
speeds under 30 miles per hour; however, tire noise
continues but with lower tone as vehicle speed is reduced.
Rear axle noise usually changes when comparing "pull" and
"coast" but tire noise remains about the same.
Engine and Transmission Noises
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 approximate car
speeds and conditions under which the noise is most
pronounced; then stop vehicle in a quiet place to avoid
interfering noises. With 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, it is
caused by the engine or transmission and not the rear axle.
Front W heel Bearing Noise
wheel bearings will cause noise which may be confused with
rear axle noises; however, front wheel bearing noise does not
change when comparing "pull" and "coast". Light
application of 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 determine if
bearings are excessively loose.
Body Boom Noise or Vibration
Objectional "body boom" noise or vibration at 55-65
mph (90-100 km /h) 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. Undercoating or mud on the shaft, causing
unbalance.
2. Shaft or companion flange balance weights missing.
3. Shaft damage, such as bending, dents, or nicks.
4. Tire-type roughness. Switch tires from a known
good car to determine tire fault.
If, after making a comprehensive check of the vehicle,
all indications point to the rear axle, further diagnostic steps
are necessary to determine the axle components 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 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 running 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 bearings, misalignment between two U-
joints, or worn differential side gears and pinions; noises
may also be caused by mismatched, improperly adjusted, or
scored ring and pinion gear set.
Rear W heel 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 approximately
-Sometimes a
-Loose or rough front
-A rough rear wheel
LIG H T T R U C K S ER V IC E M ANUAL