GMC 1982 Light Duty Truck Service Manual page 422

4 B -8 REAR A X LE
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 elim inate 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.
D eterm ining Type of Noise
E xternal Noise
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
a djustm e nt or rep lacem en t of parts in the rear axle
assembly.
Noise-Some road surfaces, such as brick or
Road
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 N oise-T ire noise may easily be mistaken for rear
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.
noise changes with different road
Tire N oise T est-T ire
surfaces, but rear axle noise does not. Temporarily inflating
all tires to appro xim ately 50 pounds pressure,
purposes only will materially alter noise caused by tires but
will not affect noise caused by the 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 Transm ission
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 N o ise -
bearings will cause noise which may be confused with rear
axle noises; however, front wheel bearing noise does not
change when com paring
application of brake, while holding vehicle speed steady, will
often cause wheel bearing noise to diminish, as this takes
for test
Noises-Sometimes a noise
Loose or rough front wheel
" p u ll" and " c o a s t" . Light
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 N oise or Vibration-
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.
O ther items that may also contribute to the noise
problem are as follows:
1. Undercoating or mud on the shaft, causing unbalance.
2. $h aft 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 A xle 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
differential or pinion shaft bearings, misalignment between
two U-joints, or worn differential side gears and pinions;
noises may also be caused by m ism atched, improperly
adjusted, or scored ring and pinion gear set.
Rear W heel Bearing N oise-A
produces a vibration or growl which continues with vehicle
coasting and transmission in neutral.
bearing causes a knock or click approximately every two
revolutions of rear wheel, since the bearing rollers do not
travel at the same speed as the rear axle and wheel. With
rear wheels jacked up, spin rear wheels by hand while
listening at hubs for evidence of rough or brinelled wheel
bearing.
Differential Side Gear and Pinion
side gears and pinions seldom cause noise since their
movement is relatively slight on straight ahead driving.
Noise produced by these gears will be most pronounced on
turns.
failures can be distinguished because
Pinion Bearing
they rotate at higher speeds than differential side bearings
and axle shaft bearings. Rough or brinelled pinion bearings
produce a continuous low pitched whirring or scraping noise
starting at relatively low speed.
Side Bearings produce a constant rough noise of a lower
pitch than pinion bearings. Side bearing noise may also
fluctuate in the above wheel bearing test.
G ear Noise
There are two basic types of gear noise. The first type is
produced by broken, bent, or forcibly damaged gear teeth
and is usually quite audible over the entire speed range and
presents no particular problem in diagnosis.
Objectional "body
rear wheel bearings, faulty
rough rear wheel bearing
brinelled rear wheel
A
Noise-Differential