Chevrolet Light Duty Truck 1973 Service Manual page 260

REAR SU S PEN SIO N AND DRIVELINE
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.
Some road surfaces, such as brick or rough-
Road Noise—
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.
Tire noise changes with different
Test for Tire Noise—
road surfaces, but rear axle noise does not. Temporarily
inflating all tires to approximately 50 pounds pressure,
will materially alter noise caused
for test purposes only
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
j "coast" but tire noise remains about the same.
Engine and Transmission Noises—
which seems to originate in the rear axle is actually
: caused by the engine or transmission. To determine
I 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 Wheel Bearing Noise—
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.
Sometimes a noise
Loose or rough front wheel
Body Boom Noise or Vibration
Objectional "body boom" 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. 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 Wheel Bearing Noise—
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
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 Noise—
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 differentia] 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.
A rough rear wheel bearing
Differential
LIGHT DUTY TRUCK SERVICE MANUAL