GMC 1986 P-Truck Service Manual page 389

4B-4 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—Tire 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 d ivisions 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 Test—Tire noise changes with different
road su rfa ce s, but rear axle noise does not.
Temporarily inflating all tires to 345 kilopascals (50
pounds per square inch) pressure, fo r test purposes
only will materially alter noise caused by tires but will
not affect noise caused by the rear axle. Rear axle
noise usually stops when coasting at speeds under 30
miles per hour; however, tire noise continues but with
lower tone as the 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 Noises—Sometimes 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 approximate car speeds and conditions under
which the noise is most pronounced; then stop the
vehicle in a quiet place to avoid interfering noises. With
the transmission in neutral, run the engine slowly up
and down through the engine speeds corresponding to
the vehicle speed at which the noise was most
pronounced. If a similar noise is produced with the
vehicle standing, it is caused by the engine or
transmission and not the rear axle.
Front Wheel Bearing Noise—Loose or rough front
wheel bearings will cause noise which may be
confused with rear axle noise; however, front wheel
bearing noise does not change when comparing " pull"
and " coast." Light application of the brake, while
holding the vehicle speed steady, will often cause the
wheel bearing noise to diminish, as this takes some
weight off of the bearing. Front wheel bearings may be
easily checked for noise by jacking up the wheels and
spinning them, and also by shaking the wheels to
determine if the 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. U ndercoating or mud on the shaft, causing
unbalance.
. Shaft or com panion flange balance weights
2
missing.
3. Shaft damage, such as bending, dents, or nicks.
4. Tire-type roughness. Switch tires from a known
good vehicle to determine a 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 rear axle noise generally
falls into two categories: gear noise and bearing noise.
Rear Axle Noises
If a careful test of the vehicle shows that the noise is
not caused by external items, it is then reasonable to
assume that noise is caused by the rear axle assembly.
Test the rear axle on a smooth level road to avoid road
noise. It is not advisable to test the rear axle for noise
by running with the rear wheels jacked up.
Noise in the 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 p in ion s; noise may also be caused by a
mismatched, improperly adjusted, or scored, ring and
pinion gear set.
Rear Wheel Bearing Noise—A rough rear wheel
bearing produces a vibration or growl which continues
with the vehicle coasting and the transmission in
neutral. A brinelled rear wheel bearing causes a knock
or click approximately every two revolutions of the rear
wheel, since the bearing rollers do not travel at the
same speed as the rear axle and wheel. With the rear
wheels jacked up, spin the rear wheels by hand while
listening at the hubs for evidence of a rough or
brinelled wheel bearing.
D iffe r e n tia l S id e
Noise—Differential 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.
P in io n B earing failures can be distinguished
because 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 a low
speed.
Side Bearings produce a constant rough noise
pitched lower than pinion bearing noise. Side bearing
noise may also fluctuate in the above rear wheel
bearing test.
G ear and P in io n