Diagnosis Of The Rear Axle; Determining The Type Of Noise - GMC G Service Manual

DIAGNOSIS OF THE REAR AXLE

The most essential part of rear axle service, as with any
mechanical repair, is proper diagnosis of the problem. In
axle work, one of the most difficult areas to diagnose is
noise. Locating a broken axle shaft or broken differential
gear presents little or no problem, but locating and isolating
axle noise can be an entirely different matter.
Degree of Noise
Any gear-driven unit, especially an automotive drive axle
where engine torque multiplication occurs at a 90 degree
turn in the driveline, produces a certain amount of noise.
Therefore, an interpretation must be made for each vehicle
to determine whether the noise is normal or if a problem
actually exists. A normal amount of noise must be expected
and cannot be eliminated by conventional repairs or
adjustment.
Acceptable noise can be defined as a slight noise heard
only at a certain speed or under unusual or remote condi­
tions. 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.
Driveline 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, and 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 driveline component instead
of making a random guess that could be a costly waste of
time.
DETERMINING THE
TYPE OF NOISE
External Noise
When a rear axle is suspected of being noisy, it is advis­
able to make a thorough test to determine whether the
noise originates in the tires, road surface, front wheel bear­
ings, 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 mis­
taken 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 divisions worn in saw-tooth fashion, are usually
noisy and may produce vibrations which seem to originate
REAR AXLE 4B1-3
elsewhere in the vehicle. This is particularly true with low
tire pressure.
Tire Noise Test — Tire noise changes with different
road surfaces, but rear axle noise does not. Temporarily
inflating all tires to 345 kilopascals (50 pounds per square
inch) pressure, for 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 con­
tinues 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 Transmission 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
vehicle 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 dimin­
ish, 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 — Objectionable "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 prob­
lem are as follows:
1. Undercoating or mud on the shaft, causing
unbalance.
2. Shaft or pinion flange balance weights 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.