Pontiac FIREBIRD 1972 Service Manual page 326

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4A-2 1972 PONTIAC SERVICE MANUAL
tires to approximately 50 pounds pressure (for test
purposes only) will materially alter noise caused by
tires, but will not affect noise caused by 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 car speed is re-
duced. Rear axle noise usually changes between pull
and coast, but tire noise remains about the same.
Distinguish between tire noise and differential noise
by noting if noise varies with various speeds or sud-
den acceleration and deceleration; exhaust and axle
noise show variations under these conditions while
tire noise remains constant and is more pronounced
at speeds of 20 to 30 miles per hour. Further check
for tire noise by driving car over smooth pavements
or dirt roads (not gravel) with tires at normal pres-
sure. If noise is caused by tires, it will noticeably
change or disappear and reappear with changes in
road surface.
5. Loose or rough front wheel bearings will cause noise
which may be confused with rear axle noises; how-
ever, front wheel bearing noise does not change when
comparing pull and coast. Light application of brake
CAUSE
Differential Side Gear and Pinion Noise
Ring and Pinion Gear Noise
Bearing Noise
while holding car speed steady will often cause wheel
bearing noise to dimish, as this takes some weight off
the bearing. Front wheel bearings may be easily
checked for noise by jacking up the wheels and spin-
ning them, also by shaking wheels to determine if
bearings are loose.
6. Rear suspension rubber bushings and spring insula-
tors dampen out rear axle noise when correctly in-
stalled. Check to see that no metallic contact exists
between the springs and spring opening in frame or
between upper and lower control arm bushings and
frame or axle housing brackets. Metal-to-metal con-
tact at those points may result in telegraphing road
noise and normal axle noise which would not be ob-
jectionable if dampened by bushing.
7. It is important that a check also be made to ensure
that the floor of body is not in metallic contact with
frame. After the noise has been determined as being
in the axle by following the above appraisal proce-
dure, the type of axle noise should be determined to
aid in making repairs if necessary.
REMEDY
Seldom cause noise on straight
ahead driving, as their movement
is relatively slight. Noise produced
by these gears will be most pronounced
on turns.
This type of rear axle noise normally
appears first as a whine, audible between
20 and 65 mph and under four driving
conditions;
1. Drive-Acceleration or heavy pull.
2. Road Load
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Car under steady
load or at constant speed.
3. Float
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Maintaining constant car
speed at light throttle on a level road, or
using enough throttle so that engine is
not actually driving the car
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car slows
down gradually while engine still pulls
slightly.
Gear noise most frequently has periods
where noise is more prominent
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usually
30 to 40 mph and 50 to 60 mph.
When objectionable axle noise is
encountered, note the driving condition
and speed range. If the problem is
diagnosed as g&r noise, remove the
differential cover and perform a red
lead check. Shim and adjust to obtain
the, best possible tooth pattern. If
noise still persists, replace the
gear set.
Bad bearings generally produce a rough
growl or grating sound, rather than the whine
typical of gear noise. Bearings fail by
lapping, spalling or locking: