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NOISE DIAGNOSIS
INDEX
Page
Axle Shaft Bearing Noise
. .
4
Backlash "Cluck"
. 4
Bearing Noise (Drive Pinion Gear Shaft and
Differential
3
Drive Shaft Vibration
4
Driveline "Snap"
4
Engine and Transmission Noise
. . . . . . . . . . . . . . .
4
GENERAL INFORMATION
The most i m p o r t a n t p a r t of rear axle service is cor-
rectly i d e n t i f y i n g the cause of failures and noise com-
plaints. T h e cause o f most rear axle failures is
r e l a t i v e l y easy to identify, b u t the cause of rear axle
noise is n o r m a l l y more difficult t o identify.
I f vehicle noise becomes objectionable, or i f i t oc-
curs at a l l vehicle speeds, a n effort should be made t o
isolate the noise to one p a r t i c u l a r area of t h e vehicle.
M a n y noises t h a t are reported as coming f r o m t h e
rear axle actually originate a t other sources (e.g.,
tires, road surfaces, wheel bearings, engine, t r a n s -
mission, exhaust, drive shaft (vibration), U - j o i n t or
vehicle body ( " d r u m m i n g " ) . A x l e noises are n o r m a l l y
divided i n t o t w o categories: gear noise or bearing
noise. A thorough and careful inspection should be
completed to determine t h e actual source of t h e noise
before axle disassembly and component inspection is
attempted.
The rear suspension rubber bushings and s p r i n g
insulators help to "dampen-out" rear axle noise w h e n
correctly installed. Inspect to confirm t h a t no m e t a l
contact exists between t h e springs and t h e s p r i n g
hangers, the shackles or the U-bolts. Metal-to-metal
contact a t a n y of these locations can r e s u l t i n a
means o f t r a n s f e r r i n g road noise and n o r m a l axle
noise t h a t w o u l d not n o r m a l l y be objectionable i f the
components were correctly i n s t a l l e d and tightened.
The complete isolation o f noise t o any one area re-
quires considerable expertise and experience. I d e n t i -
fying c e r t a i n types of vehicle noise baffles even the
most experienced mechanics. Often such practices as:
• increasing t i r e i n f l a t i o n pressure to e l i m i n a t e t i r e
noise;
• l i s t e n i n g for noise a t v a r y i n g vehicle speeds w i t h
different
driveline
load
conditions
(e.g.,
" d r i v e " ,
" f l o a t " a n d "coast") a n d w i t h different road/street
conditions; and
• " s w e r v i n g " the vehicle from left to r i g h t t o detect
wheel bearing noise w i l l a i d even t h e beginner i n
isolating axle shaft bearing noise w i t h axles shafts
t h a t have tapered roller bearings.
Page
Front Wheel Bearing Noise
3
Gear Noise (Drive Pinion and Ring)
3
General Information
2
"Knock" At Low Speeds
4
Pre-Disassembly Inspection
4
Tire Noise
. . .
3
A l l rear axles produce noise t o a c e r t a i n extent.
Axles can be noisy i f t h e y are not correctly adjusted
or i f they lack sufficient l u b r i c a t i o n . U s u a l l y , w h e n
incorrectly adjusted replacement gears are causing
noise, the noise can be e l i m i n a t e d b y correctly ad-
j u s t i n g the gears.
I f axle noise occurs w i t h i n t h e first 1,500 miles (2
413 k m ) of vehicle operation for b o t h 8 3/8-inch and
standard or heavy d u t y 9 1/4-inch (212.72-mm a n d
234.95-mm) diameter r i n g gear axles, or w i t h i n t h e
- f i r s t 5,000 miles (8 050 k m ) for Model 60, 6 0 M a n d
70 axles, and the gears are not scored because of lack
of lubrication, t h e gear adjustment should be sus-
pected and corrected, i f necessary.
U n a c c e p t a b l e g e a r m e s h i n g ( t e e t h c o n t a c t ) , i n -
c o r r e c t r i n g g e a r " b a c k l a s h " a n d a loose d r i v e
p i n i o n
g e a r s h a f t y o k e
n u t a r e t h e p r i m a r y
c a u s e s o f gear noise w i t h r e p l a c e m e n t axles.
Regardless of w h a t has been w r i t t e n to t h e con-
t r a r y , noisy axle gears w i l l not produce less noise
w i t h additional usage. T h e y w i l l e i t h e r r e m a i n the
s a m e o r become w o r s e .
S l i g h t axle noise t h a t is noticeable only a t c e r t a i n
speeds or i n isolated situations m u s t be considered
n o r m a l . A x l e noise tends t o "peak" a t a v a r i e t y of ve-
hicle speeds and noise i s N O T A L W A Y S indicative
of a problem w i t h i n the axle.
A good diagnostic test for rear axle noise includes a
thorough road test. Select a level a n d smooth "black-
t o p " road/street. This type o f surface w i l l reduce t i r e
noise
arid
vehicle
body
" d r u m m i n g " . D r i v e f a r
enough to thoroughly w a r m up t h e axle l u b r i c a n t to
t h e n o r m a l operating temperature.
Note the vehicle speed w h e n the noise occurs. T h e n
stop and, w i t h the clutch disengaged or w i t h the au-
tomatic transmission shifted to " N E U T R A L " ,
v a r y
the engine speed (rpm) slowly up a n d down i n a n at-
t e m p t to duplicate a n d identify t h e engine speed
w h e n t h e noise occurs. T h i s w i l l determine i f the
noise is caused b y the exhaust or another engine re-
lated component. I f a noise can not be produced, re-
peat the road test w h i l e engaging a n d disengaging
the clutch or s h i f t i n g the transmission i n t o and out
of " N E U T R A L " .
I f the noise can not be isolated b y
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