Diagnosing Noise, Part 1
Many things can go wrong inside a differential. Although the hints are often
subtle, most impending failures give fair warning in the form of noise.
Several situations can create ring-and-pinion noise. If the gears have been
quiet and begin to howl, they are probably worn or wearing. If the gears howl
during deceleration only, it's possible that the pinion-bearing preload has
loosened. Howling under acceleration at all speeds indicates that something in
the differential -- gears, pinion or carrier bearings -- has worn or no longer
keeps the gear alignment correct. If the gears howl while accelerating over a
certain speed range, but not all speeds, it's likely that the gears are worn
due to lubrication failure or overloading. When a newly installed gear set
howls, suspect the design or setup.
A common problem is worn carrier bearings, as indicated by a low-pitch rumble
above 20 mph. On vehicles with C-clip axles the noise may vary while
negotiating turns. Worn pinion bearings can cause whirring noises at all
speeds, under deceleration and/or acceleration. Pinion bearings tend to whir,
rather than rumble because the pinion is turning several times faster
(depending on gear ratio) that the carrier. Badly worn bearings can also cause
howl if they do not support the gears correctly.
Worn wheel bearings can be difficult to determine. A very bad wheel bearing
typically makes itself heard with great clarity; it's the bearing that is
going bad, but not destroyed that is hard to find. Turning back and forth from
hard right to hard left can identify the culprit; however, I've been fooled by
right-front wheel bearings that make noise when turning right (which heavily
loads the inside-left-front wheel bearing, but also loads the
One common situation that may not make any noise: The pinion spins, but the
tires don't rotate. Broken spider gears can render the differential immobile,
and usually make a loud, crunching sound as they make their final departure. A
broken ring gear will allow the differential to propel the vehicle for about
eight feet at a time, then bang or grind as the section with broken teeth
tries to engage the pinion. Depending on ratio, a broken pinion tooth (or
teeth) will clunk about every two or three feet.
A broken axle is easily determined. After it breaks, a C-clip design axle can
be pulled out of the housing without unbolting anything -- or may even find
it's own way out. On many bolt-in-design axles, the wheel will give the broken
axle shaft away by cambering in at an angle.
A high spot on a gear tooth may sound similar to a broken gear, but will only
make noise while accelerating or decelerating, since the spot appears on just
one side of the offending tooth. A high spot on the ring gear will make a
heavy clicking sound about every eight feet; a high spot on the pinion makes
noise every two or three feet and is much more pronounced due to its higher
Whether large or small, differential noise is telling you something. Listen
carefully! If in doubt, pull off the cover or remove the third member for a
closer look. Catching a bad part before is ruins others is definitely worth
Diagnosing Noise, Part 2
If you've been left hanging with a "mystery" differential noise that still
refuses to make itself clearly understood, then hopefully this month's info
will lend some more insight.
Anyone who has been involved with four-wheel-drive vehicles has probably heard
of or experienced positraction (posi) "chatter". Posi chatter is noise that is
very recognizable and happens when there is too much friction in the clutches.
Some hard-core offroaders set up their posi this way intentionally. The noise
sounds like someone is pounding on the rearend with a huge sledgehammer. It is
most prevalent when backing up in a parking lot (when everyone around can
stare), and gets worse as the differential heats up. It also tends to show up
on freeway off-ramps and when turning while taking off from a stop sign.
Broken spider gears can sound similar to posi chatter, only more consistent,
regardless of oil temperature. Broken spider gears will make a grinding or
banging sound any time the vehicle is making a turn, and, if they are bad
enough, even when going straight.
Driveline vibrations can be caused by several problems. Worn universal
joints or a driveline that is out of balance are often the problem, but
driveline angle can cause a balanced driveline with good U-joints to vibrate.
If the U-joints are bad, they can cause several different noises from
squeaking, to clunking, to grinding, to vibrations. If the driveline is out of
balance, it will vibrate with a steady pitch that increases as the vehicle
speed increases. If the pinion shaft is out of alignment and not parallel to
the transmission yoke, the difference in the angles between the front and back
U-joints can cause the driveline to vibrate. If the vibration is due to
improper angles, it will create a cyclic sound that increases and decreases in
intensity and is not steady. An out-of-alignment problem can also be
identified by the change in the noise when accelerating or decelerating. As
the pinion yoke torques up from acceleration or down from deceleration, the
rear U-joint angle changes and causes the vibration to change.
A worn side-gear bore in the carrier case will usually cause a clicking sound
as the vehicle is coasting down from speeds of about 20 miles per hour to a
stop. If the bore that supports the side gear becomes too worn to hold the
side gear in place the side gear will "roll over" the spider pinion gears and
will make a clicking noise.
If your differential problem is still not clear and you don't want to take the
time to look inside for more data, you can always drive it until it breaks and
the problem will be much clearer, although much more expensive.
Much more similar differential and gear tech articles at: