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North American Grand Cherokee Association
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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.

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