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I've done some reading about the vibration problem associated with many cougar/tbirds. I've got something like it that I can start to notice around 50, and gets pretty noticable/irritating anywhere above 70mph. Considering that I usually cruise more like 75-80 this has been wearing on my considerably.

From a post by yardbird http://forums.tccoa.com/showthread.php?t=58222 it seems that one cause of this could be misalignment between my rear differential and my transmission: the transmission is tilted down somewhat, and the rear diff isn't tilted up to match. But I also noticed a post by Kentwood in there suggesting that these kinds of problems could also be runout. A quick google on runout turns up a bunch of info on disc brakes. I have drums in the rear. When I step on the brakes, it doesn't get worse, which to me rules out warped rotors. But I suppose runout in the front discs is a possibility.

http://autorepair.about.com/library/glossary/bldef-454.htm

The description of the driveshaft misalignment vibration sounds somewhat like mine, except that the source doesn't seem to be moving. I can feel it from everywhere in the car. It doesn't seem to make much difference if the driveline is under load or not, or if the car is in neutral.

I've jacked the car up and rotated the "up" tire when its in park, and I seem to get about 1-1.5 inches of rotation out of the tire before the drive line stops it from rotating. The wheel and the shaft going from the wheel to the diff move nearly, if not identical amounts. There does seem to be some play in the rear diff. The driveshaft will rotate a little bit before the tranny prevents it from moving any further, but as far as I can tell the U-joints look pretty solid. IE the play seems to be in the rear diff and the yoke going into the tranny.

Anybody have any ideas? I'm getting pretty thoroughly stumped at this point. I talked to the guy at my local NAPA here in Tallahasse and he said that to check on the DS balance I could use 2 hose clamps and start them 180deg opposed, and through a process of trial-and-error see if the DS balance was the problem.

Based upon these symptoms, anybody have any suggestions? Also, if you live anywhere in Florida and would be willing to help me diagnose, please message me. I drive to tally, jax, gainesville, daytona, tampa, orlando, miami pretty regularly.

Thanks,
Mike
 

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Has the shaft been removed before or gears changed? If so it may have been reinstalled with improper orientation. I would also check to see if there is any debris or foreign matter on it. I've seen shafts vibrate so bad, taken in and had the paint stripped and voila, no more vibes. May also be a sign the u-joints are in need of replacing.
 

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I first posted this here back in 98. It is still good today... :D Here are some symptoms and problems:

Low pitch rumble at all speeds over 20 MPH; fluctuating noise when turning: Worn carrier bearings.

Banging or clunking while making a turn, backing up, and/or on freeway off ramp. Gets worse with higher temp: Posi chatter due to improper lubrication, worn clutches, worn spider gears, or improper assembly.

Banging or clunking while making a turn. Heat does not effect: Broken or badly worn spider gears.

Rumble or clicking noise that worsens on hard turns: Bad wheel bearings.

Cyclic vibration (unsteady in intensity), more pronounced over specific speed range. Worsens when decelerating: Pinion angle too low, needs to be parallel with front yoke.

Cyclic vibration (unsteady in intensity), more pronounced over specific speed range. Worsens when accelerating: Pinion angle is too high, needs to be parallel with front yoke.

Banging or heavy clicking every 8 feet during acceleration or deceleration: Broken or damaged pinion gear tooth or teeth.

Banging or heavy clicking every 2-3 feet during acceleration or deceleration: Broken or damaged pinion gear tooth or teeth.

Banging or heavy clicking every 8 feet while either accelerating or decelerating, but not both: High spot or heavy chip in pinion gear tooth.

Banging or heavy clicking every 2-3 feet while either accelerating or decelerating, but not both: High spot or heavy chip in pinion gear tooth.

Clicking while decelerating from 20 mph to a stop: Worn carrier case-side bores.

Steady vibration that increases with speed: Worn u-joint or driveshaft is out of balance.

Clunk just after taking off from a stop: Worn slip yoke splines.

Clunk when getting on the throttle (taking off): Worn U-Joints, worn spider gears, worn axle splines, excessive backlash, loose yoke splines, or worn slip yoke splines.

Driveline squeaking or grinding at speed: Worn or damaged U-Joints.

Whirling during decelerating/ accelerating above 10 mph, possible rumble: Worn pinion bearings.

Gear set howls after install: Faulty design or improper installation.

Howl while accelerating over a small speed range (previously quiet): Worn gear set due to lubrication failure or overloading.

Howl while accelerating at all speeds without whir or rumble (previously quiet): Worn gear set due to lubrication failure or overloading.

Howl while accelerating at all speeds, accompanied by a whir or rumble (previously quiet): Worn rear pinion bearing and possibly worn gear set.

Howl while decelerating (previously quiet): Loose pinion bearing preload.
 
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