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Discussion Starter #1
Wrll just installed a posi into and 3.73 gears now I have an issue with really bad vibration that shakes the whole car. Any ideas as to what could cause this ?
 

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What speed does the vibration occur?

Also, does throttle position or gear have anything to do with it? Does it sound the same under load, coasting in neutral, and decelerating?

A few hints to the problem and I'm sure the guys on here will have some good ideas. I would say the pinion nut could be loose. How did you tighten it and what did you set the pinion preload at?
 

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I'd guess is a drive line balance issue. You have increased the drive line speed at the same MPH with the lower ratio gears. Like lowrollin70gmc said we need some more details. I used to fix this stuff for a living, and a lot of guys on this board have tons of experience so we'll find it if you are patient and follow a good solid diagnostic path.

First thing I would do is flip the shaft in the yoke 180. While it's apart to do that check both u-joints again. Make sure you didn't drop a roller into the cap or otherwise mess up the joint during the swap. Make sure the wheels are installed correctly.

Drive it and see if it's speed related. Does it do it at the same speed regardless of gear? Coast, acceleration and neutral? Where do you feel it? Feet, seat etc? Any noise associated with it, a booming or drone?
 

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Discussion Starter #8
Vibration gets worse with speed and during braking when it down shifts from 2nd to 1st there is a noticeable pop. I'll try and get more info as I can
 

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Are you absolutely positive the pinion is not loose and walking? Sometimes when you tighten the crush sleeve/set pre-load the bearings will bind and don't set on the pinion correctly. After you run it a bit the bearing slides settles. The pinion is then loose in the case and can walk back and forth.

The noise is a big concern to me. It sounds like something didn't go right on the install.
 

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Discussion Starter #11
Went by the book on the entire install so it may be a possibility but not likely. It's always made a small bit of noise when doing that down shift before but it's just louder now. May need to tighten down the pinion nut then?
 

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Went by the book on the entire install so it may be a possibility but not likely. It's always made a small bit of noise when doing that down shift before but it's just louder now. May need to tighten down the pinion nut then?
Check it don't tighten anything unless there is an issue. I've been fixing cars professionally for 25+ years and I still make mistakes. You gotta go back under the car and double check everything. Grab the yoke and see if it moves. It will be obvious. IF all looks good you might consider having the drive shaft checked by a drive line company.

What size rear tire are you running and what gears did you have before the swap?
 

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When checking u joints the shaft needs to be out of the car. When the shaft is installed unless the joint is ready to fall apart any play in them can not always be felt.

One thing to be aware of when installing a u joint it should move easily, so easy it takes no pressure at all to move it. After installing a new u joint the yoke ears need to be tapped with a hammer to eliminate any binding from the ears being compressed during installation. If this is not done the joint will fail prematurely.

 

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Make sure there is Zero play in the saddle on the pinion yolk. I had my driveshaft balanced this summer, but figured out the play at the saddle was my issue. Most driveline shops can balance your shaft same day or overnight. They also have yolks on the shelf, most of today's so-called auto parts stores do not.
 

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Not to be annoying by repeating myself but what is the tire size and old gear ratio? I want to run some numbers drive shaft diameter too please.
 

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Discussion Starter #17
Found the culprit it was a broken u joint. Couldn't tell until I took the driveshaft out. But old gears were 2.41 and new ones are 3.73 wheel and tire saize is 255-50-16
 

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Found the culprit it was a broken u joint. Couldn't tell until I took the driveshaft out. But old gears were 2.41 and new ones are 3.73 wheel and tire saize is 255-50-16
Woot woot :) glad you found it. Moot point on the numbers if the vibration is gone since we don't have to look any further. :eek:
 
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