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Unread 02-01-2011, 05:29 PM   #16
50rck
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1991 YJ Wrangler 
 
Join Date: Mar 2008
Location: nc
Posts: 320
I have only had my KJ for a few months, so I haven't looked at this yet. Are the drive shafts balanced/keyed? Meaning they can only go on one way?

The reason I ask is this: I had a 1999 F250 Powerstroke that did something very similar, but only in cold winter months. It was a 6 spd manual, so it would do it each time I upshifted until after 4th gear. Turns out, it was the teeth and groves in the driveshaft where the slip yoke was. The cold weather would cause the metal to shrink a little, cause a very small amount of "slop". I replaced all universals, and the carrier bearing, but it still happened. Once the warmer spring and summer weather would arrive, I got no "thump". When the torque of the motor would kick in after an upshift, the teeth would "thump" against the groves. I actually packed a lot of thick grease in it, solved the problem for a while, too.

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Unread 02-01-2011, 08:05 PM   #17
waywardtravel
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2006 KJ Liberty 
 
Join Date: Dec 2010
Location: Panama Country and, az
Posts: 76
Quote:
Originally Posted by lilngineer View Post
What might be the problem in the rear diff?

BTW- I am mostly the mechanic on this one. The shop just took a free look at it for me on Saturday. (I also quickly took a look at the front drive shaft while on the lift. I asked him at that time if ge noticed anything else and all he commented on was the front brakes and a slight rear main leak.

I also asked about u-joints but he said they would be vibrating like crazy if they were the problem.
I would not rule out U-joints, Upper tri-link or lower control arm bushings, gear lash in the diff or anything for that matter.
Can you try to coast on a flat road and try to punch it. Then get up to the same speed in the same gear then Very lightly touch the gas (just enough to know when the engine is pushing the car a tiny bit) then punch it.
Then do the same thing leaving the corner into traffic kind of thing both ways.
Is it only one clunk Every time it does it. Can you feel it?
Usually a thump or thud description is more of a bushing or shock kind of thing (control arm or tri-link) and a bang or clank, loud crack could be more driveline, U-joints, ring and pinion lash etc,,,
It's really a crap shoot in my opinion right now given the current description.
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