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I changed my open 2.79 8" rear pumpkin to a 3.55 traction-lok rear and now have a vibration which is really noticeable. It is steady during driving and really gets bad when I let off the accelerator. I didn't change anything else except axle seals so it has got to be something with the pumpkin. My first inclination was to make sure that the u-joint was put in correctly between the "knocks" when it was reinstalled, and that is ok. I got the pumpkin from a friend who had it in his car for about 3 months and then took it out when selling the car. He says it had no problems in his car. Everything went back together perfectly with no problems, so any ideas?

Thanks,
Doug
 

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Before diving in deeper, I'd double check the U-joint connection.

Disconnect it as you did before, pull the caps off and check to make sure the needles are all lined up in place, and then re-install and test drive. A disconnected and then re-connected U joint sometimes has a way of getting messed up... are you sure the joint is fully seated/tightened?

Start out with the simple stuff.

Dave
 

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also you can try disconnecting the u joint and rotating it 180 degrees, then reinstall.

Z. Ray
 

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I just went throught this also, the yoke on the "new" ctr section I installed was worn. You could actually grab the driveshaft and wiggle it, the caps stayed in place in the yoke, but they were spread out too far, it wasn't much .040/.060 or so. I pulled the yoke out of the old rear and swapped them, vibration gone.
 

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I'd recommend that you have your driveshaft re-balanced (& checked out by a reputable shop). With the new gear ratio, your driveshaft is turning much higher rpms than before and any imbalance is going to be magnified. After 40 years, some of these old driveshafts get worn out and wont hold a balance any more. Have it checked out; at least you can eliminate that from the list of possiblities that way.
 

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the other trick to seeing if this is a balance problem is to put on one of the worm screw radiator clamps. Try it then keep trying to rotate it 180 degrees, 90 etc to see if the problem gets better or worse.
 

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It is steady during driving and really gets bad when I let off the accelerator.
Although not impossible, I doubt that your driveshaft is mysteriously out of balance from a quick center section change.

Since the vibration seems to get worse with the drivetrain "unloaded", are you sure that you have a compatable U-joint to rear-end yoke? "Loaded", the transmission is placing a rotational force on the U-joint. Even if the connection was not perfect, the rotational force would keep tension on the ill-fitting connection. "Unloaded", the transmission is NOT placing rotational force on the U-joint connection. This allows the connection to loosen up and cause a heavier vibration.

1330 U-joint to a 1350 yoke maybe?

The same symptoms could apply to a bad pinion bearing.

Focus on the U-joint connection first before delving deeper than necessary...

Dave
 

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Discussion Starter #8
I rotated the driveshaft 180 degrees today and have the exact same thing. Driving it I noticed the vibration gets really bad from about 50 mph up and yes, is definitely worse when letting off of the gas. The driveshaft is fairly new, being replaced after installing a T5 earlier this year, with only about 1000 miles on it. However, Dave is right, it should be ok since the only thing changed was the pumpkin. I checked the ujoint and it is correct for the yoke. I've also checked that the tires are bolted on tight. I was thinking about changing the yoke out from my old one since that should be the next easiest thing. How hard or what is involved to change out the pinion bearing at the same time? Are there things that can go wrong with the insides that can cause this? I know that the gears are new Richmond gears, but I don't know much about the traction lok unit.

BTW, thanks for all of the help. I really appreciate it.

Thanks,
Doug
 

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Something just came to mind. Can you measure the"depth" from the face of the diff to center of yoke. Wonder if it could be shorter and the driveshaft may be a little short if the rear yoke is shorter. Reason It may be happening on throttle off, is the pinion angle is going negative if it typically wraps up on the ring gear with power on. Could also be the binding of the u joint if there is a hange in length.
 

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i would put the rear end and tires in an off the ground condition so tires safely could not touch the ground and observe your vibration and see what it shaking the most,of course a lift,frame type would be easier. wes
 
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