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Discussion Starter #1
So, I changed the trailing arm bushings and sway bar bushings on the '91. No issues, although I was surprised that those little sway bar bushings were actually harder to re-install than the large trailing arm bushings!! After assembly, and re-tightening in the same strap drop location, the car now has a disturbing vibration in the drive line. I noticed that the center driveshaft bearing support was rotted, so I changed that, making sure that all alignments were kept. U-joints are solid. It feels like it is in the pinion/ring gear area, listening with my stethoscope. Could I have damaged or relocated the pinion gear in relation to the ring by lifting and muscling the rear end back into alignment with the bushings? I've never heard of that happening, and find it hard to believe. I plan to drop the diff inspection plate and see what I can see and look for metal shavings, etc. Of course, everything was fine prior to this fix, except the rear end swaying like a cheap hookers!

Thanks in advance for any insight!
 

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Does the vibration frequency change with road speed? Is it worse at any particular speed or under load compared with cruising or on the over-run.
It is highly unlikely that you have damaged anything in the axle.
When you wrote about alignment were you refering to phasing of the U-joints?
 

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Discussion Starter #3
It is not necessarily a speed thing, but load related. Clutch engaged, and costing, it is more noticeable. Push in the clutch, no load, it goes away. Nothing in reverse. Smooth.
Yes, alignment of the U-joints.
 

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After assembly, and re-tightening in the same strap drop location, the car now has a disturbing vibration in the drive line.
I'm not clear on what you mean by 'same strap drop location'. Anyway, when I change suspension bushings I leave them a little bit loose until the car is back on its wheels. Then I jounce the car a few times to settle suspension and then do the final tightening of the bolts in the bushings. This puts them in a neutral position at rest.
 

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Discussion Starter #5
Yeah, that's effectively what I meant. The axle straps. Measure the distance between the axle and the strap to return it to the same relative position. I did-re-loosen all the nuts and bounce it and snugged them back up.
I pried the tranny output shaft out about 1/8" and that seemed to smooth it out, but that only lasted a few hundred yards or so and it came back. That's why I think it is in the rear axle gears.
 

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Discussion Starter #6
My first thought was pinion angle, but there is no real adjustment for that.
 

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Clutch in and rev the engine - vibration or smooth?
 

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Have you shot some grease into the splined coupling lately? Ought to be a grease fitting on it. What about the condition of the giubo coupling? Maybe for some reason, it's now in compression (shouldn't be). I assume the new support bearing/housing all went back together correctly.
 

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Did you replace the center bearing along with the surround? Describe the steps in that operation?

I think that you almost certainly have a driveshaft issue although I had similar symptoms to yours when the splines were worn on the clutch plate causing it to run off center.

If you get the rear of the car up on jackstands (better still the whole car up on a lift) and have your helper drive the car in gear to reproduce the vibration while you carefully watch the driveshaft, then you may be able to see which part is moving around.
 

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Discussion Starter #11
Splined coupling properly greased. Giubo is fairly new, based on condition and the strap that was still on it! I've done all that has been suggested with watching it in gear up on stands. Nothing obvious. I'm a pretty experienced mechanic/ME, so I've been trying all the tricks. I'll dive back into it and let you know if/when I solve it (or not! :rolleyes:)
 

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Problems with the center support usually cause a thumping vibration.

You probably know that the U-joints must move through the same angle or there will be a push-pull vibration. That means that the pinion shaft should be parallel to the front drive shaft when the suspension is loaded. Adjustment in the vertical plane is made by lowering either the center support housing or the gearbox cross member using washers. In the horizontal phase it can be done by slotting the holes in the gearbox cross member.
 

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Just a thought Feex... how's your transmission mount bushing?

Going back to your description, and your using the stethoscope, is the flange loose on the pinion shaft? That nut is staked in place, so I'd be really surprised if the flange had worked loose, but I wanted to raise the question since you indicated the noise seemed to be around in that area.
 

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The strap was still on the Gibbo? (Post 11)
I trust you removed it?
A few posters have pointed to driveshaft alignment front/rear being critical, and to play with up/down and left/right adjustment of the various mounts (as described by alfaparticle).
Unless I’m missing something (entirely likely), I have some trouble reconciling an adjustment of a couple of mm making a difference when all was fine with a collapsed bearing support. Maybe the slop allows the driveshaft to centre itself whilst spinning, and a fresh support makes this harder/impossible if things aren’t perfectly aligned.
Love to know how you resolve this.
 

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It feels like it is in the pinion/ring gear area, listening with my stethoscope. Could I have damaged or relocated the pinion gear in relation to the ring by lifting and muscling the rear end back into alignment with the bushings? I've never heard of that happening, and find it hard to believe.
I've also never heard of that happening, and find it hard to believe. I'd look elsewhere.

I plan to drop the diff inspection plate and see what I can see and look for metal shavings, etc. Of course, everything was fine prior to this fix
Yes, it would be an incredible coincidence for the differential to suddenly fail at the same time that you had the driveline apart. I would focus on the driveshaft.

You said "U-joints are solid". Is it possible that they are TOO solid? They shouldn't have any play, but they should flex freely.
 

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Discussion Starter #16
Slight update. After new drive shaft bearing/carrier and U-joints, the problem persisted. From talking with other car friends, someone mentioned that the driveshaft may have pushed the tranny yoke inward while the whole rear-end was being moved around. That could cause internal tranny vibe issues. Released the center bearing mount and did all possible to pry the yoke rearward. Go it to move about 1/8" outward. Slight improvement off the bat, but then returns within a few gear changes.Still have some suspicion about the diff. in the pinion area. Plan to drain it and inspect this weekend. Had it on a lift, running in gear and the rear end sounds like there are things rolling around inside!! Could just be the inner spider gears since there is no load on them. More to come. Anyone have a picture of how much gap they have from the tranny housing to the yoke?
 

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Discussion Starter #17
Still no resolution on this. The only thing left of the driveshaft to change would be the front doughnut at the yoke. I'm wondering if my attempts to pull the tranny yoke rearward is actually contributing, and I should be moving it forward? But why wouldn't it finds its original location after driving for some time? Driving me batty! Otherwise it is running great!!
 
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