Vibration after diff rebuild

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I had a spare axle rebuild by a knowledgable guy who does diff and gearboxes for a living. He did my 95 GT years ago too. Full rebuild with new clutch packs, every bearing new and a fresh set of 3.73s (ford racing).

Prior I had a stock axle with 3.55s, all new control arms, new DSS driveshaft, pinion angle at -2 and all was good. Smooth as a babys buttocks.

Since I swapped axles, I have a vibe above 70mph. It doesnt shake the mirror but almost feels like im running open headers. Like a drony vibration throughout the car.

Today, I clocked the DSS shaft twice, 60° everytime (6 bolts). Nothing seems to change.

Any ideas? could the ring and pinion install just be bad? It seems fine at low speeds other than a very faint howl on decel with clutch in.
 

Juice

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First impression, maybe driveshaft needs to be rotated on the diff flange. But your description may also be an exhaust vibration.
So, is it speed related? What happens if you put car in neutral when you feel vibration? If putting it into N does not change the vibration, then it is likely driveshaft related. Could have a piece of dirt between yoke flange and driveshaft for example. That is the most likely spot IMO.
 
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Ill have to try popping it in neutral tomorrow.
So far I did rotate the ds at the pinion 3 times now, cant do 90 degrees with the DSS shaft. Only thing I can try is remove the DSS spacer from the pinion to see if theres any dirt between that.
 
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Update: checked pinion angle. Which was perfect and I figured unchanged since I only swapped axles and pushed a new oem rubber bushing in the diff.

I can't get a good reading on the transmission side, everytime its something between 1 and 3 degrees pointing down. The pinion was 2.5 up when I started.

So I went with pinion flange at 0, so when it torques the nose up 2 degrees on acceleration it would be 0 pinion angle if the tranny points down 2. This resulted in vibration from 20mph up.

Then I tried pinion flange at 1 degree up, same story. Also, my DSS driveshaft would be beyond its minimum of 3.25" distance on the CV.

Lastly I went pinion flange 3 up, which it still is. Couldnt test it thouroughly but seems vibration free up to 80mph. Only feel a vibration on heavy acceleration ib 3rd gear and low revs.

So far this goes against the advice of having a -2 pinion angle, but its closer to zero now.
 

rocky61201

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I just installed the FRPP aluminum driveshaft. Unfortunately I only had the magnetic dial angle finder instead of a digital one. Driveshaft angle looked to be about 1 degree down. I set my pinion at 1 degree up. Vibe free up to 100mph.
I think I might move the pinion down 1 or 2 degrees to account for pinion rise on acceleration. I can't really tell if there is strong vibes on a hard launch. Too much other stuff going on to pay attention to driveshaft vibe but there may be something there. I got BMR poly bushing control arms upper and lower with a BMR poly bushing on top of the axle so there has to be at least some pinion rise.
When you first posted I was going to suggest maybe the driveshaft got tweaked when it was unbolted, rested on a tall post jack, or suspended with some wire. Might need to be rebalanced. That would suck if it got damaged and I try not to post bad luck stuff.
 

GlassTop09

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I had a spare axle rebuild by a knowledgable guy who does diff and gearboxes for a living. He did my 95 GT years ago too. Full rebuild with new clutch packs, every bearing new and a fresh set of 3.73s (ford racing).

Prior I had a stock axle with 3.55s, all new control arms, new DSS driveshaft, pinion angle at -2 and all was good. Smooth as a babys buttocks.

Since I swapped axles, I have a vibe above 70mph. It doesnt shake the mirror but almost feels like im running open headers. Like a drony vibration throughout the car.

Today, I clocked the DSS shaft twice, 60° everytime (6 bolts). Nothing seems to change.

Any ideas? could the ring and pinion install just be bad? It seems fine at low speeds other than a very faint howl on decel with clutch in.
Just a suggestion but you might want to recheck the rear end's lateral alignment (trans center & pinion center in same vertical plane) thru your PHB since this is a different rear end housing from your original one to ensure that this is on point as well as check the rear end housing for any crab walk (rear end not perpendicular to trans shaft center) as both of these can offset the driveshaft angles & contribute to vibrations as well, especially when going to a 1-piece driveshaft design from a 2-piece design (2-piece w\ 2 CV joints can mask a lot of this). Slight variations in axle mounting hardware positions or bolt hole bore variations between different axle housings can throw this off somewhat.....don't take a lot of variation. This is somewhat geeky speak but is also an aspect of true driveline alignment geometry.

These are mass produced parts so just mentioning this as most may take these aspects of driveshaft alignment to be for granted so may overlook them when swapping out a full assembly. Doesn't remove the possibility of the driveshaft being out of balance so there's that as well...………….

For this reason is why IMHO to go the longer route & rebuild the rear axle assembly using the same housing that came out of the car.

Just a suggestion......…...............
 

07 Boss

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Just out of curiosity, do either or both your axles have those weights attached to it?

I might try swapping in the old one just to verify that it is the axle causing the issue. Can't hurt and isn't that hard to do, couple of hours.
 
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Neither axle had those weights. It's pretty much vibration free as far as I could try (90mph). But I still get a rumble strip like vibration if I give it some power in 3rd gear and low revs.

Also noticed the driveshaft hitting the tunnel on some bumps, probably because of how much nose up the pinion is now.

I'm going to raise the rear springs a little tomorrow (coilovers) and check the pinion angle again with the tremec app this time to make sure.

I find it very strange that the car is basicly undrivable with a recommended -2 pinion angle (-1 at pinion flange).
 

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