I recently replaced my entire rear subframe. While it was out I had the diff rebuilt, the seals replaced, and I installed a mazda comp front diff mount w/ MMR rear diff mounts. I also repacked the halfshafts w/ the appropriate synthetic grease.

When I first got it re-installed I went for a test, I was expected increased vibrations front the MMR mounts but it felt like the rear end was gong to separate from the car above 40 mph. (Note: it was fine below 40)

I assumed this was because my reference marks on the diff flange were erased (inadvertently by the shop) when it was serviced and when I reassembled it the drive shaft was not properly aligned.

I unbolted the drive shaft an rotated the orientation clockwise by one bolt. This seemed to increase the threshold for vibration by roughly 5-10 mph (about 50mph before it feel like it wants to come apart)

This vibration is coming exclusively from the rear so it is not an issue with the engine, clutch (10k) or trans (engine and trans mounts are MMR as well).

What's causing this insane vibration ? why is it perfectly fine up to a certain rpm/speed?

I'm leaning toward the driveshaft, but its odd that it would be bad now since it was perfectly fine before the rearend ovehaul...

 

When it starts vibrating does it increase in severity as speed increases?

 

its fine up to about 40-45mph, then it goes bat **** crazy! its like someone hit a switch. I was thinking perhaps it has a limited range, but i'm afraid to take it any higher than 50mph because it literally sounds like it might come apart.

 

you should not be able to be able to rotate the driveshaft 90 degrees and install it...
it is probably not sitting flush on the diff...

go keep turning it, hopefully, you would be able to have it at the right direction.....

looks like you really fucked something up to be able to put the driveshaft in wrong! all the bolts are off a little so it only goes on one way...

 

Just kind of throwing it out there, but whenever you mess with the suspension then something's screwed up I would get the alignment checked.

 

Just as an FYI, the MMR Rear Diff bushings cause a lot more noise to be transmitted to the cabin. I realize that this is not a vibration, but it should be considered as it may affect your perception of the severity of the vibration.

 

I have mmr rear mounts and notice very little difference at all in vibration or harshness, I definitely would not perceive it as a problem and it does not feel like the diff is going to fall out. It is worse in 4th or 5th gear though, so does your problem change depending on which gear your in?

I did not even know the driveshaft is supposed to go back in the same orientation with the diff flange. I didn't even pay attention to that when I replaced my diff mounts and I guess I got lucky putting the driveshaft back in the same orientation. Idk, I thought the driveshaft is supposed to balanced and that it doesn't matter which bolts go in which holes? GL dude.

 

I wouldn't rule out that it isn't sitting flush on the diff....

However the bolts are not off by anything. If you keep rotating the drive shaft (clockwise in this case) you can match any hole on the shaft to any hole on the diff and they all match up....hence the problem.

I've tried two positions, just 2 more to try, then I guess its off to the transmission shop ?

 

keep trying...
if not - take a close look at the ujoints???
have you looked at the not so obvious... like loose lug nuts?

 

Make sure you didnt loose a balance weight on your wheels.

 

try checking your ring and pinion mesh. just get a can of lead paint that they sell for checking it. it might have been put back together without being properly spaced

 

an improperly clearanced diff would cause noise, not so much vibration unless there are broken bits inside.

 

I'm gonna try to get it on a lift as soon as possible to look at everythign in motion, problem is convincing someone to do that while the car is running w/ just a downpipe......and not idling on its own

 

Ok, problem solved!

Turns out after rotating the drive shaft 3 positions, it is now oriented properly. It is important to make sure the bolts are seated completely on the back side of the diff flange, as they will not do so unless you rotate them when they are installed.

It still has a light vibration due to the MMR drive line mounts, that picks up w/ RPM until about 4500 or so, but thats what I was expecting and the severe vibration is now gone.



Now to get that idle fixed so I can get to the exhaust shop and stop running open downpipe like a skittle.....