My wife's 97 got rear-ended a month ago. Very little damage, you can hardly tell it was hit. Well, the diff housing broke at the little relief notch that Mazda designed in there. I got a RX-7 diff housing from panicmotorsport for cheap and transplanted the open 1.8 diff over to the rx-7 housing. It was surprisingly easy and everything matched up. That rx-7 diff housing is alot beafier.
Well the first couple of test drives seemed just fine. There is a little gear whine almost like when the diff is set too tight. I figuired this might be due to the new polyurethane mounts transmitting more vibration. It still felt real smooth though. I didn't mess with the gears at all while they were out and checked them to see if anything was damaged or worn funny. Everything looked perfect.
Then I got on the interstate, right when I hit 75mph a loud noide starts in the rear with a vibration associated with it (no wheel vibe and yes I marked the driveshaft before I removed it and installed in the same orientation). It is really weird how it kicks in very distinctively at 75mph, it doesn't build up and then hit that resonant peak like a wheel vibe does. The really strange part is that the tach will start oscillating slightly(around 4k to 5k) even though there is no load or rpm changes on the engine, it runs just smoothly. I tried it in 4th and 5th to see if it was rpm dependent, but it doesnt matter what rpm, just when it hits 75mph exactly. There are no tach oscillations at 4k to 5k when driveing at lower speeds.
The first thing I did was replace the passenger axle because I could swear the sound came from that side and I was worried that I had damaged the axle when driving the car with a broken diff housing for 2 miles on the way home, before I realized it was broken. Well, even with the replacement axle, I have the same symptoms.
I am going to replace the driveshaft and see if that changes anything. At this point I am really stumped. The diff was working fine in the old housing, everything was inspected cleaned and relubricated with new axle seals installed.
Any ideas as to what could be the cause of this strange vibe and tach oscillation would be great!

 

Marked the drive shaft? Why did you bother doing that. Its individually balanced.

I'd check wheel bearings next.

Beyond that... I don't have anything to help with.

 

Marking the driveshaft is just one of those things I have done since working on Chinook helicopters. We marked all rotating components so that if you didn't have a problem before reasembly you will not have one after. When combining several rotating components, you can affect dynamic balancing alot buy shifting the rotaing mass out of phase with everthing else. Yes, probably it doesn't mean a thing, but I am trying to eliminate possoble causes. Would the wheel bearings vibe that bad all of a sudden? What would cause the bearings to fail that suddenly? I might swap in a different hub, just to see what it does and eliminate the bearings if they are not the issue.

 

eh, your symptoms aren't exactly what mine were, but have you checked the u-joints on your driveshaft? i'd imagine the rear one could have suffered some damage.

 

I can understand if components have some balance reliance on each other. But ours is an individually balanced system. Just my .02.

I'd just jack the car up and give each rear wheel a good top/bottom/side to side wiggle and check for play. They can happen slowly or over time, sometimes you won't know its bad until its reallllly bad.

 

+1 on checking the U-joints. One of them could have gotten tweaked in the impact.

 

I checked the hub after everything was installed and there is no play whatsover in the bearings at the hub side.
I am going to try to stick in the driveshaft I have for my 1.8 upgrade in my 91 to see if that solves the problem. If it doesn't, I guess I will have to drop that diff and swap in the torsen that was also intended for my 91. I just hope the diff itself didn't get hurt some how. I just can't see what would have damaged the mechanical parts of the diff though.

 

I have this exact same problem after a Torsen swap. I don't have a clue what it is.

 

Swapped in a driveshaft from my 96 M-donor and all the problems went away. When moving the U-Joint around on the bad driveshaft I could clearly notice some "notchiness" on the one axis. I am glad it ended up this simple. It must have been due to the unusual loading on the ujoint while I drove the car with the diff hanging all crooked. Now I just need to find a cheap driveshaft to replace the one I was going to use for my torsen swap in the 91.
Thanks for the help

 

It seemed like driveshaft. my shakes were worse, but I had "2-axis notchiness"

 

I was under my car the other day and was spinning my driveshaft by hand. It seems to not spin true, has anyone else noticed this, or is my driveshaft just completely fucked? I'm looking for another driveshaft to see if my vibrations go away.

 

I would try the driveshaft first. The funny thing is I could swear the noise was coming from the passenger rear tire area, which made me think it could not be the driveshaft. I guess listening over my shoulder makes it harder to determine where the noise is coming from.
Does anyone have an idea why or how a bad u-joint vibe would cause my tach to bounce? At first I thought it was because I might have forgotten to tighten the ground wire on the PPF but it was attached properly.

 

I think its just from the driveshaft shaking the car so damn much. That thing is spinning pretty damn fast when it vibrates for me, at above 65 mph.

 

Found my problem. Switched driveshafts with a friend and the vibration was still there. Turned out one of my bolts was a different size/lighter than the rest. Used one of my friends bolts that were the same size and the vibration disappeared.

 

That will do it every time!