Fox Drive line vibration at specific speed

[FastDriver]

To cure mine I:
– Balanced my wheels/tires & set my alignment
– Checked my suspension bushings, transmission cross-member bushings, the trans mount, and engine mounts
– replaced front & rear u-joints
– set pinion angle parallel parallel to the transmission/engine
– replaced axle bearings
– ensured my pinion flange nut wasn’t loose
- took D/S to transmissions shop. Balance was fine, but D/S was not true. They used a heating and cooling technique to true it. However, they could only get it to .008" & there is still a noticeable vibration. This is/was one of the factors that eventually drove me to swap in a T56 which will end up replacing trans, both yokes, both U-joins, and the driveshaft. *fingers are crossed*

Putting it on a lift and running the drivetrain is a technique I've used... I've even crawled under the car a bit before deciding that doing so just wasn't worth the risk of some garment getting caught up in a rotating part and getting myself mangled. Plus, when the suspension is unloaded all of the angles change and bushings are unloaded. So, while it might work, it might not reveal anything.

 

[manicmechanic007]

Just put the car (rear axle) on jackstands, the diagnosis will be easier and the u-joints will like you more
Ran them up for years on a hoist with no issues but be safe on the ground use your best judgement

 

[93CalypsoConvert]

I had my u-joints replaced and drive shaft balanced. Still vibrates, but now it oscillates. So above 70 I get a short wave of vibration about once a second. Weird.

 

[AeroCoupe]

I fought a cyclic vibration like a washing machine on my T-Bird for a couple of years. Did just about everything I could to get rid of it. Dynamic wheel balancing, rebuilt the rear end and even changed the gears, got the steel DS balanced, had a custom aluminum DS built, sold the Lakewood bell housing and installed a SROD bell housing, tore the TKO down and checked the main shaft for runout, replaced the tail housing bushing, etc.

Finally got ahold of Joe Dederichs of Dederichs Motorsports and walked him through everything I did and he asked if I had checked the input shaft runout on the TKO…nope. Was waaay out and took two shim packs to get the right combination to get the end play down to 0.00” with the input shaft bearing retainer installed with no sealer. Reinstalled and vibration was gone.

After all that BS I was so glad to have it fixed as I was ready to blow the car up.

 

[FastDriver]

Mine turned out to be the pinion bearing.

 

[93CalypsoConvert]

Input shaft speed has nothing to do with it. My slip yoke bushing has very tiny amount of movement if I really push on it.

I'll have to inspect if I have any pinion play tomorrow.

 

[AeroCoupe]

Well all I can tell you is that is what solved my vibration and I chased it for two years. Good luck and I hope you find it as it was absolutely one of the most frustrating things I’ve had to deal with car wise.

 

[FastDriver]

Maybe you're more adept, but the only thing I saw at the pinion was that the seal was leaking. The end play did not seem bad to me, but I didn't have any special tools I was using to measure or anything.

I would expect that you could eliminate the clutch, & input side of the trans by going into neutral, letting the clutch pedal up, and shutting the car off on the highway. If the vibration disappears, then you've narrowed the vibration to the front end of the drivetrain. If it doesn't, then you've eliminated the front end from the list of culprits, assuming of course that you only have 1 vibration source.