Driveline vibration...stumped

[Broncojunkie]

In my 79 with T5 and 31-spline 8.8, I have a driveline vibration I can't figure out. It starts getting noticeable around 55 mph and gets progressively worse as speed increases. If you've ever experienced a driveshaft out-of-balance, that's what it feels like. It's a much higher frequency than a wheel balance issue would feel like.
Rear end was rebuilt by myself and gear was changed to 3.73. I had a few driveshafts lying around so I tried a couple of them, with no change in vibration. I took 2 of these shafts to the only driveline shop within several hours. It's an hour away and I have to take time off work to make the trip... on 2 separate days (one to drop off and another day to pick up). Not convenient at all, since the shop is actually about a 1.5 hr drive from where I work.
I suspect the driveline shop didn't do a good job balancing the shafts. U-joints, T5 tailhousing, slip yoke, etc all new and tight. No slop anywhere. I opened the 8.8 and fluid looks good. Bearings all good. Pinion doesn't show signs of play, but there looks to be a little play in the spider gears. My experience tells me this is definitely a driveshaft balance issue, just being more obvious after going from 2.73 to 3.73. Am I overlooking something or should I follow my gut instinct?

[Broncojunkie]

Well, I ended up putting the rear diff back together this evening. New fluid with friction modifier, of course. Reinstalled one of the "balanced" drive shafts. While working under the car,I decided to give it another good once-over. Stifflers crossmember and oem-style transmission mount were good. No loose bolts or play anywhere. However, I found that the T5 was still sporting the damper/weight bolted to the tail housing. The 2 bolts were tight, but maybe it was missing some washers or grommets of some sort because it was very loose. I went ahead and removed it. Will take it for a spin as soon as I get time and the roads are dry. I suppose there's a chance that could cause the issue I'm having.

[KevinK]

Try rotating the driveshaft 90 degrees. Ford usually had a yellow dot on the pinion and the driveshaft flange.

[Broncojunkie]

Good point. I knew about this but I don't think it crossed my mind this time. In the past, I never had any luck rotating the driveshaft. I'll definitely try if vibration is still there.

[wraithracing]

You don't specify what your specific setup is, but I would also check your driveline angles. If you have swapped in dual hump transmission crossmember it's possible that the angle is different than originally and/or the angle of the 8.8 compared to the original 6.75/7.5 rear end that was stock. I have also had aftermarket transmission mounts throw the driveline angles off, so don't discount that either. This could very easily explain your vibration at higher speeds. Good Luck!

[Matt_83_GT]

Trey might be onto something with the transmission mount. Which Stiffler's crossmember are you using? I picked up STF-TCBM0 (from LMR) for mine and there's a note in the Vehicle Fitment portion that states

• This crossmember requires the use of SVE's heavy-duty transmission mount

You said the transmission mount was OEM-style so not sure if the mount might be throwing things off a little.

[Broncojunkie]

The mount is an oem style aftermarket and is basically new. The Stifflers crossmember part# is ZTCB-M06 and I'm now seeing the note about the SVE Transmission mount. I don't recall seeing this when I ordered it a few years ago. In the main description on LMR's site, it says "Works with OEM and aftermarket transmission mounts". So maybe the note about using the SVE mount was added later? If you look up the same crossmember on Summit Racing, there's no mention of using an aftermarket mount.

As for driveline angles, I can check a buddy's car with similar setup, but using factory double-hump crossmember and no vibrations. I know the setup because it was my car and I fixed the same type of issue by having the driveshaft balanced lol! For that matter, I will probably just borrow his driveshaft and use it to test on my car.

[Matt_83_GT]

Good luck! Keep us posted with how you make out.

[gr79]

First thing i do is swap out tires. Never know.
Loose damper? May try to reinstall.
Was going to say clutch/flywheel but that is deep work.
Agree higher freq is clue. Does it happen in gear?
Manual says temp install and play 'index the hose clamps on d/s' to check balance.
Have had balance weights come off.
Took d/s in for bal, new u-joints too once. Same feeling about the work. Next time going somewhere else.
When working, time is sure at a premium. Still think how the heck did i do so much in the past?
Would be heaven to live nearer to multiple solid shop choice options and ride shuttles back and forth if dropping off.

[Broncojunkie]

I emailed and received the following response from LMR regarding the mount:
"Stifflers designed thier crossmembers around the SVE bushing. The OEM technically will be the same spec and dimensions, but you have rubber vs urethane construction."
It seems to me that, if anything, the oem rubber one might do a little better job of absorbing vibration. Either way, I don't think the mount is my issue. I'm taking the car to the first Cars & Coffee event this Saturday. It's just a couple miles away and no highway speeds involved, so no worries there. But that means it may be next week before I can swap in my buddy's driveshaft.

[Broncojunkie]

First thing i do is swap out tires. Never know.
Loose damper? May try to reinstall.
Was going to say clutch/flywheel but that is deep work.
Agree higher freq is clue. Does it happen in gear?
Manual says temp install and play 'index the hose clamps on d/s' to check balance.
Have had balance weights come off.
Took d/s in for bal, new u-joints too once. Same feeling about the work. Next time going somewhere else.
When working, time is sure at a premium. Still think how the heck did i do so much in the past?
Would be heaven to live nearer to multiple solid shop choice options and ride shuttles back and forth if dropping off.

If I had a 2-post lift, I would try that hose-clamp trick. I'm going to take rear wheels off today and have them balanced. I'm fairly certain they aren't the cause, but I suppose it's possible that both are out of balance causing it to feel like a faster vibration.
The vibration happens in or out of gear and doesn't follow engine speed. I can go down the road at 70mph, shift into neutral, and coast at engine idle and there's no change in vibration. I will say that the vibration either disappears or is reduced when accelerating.
And yes, I may also try reinstalling the damper, but then again, that would just be a bandaid I think.

[at_the_junkyard]

FWIW I have the Stifflers 79-81 crossmember mount on my 80 with T5 and 8.8 and Ford racing driveshaft. I believe that the instructions recommend adding the two supplied shims to the poly aftermarket mounts (such as Energy suspension) but no shims if using the original OEM mount.

[Broncojunkie]

I got the tires rebalanced. As I suspected, no change in vibration. I'm 99.9% sure it's just the driveshaft.

[Broncojunkie]

Oh... and I got home early yesterday (Friday). Just enough time to load up a driveshaft and take it back to the shop that "balanced" them. Luckily, I thought to call first. They're evidently closed on Fridays. I absolutely refuse to waste another vacation day dealing with them.

[machinedoc]

I feel your pain in trying to diagnose a vibration. Had a similar situation on a friends 91 Coupe.
Shop changed u joints and installed a 3;55 gear.
Tried clocking the driveshaft, still had vibration. Changed to another driveshaft , still had vibration.
What I found was, when gear was installed , carrier was set to zero end play, the shop did not add any preload to each carrier bearing. After I gave each bearing a preload of .006" vibration went away.
Good Luck

[scenarioL113]

Did you install new engine mounts?

You did say "tailshaft" so I assume you meant the tailshaft bushing?

How about the input bearing retainer? They go bad bc they are aluminum. Have you been able to look at it? Does the throw out bearing make any noise at all?