Driveline Vibrations?

[RonDwyer]

Has anyone got ideas about what to check for the source of an oscillating vibration that is worst at 55? I have replaced subframe bushings, transmission mount, engine mounts, and the differential carrier. Rebuilt driveshaft and new guibos. Tires test out under load at the tire store. Tire stores say its never the tires, mechanics all say its the tires. Do I take the plunge, take it back out and re-check the balance? I watched the shop balance it and it was as good as it gets. Maybe a stress-cracked driveshaft tube or it threw off a balance weight? Nothing looks amiss at this point. 1975 300D

Comments and suggestions welcomed!

[redghost]

Front or rear vibration? At speed approaching 55 or does it get better or worse? Last time fluids were changed? Are the tires in GREAT shape, or just good shape and may have a minor chunk missing from tread?

All issues I had that resulted in performance as if driveline had something to do with it.

[wpg300d]

How is the tranny? I chased a vibration on an 86 Chevy Suburban and spent a lot of money trying to fix the problem. Mechanics all had a different idea as to what was wrong. We changed tires, driveshaft, u-joints, some of the gears in the diff etc but nothing seemed to help. I don't think they were dishonest. I think they just were not good at diagnosing the problem. My auto tranny started to leak and when I got it rebuilt, most of the vibration went away as well.

My brother in law has a similar problem on an 86 GMC pickup. There is a vibration. He has changed tires, harmonic balancer, flexplate and torque converter etc but really has not improved it all that much. I suspect it too is in the tranny but since it works well enough he is letting it go. The truck is getting pretty rusty so he may retire it soon.

He is a licenced mechanic who works in a shop all day every day.

Randy

[Jim Hansz]

Have you checked the center drive shaft support?

"Rebuilt driveshaft and new guibos." And, just what the heck are "guibos"?

Did you have any work done just prior to encountering the vibration? If so, I would work backward from there to see if it could be the source.

I agree with the mechanics, it is usually the tires when it is a speed-specific vibration. If they were replaced, did the shop align the centering dots on the tires with the valve stem hole? These tire dots, a round ink mark, locate the eccentric extreme on the tire which should be aligned with the valve stem hole to allow the out-of-round differences in tire and wheel to offset each other. In some cases this mis-match can cause a speed vibration or oscillation.

Just a thought...

JCH

[pch2021]

a 55, my worst problem was always a tire issue. Uusally a bad tie rod will revela itself sooner.

[1976300d]

I have a similar issue with my 1976 300D. I have new tires, flex couplings, center carrier bearing and support bracket and yet there is a definitive "Thrum" as speed increases the "Thrum" diminishes. I have not yet tried to have the driveline dynamically balanced. I do not see irregularities in the driveline (obvious loss of balance weights) and my transmission dos not leak a drop. I have long term concerns that the "Thrum" may create issues as time passes. When I purchased the car the carrier bearing support rubber was degraded and mostly absent (nothing holding the bearing). The vibration was reduced with the introduction of the parts previously mentioned but not completely.

Any suggestions would be greatly appreciated.