Driveline Vibration
 

As the title says, I'm trying to track down a source of driveline vibration in my '86 SDL.

Vibration picks up ~57MPH and is worst ~60MPH. Largely goes away above 65MPH and turns into an audible "grumble" but can no longer be felt.

It isn't a tire issue, tires are brand new and I've had the balance checked.

Not worn shocks, they're brand new Bilstein HD's. Problem was present before and after the shocks were replaced, no change.

Latest service was flex discs and center bearing. Flex discs were cracked and ROCK HARD, bearing carrier had decomposed, bearing was rough. Replaced everything, made sure the DS was aligned. No change, but the really rough gear engagement is now gone.

Checked the U-joint while I had the DS out. Was nice and tight, didn't have any noticeable ratcheting or notchiness to it.

Car is smooth as silk up until that vibration kicks in. It comes on very suddenly when you get to the right speed.

Where should I be looking? I'd suspect if the DS had an issue (bent?) that I'd have vibration throughout the driving range. Diff issues? Halfshafts? Something else?

Transmission mounts or motor mounts??

Forgot to mention, transmission and motor mounts were replaced this past summer. Both Lemforder parts.

The diff mounts are pretty crusty though. I wouldn't expect the diff to be vibrating bad enough to vibrate the car though, unless the misalignment due to the sad mounts can contribute to that? I freely admit I am not a driveline expert!

Change the diff mount, if anything just to eliminate it. It is easy to do and fairly inexpensive.

is the driveshaft correctly oriented and was lubed at its slip joint in the middle?

Slip joint was lubricated when reassembled. I did the DS R&R procedure according to the FSM. To be clear, the vibration was present in same amplitude and speed range before touching the DS.

Diff mounts are pretty sad. Picture attached. I can see how that would transmit vibration to the cabin, but my concern is why the vibration is present in the first place.

Vibration seems to be too fast to be tires or wheels. I ran across this thread this morning: http://www.peachparts.com/shopforum/diesel-discussion/345894-w126-periodic-vibration-source-solved.html and it has me second guessing my diagnosis on the DS U-Joint. My U-Joint isn't ratchety, but it isn't silky smooth either. My vibration is also in the same range as his on the same car. Hmm...

Diff mounts definitely need replaced, they're pretty sad, but I'm not sold that it's the source of vibration, merely just transmitting it to the cabin.

U-joint, U-joint, U-joint. Replace & rebalance. Same thing with my sdl.

Replaced all the same you have. Could not detect the u-joint "notch" while in hand. Finally took the shaft in for u-joint replacement and shaft balance.

No more 55 to 65 mph vibration.


P.S there is also a drive shaft "end bushing" that should be replaced.

had a similar problem last year. turned out to be a bad wheel bearing.

have you checked yours?

I also suspected prop shaft, center bearing, flex discs... tried preloading the drive shaft carrier bearing to no avail.

replacement of the front wheel bearings took care of my problem

a notchy u joint can cause such vibration. If it had a notchy feel in the center position it might just be a toasted U joint.

They are greased at the factory and there is no way to regrease them.

sorry, re-read your post ruling out bearings/wheels

One way to discern engine & driveline from suspension is to put in N and coast w/ engine idling. If vib's go away, it was surely from the driveline. If not, it might still be from an imbalanced driveshaft since that is still spinning, though less likely from U-joints since they are affected by torque too.

Perhaps not directly related, but I had similar in my 69 Dodge Dart long ago. It would start a humming vibration right at ~60 mph and go away above that. I found the rear U-joint (cross type) was totally rusted w/ the needle bearings missing so it just wobbled around. Replaced it and no more humming, plus no more clunk when I shifted between D and R. Must have been driving that way for years since I long wondered what that clunk was.

With my battle with driveshaft vibration, I found that there were a couple of critical items. Orientation of the hardware and the driveshaft balance disc were critical. A study of the FSM is very important.

:eek:

it is critically important.

I'm leaning more and more towards the U-joint in the driveshaft being the source of vibration.

The DS is oriented correctly, and the factory alignment marks are lined up.

The vibration is not in the front end, it's definitely something you feel in the seat. Wheel is perfectly smooth and free of any vibration.

I strongly doubt a wheel bearing issue. This is not the typical "drone" that accompanies a failed wheel bearing, and the vibration is a slower speed than that. I've had a vehicle with a failed wheel bearing and this is definitely different.

The only thing that seems to matter is road speed. If the transmission is in neutral or in gear at the speeds given above there is vibration. Whether accelerating or coasting, there's vibration. Doesn't matter if you're turning or not, it never changes. Solely fixed to the road speed.

The diff turns smooth and makes no apparent noise, no gear whine or "howl" like when they usually go bad.

About the only thing really left is the U-joint in the driveshaft.

I have a plan to put the car up on jackstands and have a helper give it some gas in gear and watch to see what it is that's vibrating. I'd expect to be able to see some sort of runout, or at least try to locate where the vibration is originating!

It's more than just the alignment marks.
I'm talking about the vibration dampener and flex discs.

https://www.startekinfo.com/StarTek/outside/12253/program/Chassis/41-050.pdf
https://www.startekinfo.com/StarTek/outside/12253/program/Chassis/41-100.pdf
https://www.startekinfo.com/StarTek/outside/12253/program/Chassis/41-200.pdf