1985 W123

I have a bad vibration, almost like a strong hum, over 45MPH. Above 60MPH it gets a bit scary. After trying the more obvious things, I have now removed the driveshaft. I'd like to run the car in gear to see if the hum is there without the driveshaft in place so I know if it could be originating from the transmission. Is it safe to do this? The car is on jack stands in the back and on ramps in the front.

A bit of history:

• bought new tires and had them balanced. Vibration still there
• ran car in gear with back end up. Vibration still there.
• on highway put car in neutral. Vibration still there.

I think this eliminates anything in the engine, front end, or tire-related.

Next planned steps:

• Center bearing carrier is all ripped up so I'll replace that and the bearing.
• getting the driveshaft balanced.
• replacing rear transmission mount
• replacing the flex discs

A couple of years ago the transmission failed so I had a used one swapped in. The car hasn't been driven much since but in the little I drove it at that point I don't recall this hum/vibration. Whatever the case, I'd now like to return this vehicle back to being the comfortable, reliable daily driver in was previously.

Any further thoughts or advice is welcome.

John

When the transmission was changed was the matching of the two driveshaft components done properly? If not there is probably a balance issue..Some had existing marks and others not.

Sounds like a good way to find the source of the issue. People run engine on test stands all the time, I don't see a problem with running it without a drive shaft for a little bit.

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1983 300TD
1985 300D
1993 190E 2.6

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If you can get the car on a lift you can get underneath with a stethoscope while a helper runs it up to speed...

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That's tough, you just have to trial and error if there are no alignment marks. Both my 84 and 85 have those but they can be overlooked easily for someone who is inexperienced or old and forgetful like me.

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84 300SD
85 380SE
83 528e
95 318ic

There were alignment marks on the driveshaft and it looks like that was done correctly. I ran the car without the driveshaft but in gear and did not feel the vibration. Monday I bring the driveshaft in for balance then I will reassemble. I'll post here again with my progress or final resolution.

I'm finding that axial nut on the shaft a little confusing. Loosening it seemed to elongate the shaft. It makes sense to me though that after reinstalling the shaft I'd tighten it to make the shaft as short as possible to reduce any play.

When you pull the driveshaft, check the U-joint for binding. I've heard stories of cars eating center bearing rubber supports and having vibrations around 45, and it ends up being a binding u-joint.

I've been meaning to check mine, I've always had a slight rythmic vibration accompanied by a hum in both of my w123s that is tied to vehicle speed rather than engine speed, which rules out engine, flex plate/flywheel (one car is 4spd swapped) and a lot of the trans shafts and stuff, and tends to lean more towards driveshaft, diff and axleshafts/CVs

I've also heard what sounds like a terribly unsafe way to balance a driveshaft that involves chalk, and jacking the car up, having a helper run the car up to where it vibrates, then touch the chalk to the driveshaft, and because of the rubber mount in the center bearing, the driveshaft will deflect slightly, and wherever the chalk mark is is the heavier side, so you'd take a hose clamp and put the screw clamp part opposite the chalk mark and retest until the vibration went away.

I haven't been daring enough to try that, and I would highly recommend not doing that on jack stands or any situation where you'd be putting life at risk.

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion

I too had a worn center mount that caused slight vibration , rebuilding the driveshaft and having it balanced fixed it .

Most Heavy Duty truck shops have a good, affordable drive shaft rebuilder on speed dial, go ask one .

Did you have the vibration from the day the used tranny was installed ? .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

The transmission won't lose fluid if you run it without the driveshaft, as the driveshaft bolts to a fixed flange on the transmission output and that shaft & seal stays on the transmission. You might be thinking of 1960-80's U.S. RWD cars which had a sliding yoke output which was part of the driveshaft, so nothing held in the oil once it was removed. Interestingly, my 1965 Dodge Dart has a fixed flange, similar to my M-B, which the driveshaft with a special "ball & trunnion" "Detroit" joint bolts to, so it won't lose fluid either with the driveshaft removed.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I said I'd report back when I got the the work done. I ended up doing everything I mentioned:

• got driveshaft balanced
• replaced center bearing and carrier
• replaced flex discs
• replaced rear transmission mount

I'm happy to say that fixed the bad vibrations I was having at highway speeds!

I did a lot of other work while I was at it (valve adjustment, oil and transmission service, etc.) and there's still a lot more to do to restore it to any of its former glory but I'm on my way and driving it again. Thanks to everybody for the help, advice, encouragement.

John

Excellent. Just curious, any idea what was the main problem causing the vibration?

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1986 300SDL, 362K
1984 300D, 138K

Good to hear this ~ most of the issues you'll face are pretty simple once you get into working on them .

Simple, well engineered and TOP QUALITY build .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

The carrier bearing mount on the driveshaft was completely ripped up and the mechanic who balanced the driveshaft told me it was pretty out of balance so it seems those 2 things would have caused the vibration. The mechanic suggested that the mount being ripped up had caused the driveshaft to go out of balance. I get that the mount might start to rip out of age but the part about that throwing the driveshaft out of balance doesn't make sense to me. I suspect it more likely that the splines were not lined up on the driveshaft when they swapped out the transmission awhile ago.

The flex discs looked ok but I found one of them was missing a bolt several month ago. I'm guessing that was another overlooked error during the transmission swap. I replaced the bolt at the time but I wonder if that also contributed --- maybe causing the unbalanced driveshaft to swing around even more than it might have, leading the aging rubber carrier bearing mount to get ripped.

There's a lot more to fix on this beast but at least I have something fun to drive that can now handle the highway.

Very nice to get rid of the toothache problems, the other stuff is easier to get to in time. On my W123, I acquired it 500 miles from home with about 1500 miles ahead to finish the round trip. This after my SDL blew a radiator.

It was dicey in spots - had a lot of rust in the tank and turned out the vacuum line from intake to alda had fallen off. I made it somehow - kept swapping and cleaning inline filters - and when I took care of both problems at home ... what a difference. The rest is annoying but bearable.

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1986 300SDL, 362K
1984 300D, 138K