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  #1  
Old 05-09-2013, 05:00 PM
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Location: Saratoga Springs, NY
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W123 Propeller shaft vibration tears up rubber mounts, replaced nearly everything

Hi All,

This is regarding my 1983 300D. Sorry for the long explanation below but this problem is detailed and has some subtle distinctions. I've read many posts about this and none has yet solved my problem.

Early last July my car suddenly developed a vibration in the driveshaft that is related to engine torque and specific speeds. Everything about the car had driven smooth and fine until I turned up an unusual road where there was both a sharp right turn and a steep incline at the same time. While climbing the (rather long) hill I reached 30mph and suddenly there was a violent shaking that eventually ripped the rubber mount that holds the propeller shaft bearing.

The character of this vibration is that it occurs only between 25-30mph, only under heavier torque or climbing any incline. I can let up on the pedal and accelerate slowly and prevent the vibration. I can accelerate moderately downhill and there is no vibration. It is definitely torque-related, and related to the angle of the car.
If I turn hard left, the vibration may not happen at all through 25-30 and beyond. If I turn hard right, it will be worse. This is reminiscent of the day this first occurred.

Over the past year I have ripped through TWO center bearing mounts. I have read many posts regarding similar vibrations and changed nearly everything that wears in my driveline. I'll start from the front of the car and note whether each part replaced seemed to be needed or not:

Engine mounts - needed
Transmission mount - maybe needed
Front flex disc - needed
Front propeller shaft centering bushing - not needed
Propeller shaft bearing - not needed
Propeller shaft bearing mount - ripped twice, I'm on my third
Rear flex disc - not needed
Rear propeller shaft centering bushing - needed
Differential mount - not needed
Both rear axles - 1 had a torn outer boot
Rear tires - eventually needed but not when the problem developed
Rear alloy wheels - I have a spare set for snow tires and the problem did not change with either set of wheels

The propeller shaft U-joint feels a little less smooth when bent to extreme angles but seems smooth at the minor angles needed when driving. I believe I've replaced every other relevant part. And yes, when the propeller shaft has been separated it was marked and reassembled in the correct alignment.

The problem developed so suddenly and has stayed so consistent that it isn't like a rubber part started to wear and is slowly deteriorating. Also I'd think if the shaft itself was out of balance it would shake regardless of the amount of torque being applied. The fact that shifting the car's position (via hard left or right turns) changes how it drives when straight... it's just plain confusing.

Does anyone have any other ideas for me? My plan of last resort is to put it in the dealer and ask them to run it on the lift to see if they can see anything.

Thanks as always for your time and your Borg collective brain!

Garrison

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'83 300D
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  #2  
Old 05-09-2013, 05:11 PM
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Location: Wilmington, NC by the Atlantic ocean
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You need to find the local dyno shop and have them run the car under load. While it might show up on an unloaded situation it'll probably need to be under load. Before you book dyno time make sure that their dyno has a PAU (power absorbtion unit - not all do) so they can apply a load at a steady state cruise. It sound like something is no longer concentric (bent shaft, etc.) but that's a guess without data.

Someone will have to look under there as the car is driven.

Another brainwave! How about mounting a camera (Go-Pro or the like) under the car and taking it for a drive until the problem occurs? The goal is to see what's happening AS it's happening. Then take the vid to your local driveshaft shop.

Good luck!

Dan
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  #3  
Old 05-09-2013, 05:15 PM
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Home video- awesome idea!

Or to save myself some time: Being that the only thing I haven't yet replaced is the propeller shaft itself... does anyone out there have advice on acquiring a new (e.g. parts car) shaft? I can get the prop shaft in and out in under an hour at this point!

Thanks
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'83 300D
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  #4  
Old 05-09-2013, 05:39 PM
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You can firstly see if the shafts are aligned up with their marks and I believe its safe to assume you are using a good quality bearing mount rubber.

The sleeve nut should be tightened to approx 30 lb/ft with about a tablespoon of axle grease on the shaft splines and about a teaspoon in the centering bushings. The shaft should be able to slide a bit in and out with some force.

If you do a google search (might have to even search via google.de) you will find some similar problems, this is usually experienced by ML owners too but their solution is to buy a MB part which is actually a spicer brand center bearing and support.

MB also provided extensive procedures to remove such vibration from the driveline for the sedans, it also included shimming the carrier bearing so that the shaft sits a bit straight and also include some weighted nuts for the D/shaft bolts.

There was a thread on these forums where a user experienced an out of round flex disc causing problems.
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  #5  
Old 05-09-2013, 06:11 PM
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I would suggest you jack up the car, remove the axles from the differential, and run the driveshaft by itself. I'll bet your vibration is gone.
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  #6  
Old 05-09-2013, 07:09 PM
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Does your car have a vibration dampner that mounts to the front flange and flex disc?

(The W126 series has it but I'm not sure if the W123 might have it as well.)

If so, ensure that the arrow on the dampner points to the nub on the driveshaft yoke.
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  #7  
Old 05-10-2013, 03:43 AM
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My take on it is the turning left or right thing is the main problem => motor mounts.

It reminds me of this thread where turning one way caused a weird effect

Front Suspension Noise (VIDEO)

It has the best on car video I've seen so it is worth looking at the thread for that.

I say replace the motor mounts the transmission mounts and the flex discs.

Look on your prop shaft for marks to see if they are aligned correctly or not.

When refitting the prop shaft roll the car backwards and forwards (on its wheels) to allow for the rubber parts to settle before you tighten everything up.

Oh yeah and when you have the front flex disc off check the output shaft on the transmission - there have been reports of the nut coming loose.
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I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



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  #8  
Old 05-10-2013, 12:30 PM
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THANKS for the feedback! Okay here are some answers to the points brought up so far:

STRETCH:
- Very much agree that the turning left/right is key to finding the problem. I have been thinking more about the rear suspension shifting rather than the motor mounts. Problem persisted before & after new motor & tranny mounts.
- I did NOT move the car back & forth after replacing flex discs - I will do this
- I did NOT put a teaspoon of axle grease inside the new centering bushings - I will do this
- I did check the nut on the flange coming out of the transmission per other threads and it seemed tight; forgot to mention this above
- Prop shaft alignment marks are definitely correct and even confirmed via the red painted marks on the spline inside

ZULFIQUAR:
- Flex discs were replaced one at a time since the rear was in good shape; problem persisted before & after each
- Prop shaft center is lubricated and slides easily as it should
- Vibration occurred before every new part listed and hasn't changed much, so an out-of-round flex disc or bad motor mount in place of one of the existing parts is unlikely

DAN STOKES:
- I *think* bent metal would vibrate at given speeds regardless of torque

SD BLUE:
- No, the W123 doesn't appear to have the vibration damper though I had heard of them via other posts

Also forgot to mention that temperature affects the problem. On very cold winter mornings (<20 F) there is little or no vibration; after some amount of driving it appears. By this I became convinced that the offending part is made of rubber - or something like the grease in the CV joints - but as you can see, I think I replaced them all.

So my next steps will be to take out the shaft once more, lubricate the centering bushings, keep the flex discs nuts loose until I move the car back & forth a little. I would also like to try to track down one of those beefy center bearing supports as Zulfiquar mentioned.
At the same time I think pulling the prop shaft and having its balance checked by the dealer would be a good idea. The very sudden appearance of the problem makes me think something fell off the car. But every indication has pointed to a rubber component or something else affected by heat, torque, and the position of the car.

Does it sound like I'm thinking it through clearly? I'm very happy to entertain any more thoughts.

Thanks again for all the input so far.
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Old 05-10-2013, 12:36 PM
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Due to the fact that the u-joint virtually does not flex, it does all it's wear in the same position. They will often wear a small notch in that location, which will often cause a vibration at a specific speed and load.

A trick that has worked very well for me in the past is to loosen both carrier mount bolts on the carrier near the u-joint. Remove one of the bolts all the way and put a few washers between the carrier and the body, and then put the bolt back through. Then tighten both bolts.

This will cause the u-joint to ride in a position ever so slightly different from the wear point. It often will cure the vibration.

Hope this helps.
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Old 05-10-2013, 12:43 PM
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@air&road

your procedure is the exact one MB specify in their repair manual to straighten out the driveshaft for best angle.


@OP

does the body of the car buzz when you turn left to right, It may be caused by the engine contacting the body or something else from some point.
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  #11  
Old 05-10-2013, 01:01 PM
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@air&road
Sounds like a real good plan; less work than removing the whole shaft again for lube on centering bearings & rolling back & forth.
One note I would make is that when replacing the differential mount, the new mount makes the diff sit about 1cm higher than the original mount did. Not enough to make the sort of change you describe though.

@Zulfiqar
No vibrations on turns or anything like metal to metal contact. Turns and the rest of the driving experience all seem normal and just like my 240D.
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Old 05-10-2013, 03:23 PM
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Quote:
Originally Posted by Zulfiqar View Post
@air&road

your procedure is the exact one MB specify in their repair manual to straighten out the driveshaft for best angle.


@OP

does the body of the car buzz when you turn left to right, It may be caused by the engine contacting the body or something else from some point.

I got that trick from a dealer tech, but I had no idea that it was in a manual anywhere.
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  #13  
Old 05-10-2013, 04:03 PM
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Try to see if the factory balance weight is still present on the driveshaft. Take a dial gauge or some form of refferance. See if the driveshaft is turning in a centered position and that it is not bent. Ask a driveshaft shop or it may be in the manual of the allowable shaft run out.
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  #14  
Old 05-10-2013, 05:32 PM
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@barry12345
Are you saying you've seen the factory balance weight fall off the shaft before? That would explain quite a few things... even the "cold rubber" phenomenon could be the center bearing support and flex discs being stiff enough to counteract an imbalance while they are cold.
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  #15  
Old 05-10-2013, 05:39 PM
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...Though it doesn't speak much to the changes after left/right turns...

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