Ken,

I'm not sure I understand 100% of what you are saying. Here are some points to consider:

1. MB driveshafts RARELY fail. i.e. Mechanic replaced 2-3 BMW driveshafts a month and 2-3 MB driveshafts in 10 YEARS.

2. The center support bearings DO fail and can cause rumbling, howling, or very rapid vibration on hard acceleration, some may mistake it for the trans. In some cases the center bearing rubber separates into two pieces - but I've only seen that myself on OLD cars 20+ years old & with original part.

3. Flex discs can cause ALL sorts of vibrations. If one bolt is loose, broken, etc.

4. If the driveshaft is NOT supported in the center then I would think it WOULD flop around - possibly on both sides. To turn it without any support is not a valid test, IMHO.

5. Have you tried the PGA shaft yet? If not, I would. If it came with center support bearing and flex discs - even better. Try again and see how it goes. Note how the flex discs are removed and see if you see anything out of place or questionable.

HTH,

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Brian Toscano

I'm sure some pictures will help explain what I'm seeing - obviously. With a ruler so you can see the off-true situation as I turn it through a full turn.

The flex disk material is pretty hard rubber. The front section of driveshaft still on the car might weigh 5 pounds. It doesn't really flop around, it maintains its position as it turns around - and that position is not true to the axis of rotation.

Too tired to get under there tonight - I was putting the finishing paint coats on the replacement differential and driveshaft rust spots.

It really is hard to explain without a picture, so I understand the frustration you have in trying to help.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

Make sure you have the bolts in right way round -- they go one way for the tranny flange and the other for the driveshaft.

Check the centering sleeve -- if it shows signs or wear at all, replace it, it just presses it. If it is worn or loose, the shaft will be off center, the bolts won't hold the flex disk centered unless the sleeve is good.

Bad part to this story is that you also need to find a shop that can verify that the shaft is straight and can balance it for you -- the center u-joint isn't self centering, and if you replaced it, the shaft is almost certainly out of balance. It 's also possible the front flange is bent and the flex disk is fine. I have no idea how you could have bend the front shaft, but it sounds bent to me!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

This took a while, but I'm understanding you now. The front 1/2 of the shaft is behaving as if it is bent. Since you are so close, try a different flex disk. While you have it apart, check that shaft to make sure that it isn't bent. I've never had one of these apart, but can you stand it up on the three mount points? If so, measure the distance at the other end from a fixed position, say the wall, rotate the shaft 180 degrees and measure again. If it's an inch off like you say, that should be easy to see.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References

Ken,

I went back to read the whole thread again, and have some questions to clarify my understanding of what has happened so far.

When you first started the car with the rear driveshaft section off, you noted there was no vibration at idle. It was not until you got going pretty good that you noticed the added noise and vibration. If you run the car now with the driveshaft in the same configuration, is there a noise or vibration at idle, or do you need to see 25 mph or so on the speedometer to hear noise?

When you visually inspect the transmission stub shaft, the flex disc, and the short forward driveshaft section now you note it is visually out of alignment - was this apparent when you first disassembled the unit? How about when you put it together the first time (if you had a misalignment of an inch or so, you probably would remember having to align the connection hardware when you put the system together)?

I am not familiar with the flex disc design, guess I am lucky there and have not had to get intimate with them yet, and am curious about their basic design. Is the flexible member rubber or metal? Or metal bonded to rubber?

I would be inclined to put a dial indicator on the various flanges and shaft areas to try to find out what is misaligned. I would even take the flex disc off the present assembly, then check the parts. I would be surprised if something with a flexible coupling member could bend a rigid shaft section without damaging the flexible element. Just seems like the shaft parts, made of pretty stout steel, will be less likely to become deformed than the flexible element. Running it at a good speed with the end cantilevered and unsupported may have contributed to the magnitude of the present issue, but I would assume the consequences were confined to the flex disc.

Good luck, Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

Jim, I can answer what the flex disc is made of. Had a MB burn up and I hired a Fire Investigator to find out why.
Turned out the flex disc was defective and it shifted around between the two plates until it got hot with the friction and began to burn.
We took out the flex disc and it half of it was burned away. It is composed of a hard rubber compound. much like a tire, with steel cords wrapped around inside.

Flex disk was defective - from the manufacturer.

I knew the problem isolation was getting closer when I saw that front driveshaft section rotating off-true. To me the most likely thing was an assembly error on my part. Particularly not properly seating one of those flex disk cylindrical metal inserts into the insets in a flange.

It took some additional disassembly and testing. I mounted the just-purchased junkyard driveshaft front section on the differential flange (its the same as the transmission flange). Rotated true to axis. Mounted the old driveshaft front section to the junkyard differential just purchased, with a MB flex disk that came with the junkyard driveshaft. Rotated true to axis. This meant the driveshaft section was itself true.

Took off the front (new) flex disk from the transmission, and assembled it with the junkyard differential and the old driveshaft front section. Rotated OFF true. By process of elimination - that new flex disk was defective.

First theory of defection failed. All six cylindrical metal inserts measured nearly the same with digital calipers.

Second theory of defection - warped disk. Have not figured out an accurate way to test this without assembly to other parts. I would agree that the flex disk material closely resembles very hard tire material. Fairly hard rubber with cords inside.

You know how sometimes you get a defective tire new from the manufacturer? I got a defective flex disk. The other one on the differential was purchased at the same time and was OK.

Got it all back together and its smooth running now. Used the "like new" MB flex disk that came with the junkyard driveshaft.

Now I get to get under there again for the differential replacement. Didn't want to throw that replacement operation into the mix of variables before fixing the driveshaft vibration.

So, my advice reflects that of others to some extent:

1) Better to find a good junkyard driveshaft than try to replace the U-joint. If it should even go bad. I just thought mine was bad because it seemed the only thing left.

2) You CAN get a defective flex disk new from the manufacturer, so it seems to me at this point.

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Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles

If that was mine, I would reinstall the rear driveline, clamp a piece of something onto the car that can almost touch the front portion of the driveline (on the L of R side so you can see the gap between it and the driveshaft), run the engine slowly in gear, and observe for any runout. It seems like if there was runout, it would have to be comming from that little piolet stub on the end of the shaft or the trans flange. I can't believe that MB intended for that flex disk to keep the driveshaft aligned to center. I have an 83 TDT that I of lately have replaced the center carrier bearing & support because it had some vibration. It didn't help the vibration, but I'm hoping that if I go back into it and replace the worn, u-joint (it has a sticky spot) maybe the vibration will stop. The Mercedes u-joint that dosen't have to bend much (5 deg. or less) tends to wear groves in the cross & race at that spot where the needle bearing has been running. They (mine) now have a sticky spot, and under power, may generate some vibration.
As for phasing the u-joint, I think that requires that there are two u-joints. When a u-joint is running at an angle, the velocity changes (it speeds up of slows sown) every 1/4 turn. As it speeds up and slows down, there's another u-joint at the other end of the shaft is doing the same thing, but the cycles of the second one are reversed so you end up with a "constant velocity"