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Old 02-04-2004, 10:16 PM
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Question Intermittent Vibration above 75MPH

I've been doing a lot of highway driving lately with my '85 300D and when I get up to and over 75MPH the car starts to vibrate kind of hard but not in any constant way. It comes and goes with no real predictable frequency. Below that speed its smooth as glass. I don't think that it could be the tires since I would think they would start vibrating way earlier and with a predictable frequency. I know that its not the road surfaces either. I don't think everyone pushes their W123 that fast that often but I would think these cars are capable of going a bit faster without any problems.
Any ideas?

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84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012
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Old 02-05-2004, 08:15 AM
LarryBible
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To start the troubleshooting process, try to ascertain whether the vibration is at tire/wheel speed or engine/driveshaft speed. What do you think?

Good luck,
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Old 02-05-2004, 08:43 AM
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Larry's right, determine if it is driveline, wheel or engine source vibration.

Also see here:
http://www.peachparts.com/shopforum/diesel-discussion/61086-rear-wheel-drive-rumble.html#post559254

After replacing the driveshaft and driving for a few thousand miles, I believe that I also have a bad rear motor mount. At certain speeds, undulations in the road surface that significantly compress the suspension, will cause a 'driveshaft speed' vibration to momentarily appear. It's hard to tell but it really feels like it is eminating from the shaft tunnel area of the car. Since the driveshaft, flex disks and support bearing assy are all new, I'm guessing that when the suspension is compressed, so is the back motor mount...probably enough to allow the tranny tailpiece to touch the body somewhere.

I have a rear mount in inventory...it will get changed in the near future.
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Old 02-05-2004, 07:17 PM
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Larry, It seems to be engine/driveshaft speed related more than tire/wheel speed influenced. R Leo, your suggestion makes sense about since I think it does appear sometimes after the suspension goes into any kind of undulating movement but I don't think there is any transmission to body contact though. Maybe I'll just have to keep my speed topped out at 70 MPH. It seems to ride fairly smooth at that speed.
Thanks!
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Old 02-06-2004, 09:14 AM
LarryBible
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Yes, the transmission would not move enough to make contact with the body, but it could easily be moving enough to change the driveshaft angle enough to cause a vibration.

What happens with the u-joint is that it changes angle only a microscopic amount. This causes the u-joint to wear in a very close area. If you were to take out the driveshaft and flex the u-joint, you very likely would find each axis of the u-joint stiff with the driveshaft straight, but smooth once flexed beyond that point. By changing the trans mount, you may very well put the driveshaft into an angle that will not vibrate even when hitting a bump since the new mount would not flex downwards near as much.

If the trans mount does not cure the problem, you may need to put a few washers between one side of the carrier bearing mount and the body to put the u-joint in a slightly different area, away from the worn spot.

Good luck,
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Old 02-06-2004, 10:50 AM
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Absolutely right, the transmission can't really move so far that it would intermittently contact the body (at least not without some really wild noises and things) ....I was thinking that maybe a mealy rear mount could compress sufficiently enough to allow the metal parts of the mount to come in contact with one another.

The FSM goes into excruciating detail about driveline aligment so Larry's misalignment scenario is more plausable.

FWIW-My old driveshaft had 'harmonic' speeds where it would really get to vibrating. Of course, the 65-72 mph range was the worst.

It also had quite a bit of the u-joint wear that Larry describes. I wish I thought of/known about the support washer trick before I replaced that shaft...it would be interesting to have found out if that solved anything for me.

I realize now that in the course of my troubleshooting and repairs I was (and, am still to some degree) actually dealing with a multitude of different problems — bad center support rubber, worn u-joint and now motor mounts.

My experience is that fixing one issue will unmask the others in the system.
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Old 02-06-2004, 05:07 PM
LarryBible
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I too wish that I had tried the washer trick before pulling down the exhaust system multiple times and replacing stuff in an effort to fix it. I really suspect that if I had put in the couple of washers at the get go, the vibration would have disappeared.

Have a great day,
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Old 02-06-2004, 07:05 PM
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Larry,
Thanks a lot for the advice on the mount. At this point I have a couple of more serious items that need attention before I will go and buy a new rear mount for it. I'll just keep my highway speed at 70 or below for now. It'll get me better fuel economy as well as those higher speeds seem to really affect it. That little motor sure sounds wound up at over 70 MPH. I'd be curious to know how much but my tach circuit has a bad bug in it but that's a whole other issue.
Have a great weekend!

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