Driveline vibration in low rpm - PeachParts Mercedes-Benz Forum

acb70 · #1 09-04-2015, 12:32 AM

Hello fellow benzes, my driveline makes a clunking sound as it hits the trans tunnel in low rpms, but is totally non existent at any other speed. This started after I replaced the trans mount, and there was a plate that went between the mount and the crossmember, does anyone know if this is factory? Maybe spacer to even out sagging old mount?

I'm hoping this is not related to center driveshaft mount problems, both flex-discs look pretty new.

mannys9130 · #2 09-04-2015, 12:44 AM

Are you sure it's the driveshaft hitting the tunnel? That's real bad. Have you gotten under it and just looked around for witness marks and overall alignment of the drive line?

Ceristimo · #3 09-04-2015, 03:19 AM

Are you sure it's not the U-joint that has play in it? Perhaps it makes an audible noise now because it's at a slightly different angle with the new transmission mount? I've experienced this before with worn CV joints, where swapping out to a 1 inch smaller size rim made the noise en vibration (mostly) disappear, since it made the axles sit at a slightly different angle.

gmog220d · #4 09-04-2015, 06:39 AM

X2 on the u-joint being bad. It can also feel "wobbly" before it strikes anything, as you accelerate.

acb70 · #5 09-04-2015, 09:40 AM

Quote:

Originally Posted by mannys9130

Are you sure it's the driveshaft hitting the tunnel? That's real bad. Have you gotten under it and just looked around for witness marks and overall alignment of the drive line?

Yes or something near it, the impact sound appears to be right under the center console. I looked under the car and I could not see any clear marks, but the clearance between the flex disc and the tunnel seems really tight, I am going to remove the spacer under the transmission mount to see if this gives the transmission more room.

Sorry BTW this is a w123.193 1985 300td

charmalu · #6 09-04-2015, 10:22 AM

If the DL it banging on the floor/tunnel, you have some serious problems.

Jack up the vehicle enough (safely and securely) so the back wheels are off the ground.
Slide under the vehicle and start turning the DL, wiggle it, push up and down, use a pry bar etc...
Also grab the DL on either side of the U-Joint and wiggle it to see if it does have any looseness.
Look at the Flex Disks real close for cracks etc....

I would not drive it till it was sorted out.

Charlie

sent from my pos computer

Stretch · #7 09-04-2015, 12:54 PM

I think you'd have a broken flex disc or a broken something else if the propshaft was actually making contact with the chassis - the MB design is not a "conventional" US car style configuration (live axle - horse cart springs etc) but if it were to do this and by some miracle stay in one piece you'd see marks where it hit on the underside of the car.

Check for the things mentioned above

An alternative might be the nut on the end of the transmission yoke coming off

leathermang · #8 09-04-2015, 02:19 PM

As Charlie said...
do not drive it until you get this figured out...
there are pictures in the archives where the floor board was BREACHED .... very scary.

Mxfrank · #9 09-04-2015, 03:01 PM

The Mercedes driveline is different than US or Japanese car drivelines. The engine and differential are mounted exactly in line, which means that there is really no need for U-joints. The only movement is flex due to motor, tranny and differential mounts compressing under load. This is absorbed by the flex disks. The purpose of the single u-joint seems to be to ease assembly. If everything else is in good shape and properly aligned, the u-joint will move at constant velocity, and there will be no wobble or vibration.

The whole thing hinges on the integrity of your mounts. I like to replace motor, transmission and differential mounts all at once, with parts from the same supplier. That way, I know everything is lined up correctly and the parts have similar compliance. If you have a misaligned driveline, then there will be a problem.

I've never seen a transmission mount with a plate under it, but that's just one anecdote. I don't know what's right for your car. If it creates a misalignment between the transmission and the differential, the u-joint will react to the mismatch. Unfortunately, a misaligned u-joint doesn't move at a constant speed, it accelerates for part of it's rotation, and decelerates for part. That's why you have two out of phase joints on a typical US driveshaft. Without the second U-joint, the vibration would be horrible.

All that said, I'd say remove the plate and see what happens.

acb70 · #10 09-04-2015, 05:34 PM

Thank you all for your input- OK- update- So after close inspection under the car, the drive shaft center support has disintegrated leaving the drive shaft with about 1/2" of play on either side inside the center support. Looks like I am in for a fairly long repair job, and I will be replacing the center support and bearing, and at least one flexdisc (however they are both reasonable shape, its just a "while you are there" thing for me.).

Thanks for all your safety input, however fun fact: I drove 1,200mi with a loose drive shaft center support. W123 durability.

I will update this thread hopefully with some photos of the job, its gonna be a &#($ but will be nice when its finished.

Ceristimo · #11 09-04-2015, 05:40 PM

Once the drive line is out, fixing the center support and bearing is not that hard. I had to rent a bearing puller to get the old bearing off, but didn't need special tools to get the new one on. Getting the drive line out was a b*tch, but all in all the job isn't hard. If the center bushings on your drive line look fine, then don't attempt to change them, I had a really bad time getting those out. Just add some grease and put it back if they look OK.

gmog220d · #12 09-04-2015, 05:59 PM

The u-joint in the '73's drive shaft went bad not too long ago. It was a long process that started with a noticeable wobble taking off in 1st gear. It ran great otherwise. Over time the wobble got worse, and finally reached the point the shaft started hitting something in the tunnel. It sure sounded bad, as if the chassis itself was getting hit. I thought the center support bushing was bad and letting the shaft wobble around. I also wondered about the guibos.

I pulled the assembly but the only thing the shaft appeared to be touching was the inner most shift lever. The rubber center support bushing and bearing were fine. The guibos are older but still seemed OK. The u-joint was toast, with considerable play in the bearings (rusty needles, pitted and worn surfaces).

I replaced the u-joint (modified drive shaft that takes a common joint) and it runs fine now.

leathermang · #13 09-04-2015, 06:29 PM

Quote:

Originally Posted by Mxfrank

The Mercedes driveline is different than US or Japanese car drivelines. The engine and differential are mounted exactly in line, which means that there is really no need for U-joints. The only movement is flex due to motor, tranny and differential mounts compressing under load. This is absorbed by the flex disks. The purpose of the single u-joint seems to be to ease assembly. If everything else is in good shape and properly aligned, the u-joint will move at constant velocity, and there will be no wobble or vibration.

The whole thing hinges on the integrity of your mounts. I like to replace motor, transmission and differential mounts all at once, with parts from the same supplier. That way, I know everything is lined up correctly and the parts have similar compliance. If you have a misaligned driveline, then there will be a problem.

..... Unfortunately, a misaligned u-joint doesn't move at a constant speed, it accelerates for part of it's rotation, and decelerates for part. That's why you have two out of phase joints on a typical US driveshaft. Without the second U-joint, the vibration would be horrible......

Ok... some little corrections..... I do not think an actual u joint can avoid that speed up and slow down in turning.... that is why they are not called ' constant velocity ' joints.... which are made very differently.
An actual U Joint NEEDS to be out of line... or the lubrication inside it does not move around... greatly decreasing its lifespan.
When two U joints are used in a driveshaft they are usually set to where they are equal in offset from straight.... that is not well written description.. will upgrade if I can later... I think the pictures in the hotrodhotline url show that....

http://www2.dana.com/pdf/J3311-1-HVTSS.PDF

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