Potential Driveshaft Problems... or Not?
 

My SD seems to vibrate around 70mph and up on the freeway, also when I come to a dead stop at times it seems to "clunk" if you will back to first. Also if you drive about around 35mph in 3rd, then shift into 4th it'll clunk.

The 722 trans shifts fine, so I don't believe it's anything internal. I do know my trans mount is pretty much dead... so I plan to replace that ASAP here. I had my wheels and tires dynamic balanced high speed so it's not them either.

I tried to look at my drive shaft under the vehicle and at some exploded diagrams of it and it appears I have one universal joint by the resonator section right? :confused:

Would a bad flex disc cause these problems I'm facing... or is it probably a combo of the trans mount and u-joint I believe I saw in the diagram?

I had that issue on my 91 190E 2.6. I believe the soution was the rubber flex disk. See if you have that and see if it is cracked. You would see one yoke near the tranny and the other is a flex disk on the other end of the DS if your system is similar to my old car.

You the guy on the full sized chevy boards?

Yes I was checking for play in the shaft and was peeping to see if perhaps it was the center bearing was going also. I couldn't tell if those flex discs were similar to u-joints or not from the exploded diagram.

I suppose it'd be best to rebuild it all as opposed to part of it. :(



Yup... I sure am. :D

I'd say you might be able to take the entire DS out and have the it checked and balanced and see if it goes away. Flex discs are there to take up some of the shock, AFAIK. When my flex disc had a crack in it, going from neutral to reverse was jerky.

It appears these bolt to the drive shaft and to the input of the rear yolk and to the output of the trans yolk correct... basically like a "sandwich" piece if you can call it that.

6 bolts seem to be all there is to it.

Now, any clues as to how big of a PITA it is to pull the driveshaft or can you do these with it still in the vehicle? I did see a big heat shield up in there that looks to be a mofo to remove due to the bolts and the exhaust under it.

Fairly easy driveshaft to pull but mark it well so you do not loose refference between the front and back shafts. Also would not jump to the conclusion it is the driveshaft itself so quickly. Check everything out well first. I assume car has developed this problem since you purchased it? :)

To get to the center bearing you have to drop the exhaust, remove the heat shield then remove the drive shaft. Work on it away from the car.

I don't know if you can push the driveshaft enough to the front and rear to do just the flex couplings. It's probably worth trying.

While you're down there, see how much play there is in the diff.

Sixto
95 S420
87 300SDL

Nah, the car always did it... if I didn't rescue it, it'd have been out in a boneyard for sure. I think the solid motor and trans kept it out of there. My hats off to the builder in Stuggart for this one. :) I even had the tires from the wheels broken down to make sure there was no H2O within them to verify this. :(



I plan to mark the DS reference for sure prior to removal, no sense in losing any balance I have.

Looks like I need to find someone with a lift to borrow for about two hours. Doing this on my back in the garage seems to pose a nice safety hazard I'm not fond of nor spending 2-3 times a long to complete the task.

Since it always had the vibration from the day you purchased the car it does change things a little. Someone in the past may not have indexed the driveshafts properly when replacing the centre carrier or bearing. Or one of the shafts may even be bent.To prove imbalance you might want to put two screw clamps on the driveshaft and rotate them to different positions to see if vibration is reduced or perhaps get another intact shaft from a salvage car if cheap enough. First again though you want to critically check centre universal and rubber end disks etc. You can check for bent shaft on car though quite easily. Shame to have to pay the bucks out to have the shaft balance checked. As problem did not develop in your possesion it may be required. Know it bugs you as you mentioned you had a dynamic balance of all wheels done already to try to eliminate the problem. :sun_smile

If the shaft(s) were bent, wouldn't it do it at the same RPM in each gear?

Or is it because it hits the driveshafts critical speed in 4th (1:1) that it will vibrate at lets say 3000rpm that it'd have to do it at ~4400rpm in 3rd (1.44:1) to do the same thing?

This car, aside from oil changes, was pretty much never taken care of. Paint sucks, interior is shot, brakes have new pads thanks to me now, etc. Most stuff needs to be rebuilt so chances of stuff being OEM are pretty good that most have never been touched or removed from below. :(

No if you had a bent shaft (unlikely but possible if car was rescued and this applies to just out of balance shaft as well) the crucial speed is not the engine or gearing speed prior to the driveshaft. The only time the driveshaft turns at the critical speed is when the rear wheels are turning 70mph or above if it is the driveshaft problem. The ratio of rear end to driveshaft is fixed. But does make me think of something. (usually takes dynamite) Try driving car at vibration speed on downhill grade. Shift automatic to neutral and make sure vibration still persists or not as may give a valuable clue. Also when you get car on hoist to inspect things just do not look. Pry things and push and pull on areas like your centre bearing. Make sure bearing on yoke end of transmission is not abnormally worn. Any wear that causes the driveshaft to run offcentre is going to induce imbalanceas well. You do have this clunking noise that may or may not be related to your vibration problem. As you are driving a car with a lot of miles on the chassis you have to keep an open mind. Even a shot transmission mount might cause some symptoms like you describe. Another thought is to let a mechanic with a lot of mercedes experience have a quick look. Things that vibrate excessively cause wear. May be cheaper to localise problem earlier than later. Would be an easier world if everything was straightforward. Experience teaches us to expect even things that seem to almost defy logic from time to time. :sun_smile

Well it happens more under load, when climbing hills and such. I am going to change the trans mount first as I can tell it's FUBAR buy visual exam. Once that's done, I'll get it on a lift and go from there.

I'll ask around and see if someone will let me bum their lift for a few hours. I can't fathom spending $300 in labor to fix something I can do myself. But I may have to if I can't get a lift. :(

trans mount is easy...it takes 10 minutes to change...15 if you take your time.

Yeah I've changed them before on domestic vehicles, this one looks super easy too. :D

Its even easier than it looks....will take longer to jack it up and get a jack stand under it than to change it.