Does this sound like a flex disk problem on my 126?
 

BACKGROUND:
Last month, I swapped a new German Star rebuilt tranny into my '89 420SEL. Too big a job for me to handle on my own, so I turned it over to a HIGHLY experienced Benz indy (highest references.) As long as he was going to be in the neighborhood, I provided him with new flex disks and a replacement tranny mount to install as well.

New transmission seems okay (great actually), BUT here's the problem I'm now having:

PROBLEM:
Only when the engine is fully warmed-up, and only upon take-off (in 2nd gear), there's a VERY obvious thump-thump-thump that you can both hear and feel coming from the floor above the driveshaft in the vicinity of the firewall. If I back off on the accelerator, the problem stops at once.

That's it... No other symptoms: Only when engine is completely warm; only when starting from a dead stop. Otherwise, car rides like new--no vibrations or other telltale signs of faulty driveshaft balance--whether driving in town or on the highway.

My guess is its the replaced front flex disk, and I intend to get under there this week to check it out, but wanted to ask all of you for any insights you can offer...

Thanks for any guidance here!

Have you discussed the matter with the person who performed the work?

I would start there first.

Whenever possible, I prefer to do my own work on the car, but I'm no fool: If it's more than I feel capable of safely and successfully handling myself, I have no problem calling in an expert. Nonetheless, hope springs eternal that I'll get under there and find that it's an obvious fix (i.e. a loose/missing hex screw.)

My primary reason for posting here is to obtain some help in accurately diagnosing the problem first. Don't worry, if I (we) can't readily pinpoint and solve the problem, it's back to the indy.

Either way, I'll follow-up (as usual) with the resolution--to possibly assist others in the future with the same symptoms...

I had a similar sound on my 83 300cd and it was the center driveshaft support which had completely disintegrated and was allowing the driveshaft to flex when accelerating....The only difference was my car did it all the time and not related to temperature or other things...If that is whats wrong its too bad it didn't get it replaced at the time you had this other work done as it is easier to do at that time. I am also suprised the mechanic didn't notice it was bad. As a test you can get under the car and push up on the center of the driveshaft and if it moves up and down that support is bad.....

Hi Eric
I never had this problem, so I am only guessing:
(1) Wrong orientation of front flex disc (warm -> rubber is softer?), usually
there is a label on the flex disc stating: "Diese Seite zur GLKW=This side towards drivetrain". If not, you can determine orientation by checking the rubber enforcement of the holes: they should be after the holes in reference to turning direction.
(2) shot transmission mount (warm -> softer rubber, more sagging?)
(3) less likely IMHO exhaust mounting bracket, from tranny to pipes
Good luck, Bruno

UPDATE: And the answer is...
 

Thank you for the various recommendations. I called my technician and he initially thought it was the center driveshaft support and bearing, as suggested by both RAMBLIN and BRUNO_300TE; but after I told him the symptoms appeared only when the engine was fully warm, he thought that was very odd and backed away from this initial diagnosis.

(RAMBLIN, regarding the center support, you stated that you were "suprised the mechanic didn't notice [whether or not] it was bad..." when he swapped the transmission... I asked him, and he informed me that he never had the opportunity to get a look at it, since he simply pushes the entire shaft backwards and out the way. The mount was therefore never exposed for his inspection. And since I'd had absolutely no problems prior to the tranny-swap, he had no reason to check back there.)

Anyway, I returned the car to my technician and he tracked down the source of the problem easily enough: the new front flex disc was thumping up against the tunnel, which was what I could feel inside. He said that in over 25 years of working on Mercedes vehicles, he'd never before seen this occur.

Without actually determining why this was happening, he opted to simply treat the symptom. He marginally lowered the transmission by using two longer bolts and some spacers on the tranny mount assembly. By dropping the mount a fraction of an inch (and therefore the front end of the driveshaft), the disc can no longer physically make contact overhead and the horrible thumping is gone.

He said I should drive it for awhile and see how it goes. If I still notice problems, he wants me to bring it back and he'll check engine mounts and other possible root causes.

On the way home (about 25 miles), things were MUCH better. No thumping at all; however, when I hit the gas from a standstill, I can still note a minor vibration, so I know there remains instability, but it's certainly a far cry better than before.

One more thing: While my mechanic thought it a long shot, he did note that at the location where contact was being made--immediately above the front flex disc--resides the SRS electronics, and that an unanticipated triggering of the airbag(s) could have resulted from the shock. :eek:

Any additional thoughts or insights based on the new facts? Anyone ever seen something like this before??? Should I leave well-enough alone, or should I pursue perfection here?

(By the way, the center support was in great condition, so rule that out.)

Follow-Up:

After a few days, the "slight vibration" that remained in the drivetrain is gone. I guess something that was slightly askew must have settled back into proper position down there. Looks like this one's a wrap... :)

The angle of the drive train to the diff' is critical. changing the angle by installing spaces is bad karma.

Anyway, I returned the car to my technician and he tracked down the source of the problem easily enough: the new front flex disc was thumping up against the tunnel, which was what I could feel inside. He said that in over 25 years of working on Mercedes vehicles, he'd never before seen this occur

The new roto coupling is touching the body ?
Hmmm....



Any additional thoughts or insights based on the new facts? Anyone ever seen something like this before??? Should I leave well-enough alone, or should I pursue perfection here?

If this air bags deployfrom this...someone could be dead !
Air bag envelope travels at better than 200 mph for a distance of 12 to 14 inches.

If you or anyone elses head is in this area during deployment when not being thrown forward by rapid deceleration, serious injuries will result.
However, it is unlikely the box in question will activate the bags as it needs initiation by the accelerometer sensors to do that. These are usually on the inner front fenders.

I would make sure the roto coupling is the correct one and if so, the problem lays elsewhere.

PS.......The vibration would be from the new angle of the drive shaft after he added spacers. Also, the drive shaft nose fits in a bushing of the output flange from the tranny....if this is oval, the shaft will vibrate.

The rear differential mount is bad.......my W116 did the very same thing before I replaced it....now it is fine. and never does it. And it never felt like it came from the rear end either.

noise
 

Hi Eric Did your tech make sure the trans mount was the right one.joeym ps was your trans rebuilt or did he exchange it with another one

Rebuilt tranny (from German Star).

Very experienced mechanic--didn't note any problem with the new mount I provided. The part numbers on the packaging are correct; though I suppose any of the provided parts (flex discs or mount) could conceivably be defective.

The "doc's" assertion that the problem is rooted in the differential mount is both interesting and logical. As noted above, all the symptoms have vanished and I'm reasonably comfortable that there's no urgency here. When I'm down under the next time (fuel filter's about due for a change), I'll definitely check-out that diff mount.

Thanks to all for the time and effort on my behalf. ;)

Clarifying previous post: I purchased a "new" rebuilt tranny from Ron at German Star--(I did NOT have my original rebuilt.)

Important follow-up (4-months later)
 

Well, everything WAS fine until last week... I was cruising down the parkway when there was this sudden single startling thud from the same location that had produced the original problem discussed above. Believe you me, my heart missed a few beats!

Something physically broke loose and went flying out from behind the car. I glimpsed it rolling off the side of the highway, a black chunk of something I couldn't identify at the time, but which turned out to be about 1/8th of the new front flex disk discussed at length above; the thing was unquestionably defective, the cause of ALL the previous problems, and had just disinintegrated. (Fortunately no one was behind me.)

Anyway, the car continued to run alright, though I was understandably very cautious in applying throttle. I crawled to the next exit and called the mechanic from my cellphone. With great care, I took a calculated risk and slowly drove the car to his location via the back roads.

:bulb: Users of this board need to know that this defective front flex disk was the cheaper after-market version that's alternatively available for purchase online here and elsewhere and not the OEM replacement part.
(I figured it was just a hunk of rubber, so what could go wrong... Live and learn!)

Evaluating the defective old part alongside the new OEM flex disk my mechanic has now installed revealed a significant difference in the stiffness of the two versions. The after-market disk that failed is made of rubber that is significantly less firm than that used in the OEM flex disk. I'd say it's like comparing a tennis ball to a softball in terms of rigidity.

With a new OEM flex disk in place, my mechanic was able to remove those spacers he'd originally inserted in the tranny mount [see above] without any problems. Everything seems to be great now, though I'll post if there are any new developments.

Thanks to everyone who contributed their excellent insights in hopes of helping out here. You guys are the best!

No doubt those extra spacers caused the flex disc to come apart. Since you said the flex disc was hitting the trans tunnel, it should have been returned for a replacement or use th OEM version.

IMHO.

Thanks for the response, Warren, but hindsight is 20:20.

The flex disk was hitting the tunnel only under very specific conditions. This led the tech to logically conclude it was symptomatic of some other undetected problem. (The fact that the situation coincided with the swapping-in of a new transmission only fed the assumption that something more complex was the root cause.)

You're conclusion about what I "should" have done is undoubtedly correct--and I'm kicking myself now for not having had the wisdom to foresee all of this. But, realistically, there were a lot of other things besides a defective new flex disk it could have been...

Sure do wish you'd served-up your valuable opinion earlier though! ;)

Regards!