In less than two years, yet another flex disk! - PeachParts Mercedes-Benz Forum

diesel diehard · #1 10-05-2004, 11:57 AM

Hello all!

I'm having yet another flex disc (?) issue.

My problems started, I would say, about 3 years ago when an old flex disc (the one directly behind the tranny) got all busted. No problem, I thought, as I cannot even recall the last time it was replaced, or if it had been replaced at all. A year or so however after the replacing it, the "new" flex disc got busted again! This time, some of the tranny links were damaged. So okay, maybe it was a simple fluke or an unidentified parts problem.

Just the other day however, I would say less than two years with the replaced flex disc, I felt that familiar vibration again. I had the car raised up in a gas station and saw that, yep, the flex disc is torn up again! This time, it wasn't totally obliterated though which is why I was still able to bring it home.

So here I am not knowing where to begin.

I'm sure flex discs should last longer than what I experienced! All I do is city driving though there are times when I do mountain driving as well. I drive quite fast/hard but not enough make any burnouts (not even possible I suppose with a 123 240D). Just enough to keep the engine in good, diesel-ran condition.

I'm sure I used the correct, OEM parts. What about tightening the bolts? I've read somewhere here that the bolts are not supposed to be over-tightened (?), about 1/4 turn after hand-tightening the nuts. Is this right? The reason supposedly is to allow for minute adjustments as the car runs as the driveline should be expected NOT to be perfectly aligned or balanced for its age. Could using a lower ratio differential (from 300D) have that much effect? If that's so, then why is my clutch (manual tranny) still tops? Shouldn't that be the one to give in first?

BTW, I had my exhaust pipes checked about half a year ago and I specifically do not remember seeing problems or signs of wear and tear with the flex disc then! The other disc, the one nearer to the differential, still looks good as none of its threads are showing.

Im sorry for the exasperating thread. All help will be very much appreciated.

BTW, the car is a '78 123 240D.

Thanks!

Old300D · #2 10-05-2004, 12:14 PM

I'm very much a "shade-tree" mechanic, however I have seen the shop manual for removing the driveline. In my haste, I've never remembered to match-mark the splined section of the driveshaft. However, I do not believe this is necessary for shaft balance, rather for the tripod timing at each flexible joint end.

I would check the to make sure the tripods at each flexible joint are in line with each other. This is very important in normal U-joints, and may apply here. If they are not, this may be causing the discs to go out.

tangofox007 · #3 10-05-2004, 12:33 PM

The service manual calls for a torque of 45 NM on the flex disc bolts. Were the flex disc bushings indexed and seated properly? If not, I can easily see that the flex disc could fail prematurely.

bill murrow · #4 10-05-2004, 12:52 PM

Quote:

Originally Posted by Old300D

I'm very much a "shade-tree" mechanic, however I have seen the shop manual for removing the driveline. In my haste, I've never remembered to match-mark the splined section of the driveshaft. However, I do not believe this is necessary for shaft balance, rather for the tripod timing at each flexible joint end.

I would check the to make sure the tripods at each flexible joint are in line with each other. This is very important in normal U-joints, and may apply here. If they are not, this may be causing the discs to go out.

I'm just a shade tree mechanic too, but I beg to differ with you on the balance of the driveshaft, or propeller as the Europeans call it. I am quite certain that every driveshaft must be balanced and have been told and read on this forum as well to make sure you mark the driveshaft before removal. There are no marks on 123s from the factory but the later models are marked.

Am I right experts?

Cheers,

Bill

tangofox007 · #5 10-05-2004, 01:30 PM

According to the service manual, the shafts on the 123 diesels were marked starting in August 1982. Prior to that date, some shafts had alignment marks, but they were not utilized during assembly. Those without marks were to be marked prior to separation and reassembled in the original relative positions.

leathermang · #6 10-05-2004, 01:52 PM

It is easy to find something like a flexdisc torn up after experiencing vibration...and think that it was the cause of the problem.... when in fact it may, being a flexible, soft item, have been the victum of the actual problem.
If I had to replace one flex disc I would replace the other one at the same time. This is much like replacing one tie rod end... things like this are usually replaced " on principle" together...
This is aside from trying to find it something is out of balance or reinstalled out of phase at some time in the past....
Have you checked the part which goes into the back of the transmission to see if you have excess play there... or your center bearing ? ( all depending on whether your car has those items.. ).

By the way... there are different ' strengths' of rubber used on some flex discs than others.... and changes were made in which the factory wanted installed on some cars... long after they left the factory...
Check with Alldata.com for your car and read the TSB's to find out...

michael cole · #7 10-05-2004, 02:19 PM

driveshaft alignment is also a very critical factor.there are specified angles between the various pieces of the driveline.check your tranny mount as well.if its collasped or allows the driveshaft to come out of alignment during periods of high torque this could cause vibration and rapid wear of the flex disk

Old300D · #8 10-05-2004, 02:29 PM

Quote:

Originally Posted by bill murrow

I'm just a shade tree mechanic too, but I beg to differ with you on the balance of the driveshaft, or propeller as the Europeans call it. I am quite certain that every driveshaft must be balanced and have been told and read on this forum as well to make sure you mark the driveshaft before removal. There are no marks on 123s from the factory but the later models are marked.

Am I right experts?

Cheers,

Bill

I do not question driveshafts must be balanced. However, the front part of the shaft is of small diameter and is quite symmetrical. It seems unlikely to me that even if it was slightly unbalanced from a casting or machining defect it would affect the balance significantly. The indexing of the tripods is far more important to curbing vibrations, IMHO.

And, BTW, I just installed the 300D front shaft in my 240D to accomodate the 617.952, and aligned the tripods accordingly. I even used old rubber discs that seem to be in good shape. The two pieces of the shaft are from different cars, so we will see how this arrangement lasts.

tangofox007 · #9 10-05-2004, 03:16 PM

Diesel Diehard, could you provide a photo of the offending disc? Or a more detailed description of the failure mode.

dieseldiehard · #10 10-05-2004, 06:17 PM

Hello Diesel Diehard

another moniker similar to mine, yikes! I hope we don't get confused with each other's posts

I am sure that something is wrong that is allowing the disc to fail prematurely, the driveshaft might be involved. Also the carrier bearing and support (if there is one) may be worn, if you would feel a wobbling or hear something I'd suspect that. I had that problem on an old W115 car (not the one I own presently), it made a noise that I can only describe as a boogity-boogity during acceleration, more so in low gear and 2nd gear BTW.

The front flex disc fails more often then the rear one, I attribute this to it being flexed more when the engine moves. If the motor mounts are bad, and maybe the engine shocks are a factor in this also, then the flex disc will see more flexing.
Dieseldiehard (the Oldriginal DDH!)
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ ’85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 258230

bill murrow · #11 10-05-2004, 06:56 PM

Quote:

Originally Posted by Old300D

I do not question driveshafts must be balanced. However, the front part of the shaft is of small diameter and is quite symmetrical. It seems unlikely to me that even if it was slightly unbalanced from a casting or machining defect it would affect the balance significantly. The indexing of the tripods is far more important to curbing vibrations, IMHO.

And, BTW, I just installed the 300D front shaft in my 240D to accomodate the 617.952, and aligned the tripods accordingly. I even used old rubber discs that seem to be in good shape. The two pieces of the shaft are from different cars, so we will see how this arrangement lasts.

Ah, you are correct on the diameter of the front part of the shaft. And I wasn't thinking defect. I was thinking if it had been off and not put back in the same spot, an important detail when removing the rear, larger part of the shaft. And I certainly wasn't arguing at all about the tripods. Very crucial indeed.

Let us know how your driveshaft exchange holds up.

Cheers,

Bill

diesel diehard · #12 10-06-2004, 02:25 AM

Thank you all!

Dieseldiehard, I'm so sorry to have copied (I swear, it was inadvertent!

) your monicker. If can change it, I will. just dont sue me just yet over some patent or trademark violation.

I can't post some pics now because my cellphone cam got busted due to some virus probably due to bluetooth but that's another story...

Anyways, I too am a bit if a DIYer. I inspected the car in the garage this afternoon and found out that due to the nuts rubbing against the, I believe that would be the transmission mount/support, it all got rounded and that would take me some time to remove those as well.

What I do remember was that there were no noticeable vibrations before the flex disc broke down. I felt the vibrations after it broke. It didn't give any signals at all before it failed, or they were too mild for me to notice.

Also, would I be correct to assume that if the motormounts were the ones problematic which then caused the flex disc failure, then I would have felt vibrations form the engine first?

What I'm gonna do first is to change the offending flex disc and use the necessary torque in tightening them bolts (thanks tangofox).

Thanks guys!

bill murrow · #13 10-06-2004, 06:58 AM

Diehard--Once again, I'm no expert, but I don't think motor mounts would cause your f.d. to break un less they're extremely bad. I've got one with bad m.m. causing the car to shake at idle. It's not bad enough to make the familiar "motor mount knock", which happens when the m.m. get really bad. My discs are fine in this car which has been driven for several thousand miles with this conditon. And will continue to be driven until I hear the "knock". I will keep a close eye on my discs after this thread!

Cheers,

Bill

dmorrison · #14 10-06-2004, 08:32 PM

Reading the posts I would check a couple of things.
On the 240D check the motor mounts and the transmission mount. If they are old and collapsed they will affect the alignment of the drive shaft vs. flywheel. Check the front engine stop and the tranny mount. Again if these are bad they affect the position of the flywheel and the output to the transmission/driveshaft.
Check the drive shaft center bearing and rubber sleeve. The center bearing/sleeve could allow a repositioning of the driveshaft.
Check the rear differential mount. Is it broken, againg allowing the driveshaft to not be aligned correctly.
If all of these items are OK then the only other thing I could think of is was it a OEM flexdisk? or after market?

http://skinnerbox.steaky.org/Service/W123/Index/616index.html

jobs

00-211 - Removal and installation of front engine mounts

00-212 - Removal and installation of rear engine mount

00-215 - Removal and installation of front engine stop

00-220 - Adjustment of engine stop

then

Check all of these job items

http://skinnerbox.steaky.org/Service/W123/Index/Groups/41PropellerShaft.htm

Again you may find the other components of the engine/tranny/driveshaft/rear axel support system are your problems.

Dave