Checking flex disks - PeachParts Mercedes-Benz Forum

psaboic · #1 09-15-2014, 10:15 PM

Can someone give me a bit of a tutorial on checking the flex discs outside of just looking at them for cracks? I'm getting a rumbling sound and a vibration/thumping feeling at the floor by the transmission shifter when the car is in second or third gear and you step on it like you want it to downshift or if you are going uphill. Funny thing is it only happens between about 25 and 35MPH. Any faster or slower and no issue. Also when it starts vibrating, you back off the accelerator and it goes right away. There is no vibration, noise, or anything at highway speeds (smooth as silk)

I'm thinking flex discs, driveshaft bearing, or maybe the U-joints (they do clunk).

BTW, car is a 1983 300D with 278K on it.

Thanks,

Glenn

uberwasser · #2 09-15-2014, 10:22 PM

The best way is to use a small pry-bar to pry on the discs. If you see cracking, they need to be replaced.

uberwasser · #3 09-15-2014, 10:40 PM

I know he's everyone's favorite - regardless, here is a visual:

https://www.youtube.com/watch?v=v71KGSCjBrQ

DrLou · #4 09-15-2014, 11:07 PM

While you're under there, really jostle the driveshaft around, looking for any play. Center support bearing and support bearing mount likely deserve replacement if miles are high.

In my case, the bearing mount was completely worn away; the driveshaft could be deflected up and down a coupla centimeters or more. Bad! Still, the car was running pretty quietly. Interestingly, the bearing itself showed no evidence of slop. Replaced on principle. Both flex disks and shaft coupling boot, too.

Next (when?) I gotta replace the transmission output seal and re-seat/torque down the output shaft yoke. Play in the output shaft was, I'm sure, responsible for much of the wear in the driveshaft components.

Still not clear if there's a bearing on the trans output shaft to be replaced.

psaboic · #5 09-16-2014, 12:19 AM

I'll check things out this weekend when I pull the car off the road. Based on what I described, any ideas on what it MIGHT be? I looked at the stack of receipts that came with the car and it looks like the both flex disks were replaced by the previous owner about 40 - 45K ago. No mention of U-joints or driveshaft bearing.

Diesel911 · #6 09-16-2014, 12:30 AM

I think you may need to support the Transmission and drop the Steel Plate that that Transmission Mount attaches to in order to see the front of the Flex Disc unless you are can see well with a Mirror.

There is also a Centering Bearing on each end of the Drive Shaft in the Middle of the Flex Discs but not attached to them. I don't know how to inspect them on the Car.

One of mine in the Pics below. They were not making noise but I saw cracks on the Rear one so I checked the front also.

Notice that on one side you can see the Chords of the Disc showing around the Washers.

I replaced them with Lemförder/Lemfoerder/Lemforder Flex Discs.

Since the Drive Shaft was dropped I inspected the Centering Bearings and they looked OK but the Grease was all dried up so I put some Wheel Bearing Grease inside and reused them.

During the Inspection none of the Bolts and Nuts going through the Flex Disc should be loose. If any are loose it could also be the Holes on the Yoke are elongated.

Stretch · #7 09-16-2014, 01:22 AM

Are you saying the universal joint on the propshaft clunks or that you've heard that they can?

I'd check motor mounts as well

cooljjay · #8 09-16-2014, 04:14 AM

Quote:

Originally Posted by DrLou

Next (when?) I gotta replace the transmission output seal and re-seat/torque down the output shaft yoke. Play in the output shaft was, I'm sure, responsible for much of the wear in the driveshaft components.

Still not clear if there's a bearing on the trans output shaft to be replaced.

I just did this on my 78, the nut had backed off...causing the yoke/flange to come off thus the trans stop working. There is no bearing that needs replacing just the seal. You also don't want to reuse the nut. Get a new one and then torque it down....chances are you will have the older slot nut needing a special socket.

DrLou · #9 09-16-2014, 07:40 AM

Quote:

Originally Posted by cooljjay

I just did this on my 78, the nut had backed off...causing the yoke/flange to come off thus the trans stop working. There is no bearing that needs replacing just the seal. You also don't want to reuse the nut. Get a new one and then torque it down....chances are you will have the older slot nut needing a special socket.

Thanks, coojjay - Luckily, my flange has not come off; have read some of the horror stories here regarding damage this can cause. In fact, it doesn't look like the nut has loosened at all. (Does this happen very gradually; ie, could it be almost imperceptible? !) And I do have the replacement nut.

What I'm missing, though, is what accounts for the up/down, right/left slop in the output shaft? Surely will replace the seal - but if there's no bearing under it, what effect will tightening the yoke nut have on this play?

Further luck: I have the 12 pt (30mm?) nut on there, not the freaky slotted one. Project for the fall...

DrLou · #10 09-16-2014, 08:06 AM

psaboic,

Pictures (front/back) of my front flex disk below. Even in this condition, there was no slop in the disk. In fact, even with total loss of the driveshaft support, the car was running pretty quietly. There was the low-frequency hum at certain speeds, indicating something clearly wrong, but nothing like the thumping through the floor you describe... The rear disk looked nearly perfect.

I'm definitely with Stretch on checking your motor mounts at this point. This would both put your drivetrain out of alignment, and could account for the drivetrain gettin' a little closer to the floor than it should be! Might also put your exhaust system in a new position!

So also inspect exhaust. When my header pipe broke (above the mount U-bracket), the header-resonator joint was forced upward, occasionally vibrating/thumping against the driveshaft tunnel. Happened only at certain speeds, too, when the frequencies were just right.

For point of reference: These flex disks have been in service for at least the 200k+ miles I've owned the car. Likely they're the '83 originals... 400k+ Also, the U-joint showed no signs of wear; I could not produce even the slightest click or play in the joint. (Should I have replaced it just for good measure?)

psaboic · #11 09-16-2014, 09:04 AM

Stretch,

I guess what I meant to say is that the drivetrain clunks when it is cold, sort of like worn U-joints can. Here is what it is doing in a nutshell. The drive train will make a clunk when shifting from Park to Reverse or Park to Drive after the car has been sitting for awhile. The clunk when shifting will pretty much go away after the car has warmed up, but it still clunks a little but only when shifting from park to reverse or drive. On the road it shifts quietly.

As for the thumping/vibration. It will only do it if I am in 2nd or 3rd gear at about 25-30 MPH going up an slight incline and I am pushing it a bit (or if I have a good bit of weight in the trunk) If I back off the power it will go away It sounds and feels like a manual transmission car would if the clutch was slipping. Again, on the open road no problem, except for a minor humming that comes from the rear end. Hope that helps.

I'm planning on checking/replacing both flex disks and the center bearing/support as soon as I get time and some funds. I got a stack of paperwork with the car showing damn near every service it had since 1983 and I only found one receipt for flex disks and that was over 50K and 8 years ago. Nothing for the bearing.

Last question. I was reading the how-to on removing the center bearing,and was wondering, do the drive shaft parts have to be separated while it is on the car, or can I just remove both flex plates and the bearing support, and remove the drive shaft as a whole unit, then mark and separate them on the bench?

Thanks,

Glenn

DrLou · #12 09-16-2014, 09:39 AM

Quote:

Originally Posted by psaboic

As for the thumping/vibration. It will only do it if I am in 2nd or 3rd gear at about 25-30 MPH going up an slight incline and I am pushing it a bit (or if I have a good bit of weight in the trunk) If I back off the power it will go away ... do the drive shaft parts have to be separated while it is on the car, or can I just remove both flex plates and the bearing support, and remove the drive shaft as a whole unit, then mark and separate them on the bench?

I think you asked Stretch - so, apologies if I'm threadjacking:

Lot of above sounds like motor mounts - (though I don't see any connection with the engine being warm...), because all of those shifts will torque the engine around, and only do so more under load. In fact, if you do need motor mounts, be sure to do this before reinstalling the shaft. You'll want all the alignments/distances to be correct. Your rear transmission mount should also be inspected; this is a cheap enough replacement.

Hope none of this sound is coming from inside the trans (and your trans fluid level is AOK?)

Yes, I removed my entire driveshaft as one unit - did all work on the bench (which looked a lot more like a couple of 2x8s on my driveway than a 'bench'!). Since you're re-furbing the whole thing, this seems the best way to do this job.

Be very sure to mark spline alignment of front/rear shafts. Purists will also match up the front and rear yokes to their respective mounts on trans/diff - this doesn't seem important to me, but; could it hurt?(!)

Stretch · #13 09-16-2014, 11:23 AM

Quote:

Originally Posted by psaboic

Stretch,

I guess what I meant to say is that the drivetrain clunks when it is cold, sort of like worn U-joints can. Here is what it is doing in a nutshell. The drive train will make a clunk when shifting from Park to Reverse or Park to Drive after the car has been sitting for awhile. The clunk when shifting will pretty much go away after the car has warmed up, but it still clunks a little but only when shifting from park to reverse or drive. On the road it shifts quietly.

...

That sounds more like a transmission problem to me - but let's focus on the vibration first! Who knows may be that problem will be fixed after the vibration trouble is solved.

Quote:

Originally Posted by DrLou

I think you asked Stretch - so, apologies if I'm threadjacking:

...

This is a forum not "askStretch.com"!

psaboic · #14 09-16-2014, 09:10 PM

I'm working straight through for the next 10 days, so it will have to wait until the weekend after next to start looking at it, but here is my plan so to speak

I'm going to jack the car up and use a pry bar to check the flex discs for cracks, etc, but mainly the work is going to be 1. Drop the drive shaft as a complete whole unit and change both flex discs, 2. Change the driveshaft center bearing and the center bearing mount. 3. Change both motor mounts, 4. Change the transmission mount.

That said, I have looked at all the wiki's and how-to's and all of it looks fairly straightforward. I do have a couple questions though..... 1. Is there a good pictorial how-to on how the change the motor mounts? I have read the write up, but photos would be a big help. 2. How many transmission mounts are there? I'm assuming only the one at the rear, and is there any special things to know about changing it.

Thanks,

Glenn

DrLou · #15 09-16-2014, 10:22 PM

Sounds like a good plan - some notes:

1) Exactly. Yes.
2) There is also a rubber 'bearing mount boot' you'll want to replace. I'll send you the extra one I have, if yours is the same as my car ('83 SD) uses.
Also, I believe you need a big circlip remover for the bearing on your car; Stretch* can verify.
3) Motor mounts (on my car) are mounted with a large hex key; be sure you're ready with tools. Issue here is simply being able to safely jack and support the engine. Otherwise not a difficult job.
4) Same is true for trans - jacking and safe support. But you'll have to do this to remove the driveshaft, anyway. Trans mount is easy to replace; standard nuts and bolts.

1a)? One thing I regret not doing on the same project was replacement of the trans output shaft seal - and perhaps replacing/tightening the yoke nut at the same time. I just wasn't prepared with parts and tools for that; a shame with everything apart.

* I like the sound of AskStretch.com !!