Strange Flex Disk Alignment Problem - PeachParts Mercedes-Benz Forum

Ken300D · #1 05-08-2003, 09:26 AM

I decided to do some more testing to isolate the driveshaft vibration problem I've been having. All along I have been suspicious of some kind of "binding" issue rather than imbalance, because the vibrations start at a low speed (35mph) and go away at interstate speeds.

Pulled the rear driveshaft segment off and decided to start the car and run it in drive. In doing this, I thought it would be a control case for making sure there is no vibration whatsoever in the transmission and the transmission mount and support bracket. Keep in mind that the front section of the driveshaft is still attached.

So, I put it in drive, no vibrations. Now I rev up a bit and still nothing, but the speedometer starts to move (since its geared from the transmission). Then I get a gear change, and the speedo runs up to 25-mph. At this point I get metal hitting metal. I know its that short piece of driveshaft.

With the car back to idle in drive, I get under and take another look. Plain as can be, there is the short front driveshaft section wobbling all around. Stop the car and get under again. As I slowly rotate the driveshaft section, it is clearly mounted off-center somehow such that the thing has a pronounced wobble.

I can't see anything wrong with the bolts on the flex disk. They all look seated properly, the flex disk seams to be seated properly into all the insets of the transmission and driveshaft flanges. But its misaligned somehow. Any misalignment at the flex disk translates into nearly an inch of wobble at the far end of the front driveshaft segment (about a foot and a half long).

It's a FEBI flex disk - should have been well made.

Did I do something wrong? Is it not seated correctly? Could I possibly have overtorqued a bolt (by hand) enough to have crushed one of the cylindric inserts that are in the flex disk?

My flex disk replacement (done at the same time as the driveshaft center bearing) is apparently the source of the vibration issues I've been trying to eliminate.

Besides this issue, I took a little trip to PG Auto and got a replacement driveshaft, and an excellent low-mileage 2.88:1 differential (with no slack) to install. The existing differential has a lot of slack (about 50 degrees driveshaft rotation) and whines.

But what did I do wrong on the front flex disk installation?

Ken 300D

md21722 · #2 05-08-2003, 10:20 AM

Doesn't make any sense to me. There is a centering bushing in the end of the driveshaft that goes into the trans output shaft (thru hole of the flex disc). Might also compare against any old flex discs you have laying around. The flex disc has "centering" sleeves in the holes on each side that stick out 2mm or so. The sleves are to go into the flanges of the driveshaft and trans and make everything tight.

Ken300D · #3 05-08-2003, 10:25 AM

The rear driveshaft assembly section is removed from the car, along with the center bearing assembly and U-Joint. (Those two items have been replaced previously.)

It's pretty clear to me that the wobble I'm observing with just the front section of driveshaft attached to the transmission output (through the flex disk) is large enough to cause vibration.

I plan to do more testing, but the main variable that would cause the wobble would be the flex disk - I should think......

Possibly something didn't seat properly / completely, or I crushed something during assembly. Hard to see how I could crush anything, as the metal cylindrical inserts in the flex disk look pretty sturdy.

But plain as day, it wobbles.

Ken300D

kerry · #4 05-08-2003, 10:29 AM

Is it possible that the wobble is coming from the output shaft on the transmission and not from a misaligned flexdisk?

Ken300D · #5 05-08-2003, 10:35 AM

It is possible. Hope that's not it!

There was no vibration before I did the transmission mount, driveshaft center bearing, and front and rear flex disks. So I would look to things that I did in this work first.

I suppose the next step is to take that flex disk out for inspection, as its still on the transmission flange. Perhaps I'll try replacing it with another - there's a good OEM flex disk that came with the PG Auto driveshaft.

I've seen other posts here where people complained about vibration just after their flex disks were changed, so I interested in narrowing down the "Do's" and "Don'ts" of flex disk maintenance.

It sure seemed a simple thing. Then I got involved.

Ken300D

md21722 · #6 05-08-2003, 10:45 AM

A bent tranny output shaft is very, VERY rare.

#7 05-08-2003, 12:54 PM

Those bolts under there are fine thread on my old diesels. Easy to overtighen fine thread bolts and stip them.

Beagle · #8 05-08-2003, 01:33 PM

Ken

Check that the taper sleeve nut is tight (ie on the front short section of the driveshaft just in front of the centre mount bearing). The female spline on the front section is split and the sleeve nut clamps t onto the rear spline.

If this has been running loose the splines will wear quickly to the point where the sleeve nut will no longer clamp it onto the rear male spline.

Ken300D · #9 05-08-2003, 03:00 PM

Yep, that's not causing a problem.

Picture this: I have removed the back section of the driveshaft. There's no U-Joint, no center bearing left on the car. Just a foot and a half long pipe - the front section of the driveshaft.

My feeling is that this should rotate "true" without wobbling.

In fact it is wobbling badly, like an inch out of true at the "loose" end.

I think this is the source of my vibration problem, yes?

Ken300D

kerry · #10 05-08-2003, 03:40 PM

I am skeptical about the assumption that it should rotate true without wobbling at the unattached end. I would think that a very very minor imbalance at the attached end could result in a significant wobble at the other. I am not a physicist, however.

md21722 · #11 05-08-2003, 04:34 PM

Well, the driveshafts are balances as halves and then rebalanced as a complete assembly. However, I'm not sure your test is valid. I'd attach everything and run the car with the wheels off the ground and look for vibrations.

md21722 · #12 05-08-2003, 04:37 PM

Why'd you replace the driveshaft in the first place?

Perhaps the center bearing is installed incorrectly, not seated on in the rubber boot evenly?

I can spin the flex disc without the driveshaft on the trans on my cars and it looks. You might could try that too.

Ken300D · #13 05-08-2003, 05:52 PM

Yes - I see the point about wobbling at speed not being a valid test, because just the front pipe is not a balanced assembly.

But, I can see this wobble with slow hand-turned rotation, where balance is not the issue. It is physically not aligned "true" to the axis of rotation (which I presume is the transmission output shaft). To force the pipe "true" to the axis of rotation requires an effort to flex the flex disk.

It's almost like one of those flex disk inserts did not seat properly in the transmission (or front driveshaft) flange recess. The alignment bushing looks good - its function is to align the very end of the driveshaft "center" to the transmission flange. And there's another one at the differential of course.

So I blame myself for doing something wrong in assembly. But I have not found it yet. Maybe later this evening after some more quality time under the car I'll figure it out.

Originally, I changed the transmission mount. At this point I felt a slight grinding through the transmission hump at startup. The center bearing was really bad - so I changed it, the support for it, and the flex disks at both ends. After that I had a vibration as described in more detail earlier in this thread. I had noticed a "center detent" in the U-Joint, so I tracked down a replacement, took the driveshaft rear section out, and replaced the U-Joint. Nice and smooth. But still vibrates.

Naturally if I did something wrong in assembling the front flex disk that caused a wobble, my follow-on efforts at U-Joint replacement were for no effect. However, I thought perhaps I messed up the overall driveshaft balance by replacing the U-Joint, so I got the replacement at PG Auto to give it a try.

Thanks for the thoughts - I'm just trying to develop a checklist of things that would potentially resolve the problem.

Ken300D

Beagle · #14 05-08-2003, 06:04 PM

With the centre bearing and the rear section of the drive shaft removed the shaft will move around - the only connection between the front section and the tran. is the rubber between the 6 bolts and it will move at least an inch at the rear of the shaft.

Have you checked the rubber bonding on the torsional damper? This is the steel ring with 3 rubber "spokes" directly behind the flex plate which is attached with three of the flex plate bolts. If there are any splits this can give severe vibration.

Ken300D · #15 05-08-2003, 06:19 PM

I've seen pictures of those in the manual pages. But this car (the 1982 300D with automatic transmission) does not have anything like a torsional dampener behind the flex disk.

Yes, you can move the front shaft by itself all around by grabbing the loose end and effectively flexing the flex disk. I just thought that it should naturally return to a position of being "true" to the axis of rotation. It is a flexible rubber material, but I'd think its thick enough to hold that short front pipe straight.

I should be able to tell more about it with some more quality time. I've got extra flex disks to try if necessary.

Thanks,
Ken300D