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

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles