I replaced the flex discs as well as the center support, and the center support bearing. Yet when my 87 SDL downshifts and up shifts slowly there is still a clank in the drive shaft. Not as bad as before, but i want it gone.

Any ideas where this is coming from?

Are you SURE that it is the driveshaft? Have you checked your differential mounts?

The best way to find such a problem is with the 4 P's: Push, Pull, Pry and Prod.

The clank may be the trans itself downshifting. Mine does/did that. It actually stopped doing it once the trans began dying......

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It makes the same exact sound as when i took the drive shaft out and flexed it to come out. We are thinking it might be the transmission mounts that are causing to to move a bit..

Any input on this idea?

All of them will make that sound if the shifts are too quick.

Put a vacuum gauge on the modulator line and take it for a drive. Report back with some data on vacuum at various operating modes.

My suspicion is that you've got a serious vacuum leak.

I agree with Brian, my 86 300sdl had a vacuum leak and it made the worst sound when it downshifted while coasting to a red light. It literally banged on the downshifts, and you could really feel the shifts, up and down. It turned out that i dislodged a vacuum line while changing my fuel pre-filter. Hooked it back up and all was well.

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John
1986 300sdl
1985 380se

with a gauge on the vacuum line it sits around -15 psi at idle and will go to -13/14 when the rpms are about 2200 and up. Any suggestions as to where a leak may be? I have quickly inspected them and they all seem good. How can a leak be properly identified?