My 81 300SD started acting funny a few weeks ago. I was driving it home from my wifes office and right as i turned in to the neighborhood the transmission started acting funny. It was slipping, or flaring during 2-3 3-4 shifts. 1-2 shift feels almost normal now, not harsh at all. I figured that maybe it was just low on fluid. I checked the fluid level and it's fine. I disconnected the vacuum line to the modulator and it seems to be ok. Shifts a little quick and firm through all gears and downshifts beautifully. But, as soon as i get that line reconnected all hell breaks loose. I adjusted all of the vacuum components about 3 months ago and they were all within spec. Everything worked great up until this. I'm trying to find the modulator locally but not having any luck. Can anyone convince me that the modulator isn't the culprit?

No, I couldn't say it isn't a bad mod valve, sound's like it!

Try asking Phil here at Fastlane, I'm sure he can find you one!

A good modulator with low, or no vacuum is supposed to shift firm. Applying vacuum to the modulator softens the shifts...

Before you replace anything, measure the vacuum signal TO the modulator under driving contitions to be sure it is proper. If the proportioning valve adjustment has moved, it might be sending too much vacuum to the modulator at a given accelerator pedal position. More vacuum gives softer, or flaring shifts...

So, it may not be the modulator.

Check that vacuum again!

Thanks for the help guys. Turns out the little clip that holds the piece of linkage from the ip to the little valve above slipped off causing the rod to fall out. That will do it! I dont know how i didnt notice that before.