I have found a vacuum leak from behind the auto climate control panel on my 1983 300D. It seems to be intermittent only coming on when the heater/temp dial on the climate system is turned up or when the ignition is turned off.
When the vaccuum leak starts, it causes the transmission to begin shifting hard, probably because the modulator is running on less vacuum.
I have access to behind the climate system controls and found a small hole in a hard to describe part...doesn't look like anything was supposed to be hooked in there and it seems odd how there appears to be a cycling switch in direct relation to the climate system which opens and causes the vacuum leak...
As long as the climate system is set to the coldest setting on the dial, or just turned off, the vacuum leak does not happen until the ignition is turned off and then the engine continues to idle until I go under the hood and manually switch the engine off...
Any Ideas??

There are switchover valves behind the climate control panel. One of those could be leaking.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I do believe you are right about the "switchover valves." I snooped behind the control panel and found a small hole which I can put my finger over to stop the vac leak. The hold only about 1/8" diameter. Is there a fix for this or is it likely the valve is defective??

I've never had one out or troubleshot a problem with one. I've only seen them. Unless there's an obvious vacuum line disconnected from the valve, I'd probably replace it.
By the way, to confirm that your climate control is causing the hard shifts, you should be able to hit the far right button, shutting off vacuum to the climate control system and your shifting should improve.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Thanks for your input...I am also noticing the engine will not shut off when I turn the key to off postion. When I turn the key off, the same vacuum leak starts and the engine will not die.
I have to open the hood and turn off the engine manually using the "stop" lever on the throttle linkage...
Any ideas???

Push the far right button on the climate control and see if it stops the vacuum leak.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Pushing the far right button does not stop or prevent the vacuum leak..
Strange though, the temp dial on the controls seems to affect when the leak occurs. if i run the AC the vacuum leak does not occur, but if I turn up the heat dial (warmer) you can hear a faint switching sound and the vacuum leak starts.

As with turning off the key, doesn't seem to matter what I do, it just leaks and the engine generally will not stop.