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??