Don't know about 83 300SD, but my 84 300D has a relay for the AC clutch. It is under the plastic cover on the L inner fender. The other relay there is for the radiator fan. My 85 300D has just the fan relay since a Klima box controls the AC clutch. I know SD's have a relay box inside the wiper tray. In many cars, the lid of the relay box documents each.

Why not check battery draw-down w/ the starter out? Unlikely it would be a draw since the solenoid is a pretty robust on or off connection. I agree that the alternator could be a draw since all that prevents reverse current flow is the diodes, which always have some leak (usually negligible). Main suspects are always the glove-box lamp and trunk lamp. Insure those switches close when the lids are closed.

I read a while ago that newer cars (say 2005+) have many more parasitic loads. So much so, that when people park for 2 weeks, their car won't start. That keeps airport parking lots busy giving jumps. But, there are now small lithium jumper batteries which fit in a glovebox. They will start a gas V-8, but don't know about an old diesel.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans