I, too, am troubleshooting an 85 300D A/C challenge...here are some places to start that I'm currently going through...

Low Charge switch (on receiver) keeps compressor from engaging due to low freon - easy check with meter for electric side, check pressure in system for appropriate charge.

Evaporator switch kicks compressor off if evaporator temp <32* to prevent icing of evaporator - easy check w/meter

Throttle position microswitch on valve cover - easy check w/meter

KLIMA relay located just in front of fuse box / brake booster under a plastic cover, digests info from kickdown, throttle, pushbutton unit, low charge & evaporator temp switches to determine whether to engage clutch on compressor - wish it was easy to verify working order, I havent been able to find a good wiring diagram or test procedure specifically geared to 1985 300D / KLIMA # 001 545 74 05

Push Button Unit - poor/broken internal solder connection(s)

Climate control unit (solid state unit behind glove box) poor/broken internal solder connection(s)

Most all of these have extensive threads in the forums, that said I'm still diagnosing min. When/if I figure it out I'll post the results.

I'm all ears for where else to look, forum guru's...