Anyone with an 85 123 chassis Mercedes may benefit from my experience with the Klima system. After much help on this forum and by checking previous posts, I decided that any investment in making the original Klima provide airconditioning was a waste of money.
Here’s what I did.
I bought a Bosch ice cube relay, 30 amps, same as the one used to switch the aux fan. I removed the Klima (a $225 replacement that did no good whatsoever to restore the a/c) and using Radio Shack banana plugs, made three jumpers.
You don't need a Bosch relay; one from Auto zone will do just as well as long as it will switch more than 10 amps. More is better.
Using female crimp bullet connectors on the other end, I pushed the banana plugs into the following Klima terminals: the relay coil into ground and the 12 volts from the ccu. Everything fits nice and tight. The switch side of the relay went to 12 volt constant, and I wired a long 14 guage lead to the normally open side of the switch. Ergo, when the ccu selects anything except the economy mode, the relay kicks in. The long lead (I used a wire wrap from Auto Zone to protect it and make it look neat) goes right to the positive lead of the compressor instead of the blue lead from the Klima. Brown (ground) stays the same. I used a female spade lug for the compressor connection.
Voila. Select air and the relay kicks in the compressor. The blower speed and defrost work as normal/ On the advise of my parts supplier (Rick at Newton
******** in N.C.,) I wired the aux fan into the same hot lead. The relay will handle it, don't worrry.
Good bye flakey Klima, with its intermittent and inexplicable shutdowns.
You will loose the automatic temp control, but I have a simple answer. I select the lower fan speed and the hi-low button (to the right of defrost) to keep from freezing. Everything else works normally.
To keep things neat, I stuffed the whole works back into the Klima relay cover and replaced it.
Instead off sweltering, cool air.
Your compressor is still protected by the high and low pressure switches upstream (which cut off the ccu). Your fan comes on right away without waiting for the temperature switch to kick in. My engine temp runs below 120 even in traffic while I enjoy cool breezes.
Several things to watch for:
Use a meter or test light to isolate the control voltage from the climate control unit (the thing in the dashboard that looks like an old Grundig radio and is probably made from the same crappy components). Don't allow the ccu to switch the compressor or fan directly. It will blow out big time.
Mark the control terminal and allow it to switch the relay only. There's a 12 volt constant on the Klima socket that will be ample to power the compressor and fan.
You don't even have to buy a relay. If you are bypassing the aux fan relay, you can use that one.
Keep your wiring neat and watch for shorts. Use short leads on the bullet connectors and banana plugs. Be sure the insulation covers them and crimp well.
And this solution is only really safe for the 1985 123, as far as I know, because some later models used the Klima system to monitor engine speed and protect the engine from belt failure. If the serpentine belt breaks or slips because of compressor problems, you may have a long walk home.
The Klima system on the 85 is much more complex and expensive than that of the preceeding 123 models, which I fixed by jumping the temperature sensor behind the glove box.
This fix is simple, cheap and reliable.
And if anyone wants to buy a new Klima relay before I put it on ebay, lt me know. If you want the terminal identifications, email me. I can only tell you what works for me.

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Stage Wizards Lighting and sound designer
1981 SD 328,000
1985 300TD, 266,000
1984 300 TD, 267,000