I have spent the past three days pondering the ACC woes on my '84 300D (123). I just bought the car, so I don't know how this problem developed, but right now there is no response from any of the buttons on the automatic climate control unit. That's not true. I can get one response, but I'll tell you about that later.
Nothing on that unit makes the blower blow, and no I don't hear any clicks when I press any buttons. I took the darned thing apart and checked for poor solder as per the tutorial found here: http://207.210.95.34/~boostd/Climate/. Unfortunately, I saw nothing (and, actually, I didn't know what to look for since that tutorial is a little vague about that). So, there doesn't seem to be a problem with solder, but who knows?
Anyway, maybe you're thinking the blower doesn't work. It does, actually. I know because I can rig it up to blow by connecting a wire from the battery to the upper right pin of the connector that should attach to the Blower Resistor (none of these pins light my test light). When I rig this up, I get hot air out of the vents. Which vents depend on what I do with the ACC buttons (this is where the reponse comes in). When the empty circle button is depressed
(off), I do get hot air out of the center vents and the sides. When any of the other buttons are depressed instead, I get hot air out of only the side and defrost vents. Folks have suggested that this is vacuum-related, but I don't have a mightivac or the money for one right now, so I am hoping somebody will read this an recognize that the confluence of problems in the ACC is indicative of one, overarching problem which superseeds that of the vacuum.
By the way, I checked the heck out of the fuse(s), so that's not the problem.
Now, I live in San Diego. I have no interest in hot air coming from any vents in June, so this rigging up does not appeal to me. I also have no interest in running cold AC (I never use it in my other vehicle). I just want a fan to blow fresh air on me when I am driving.
None of the other threads about ACC seem to apply to me, so I ask you, strangers, have you any idea what's up with my ACC?
Mike Rancourt
ACC = strong college basketball onference; weak climate control unit.

Get a fresh set of fuses. I mean replace absolutely every fuse in the fusebox. Don't look at the condition of what you're replacing. Throw out the old, install new.

You can get fuses through the FastLane link above. If you want to do this now and don't like MB dealership prices, try a VW dealership or a store that caters to aircooled VWs. There must be such stores on every block in SD

Sixto

I can not help you on the switches. But I do know a common problem is a valve in the heater hose. It mounts on the firewall I believe just to the right side of the engine just below hood level. There are two wires that plug onto it. It is a shutoff for the hot water supply. The diaphragm goes bad. Most of the time it gets stuck shutoff but it could be stuck open giving you plenty of heat to blow around. There is a metal cover that removes with four screws. You can take this off to check it out. But be careful If I remember right there is a spring and other components you could loose. If it is bad you can usually pick up a kit to repair it on ebay for not to much cash. At least I did for my 81 300d. But mine was in northern MN in December. I wanted Heat! : )

I hope this helps. at least a little.