I've been slowly chasing a climate control problem in my 1984 300SD for about 6 months...
http://www.peachparts.com/shopforum/showthread.php?t=68325
When the A/C kicks in - it cools very well. I thought I had an issue with the push-button climate control unit as it would cool the car down to about the point where you had the temperature dialed on the temp wheel and then the whole system would go "click" and go dead. Dead as in no control of the climate control using the push buttons or the fan speed (no response). Then after 15 seconds or so the unit would "click" and slowly come back to life and start cooling the cabin down.
In an unrelated event, at some point this summer the A/C stopped being directed towards the center vent and now goes through the dash defrost vents (ugh).
My Climate Control was actually a Programa rebuilt unit out of Boca Raton FL. Luckily I had 6 months left on my 3 year warranty and I sent it in. About 10 days later I get my unit back with no problem found. Hmmm.... They suggest unplugging the auxiliary water pump as it tends to give the Climate Control fits (if not damage the unit) if it has seized.
I test the system with and without the aux pump plugged in. I still think my Climate Control unit is the issue and send it back in. The folks at Programa must have felt sorry for me and send me back another unit (exchange) as a courtesy. Nice customer service – it makes me feel better at least.
So by the time the next Climate Control came in I have a new auxiliary pump installed (and I did a monovalve rebuild kit at the same time since it's all on the same bracket). No difference. Programa said the next suspect part to investigate would be the Blower regulator or the temperature regulator. It took me some looking, but I found both of these parts attached to either side of the blower under the glove box.
This past week or so I also diagnosed and replaced two of my switchover units (#1 and #3) on that common vacuum rail thingy. Now I'm able to get air through the center vents on max (push button setting right of the defrost setting), but as soon as I push the normal A/C button, the center vents slip close. With the glovebox liner out I can watch the flaps behind the glove box open on max and slowly slip close on normal A/C. If I put a vacuum directly on the pink egg vacuum pod thingy deep in the center of the dash (hooking directly to the orifice that leads to the #3 switchover), it won't hold vacuum. So I assume that pink / green pod diaphragm is shot????
Tonight as I tried to see what flaps were working the climate control was really doing odd things. I had the car running and set the climate control on Max A/C (temp dial all the way blue), and the compressor never kicks on. I just get cool outside air. When I move the dial all the way to heat I hear a weird little cycling sound and no heat (or blower speeds). I turn off the engine but keep the key engaged and I track the cycling hum to the monovalve opening and closing fairly quickly. Super weird! I give the car a few minutes break. Start it up again and I get the compressor to kick in and A/C runs cold and the heat works as well. I swear it's possessed!
So.... back to my maddening climate control. Do you think it is the blower regulator or temperature regulator, both.... or something else?