Heater/AC Problem, HELP !

[Leewolf]

I have a 79 450 SL (grey). there are two problems with the climate control system that seem related but I can not find the common thread:
1. The AC does not work from the control panel "normally". If I "hotwire" 12 volts to the compressor clutch the AC works but the compressor remains engaged until I disconnect it from the "hotwire".
2. The heater does not work until I disable the vacume to the hot water input valve (located at right rear/top of engine) that allows water to the heater core. Then everything works at the control panel.
Both problems seem to stem from some control related problem.
I'm stumped, any help/suggestions would be appreciated. I live over 100 miles from the nearest mecedes dealer/garage.

Heat/AC on my 1980 450SL would not operate at all. My mechanic bypassed the "main" vacuum switch with a toggle switch mounted under the dash and everything started working fine. He indicated that sometimes the older the car the greater the tendency for the vacuum level to flucuate causing problems. This solution was ok with me since it was cheaper and could be reversed and fixed according to the book later if I wanted. Hope this may give you some options.

[Leewolf]

Thanks very much for your reply. I have the same problem you described, nothing worked and as I indicated I located two "fixes" to at least find that the compressor worked and I can get heat. BUT, I also have problems with opening my trunk lid after locking the car I suspect that a vacume problem does exist. Could you tell me where the "main vacume switch" you refer to is located and mabey how the toggle switch was installed, that is, into what circuit and where located. Even if I can't do it myself I have a mechanic a few miles away who used to work in a mercedes shop in Kansas City. He told me he wouldnt touch anything climate control related that was under the dash, but if I could point the way he would probably help me out. Again I really appreciate you response and hope you may be able to supply the additional information
Sincerely Lee Hazel

Send me an email and I will email a printscreen from the CD manual showing the 3 (or 4?) vacuum switches under the dash behind the glove box. It may take a couple of days for me to reply. Not having actually seen the way he fixed the wiring, my understanding is that the wire leads were removed from the vacuum switch and redirected to the toggle switch mounted in a small hole drilled in the lower dash panel next to the console.