I bought this 1985 300DT 173K miles recently knowing that it had AC issues. I had done some reading online and found that often when people say that the AC blows cold then hot it is the monovalve sticking open. I was planning on replacing this, however, recently I wanted to turn on the HEATER to get newly added coolant distributed through the whole system, and when I tried to turn on the heater the AC came on and no matter where I put the internal thermostat (all the way hot or all the way cold) it stayed on AC and blew cold. Now, am I right in assuming that this blows the monovalve diagnosis out of the water?

I know it isn't the compressor, because I can hear it engauge (at times). I know it isn't freon, because it blows cold sometimes (not in any predictable way). So, what could it be? Is there a thermostat somewhere in the system that tells the car to blow colder or warmer air based on the position of the internal thermostat that could be blown? I still might change out the monovalve just to see if this helps. I invite ya'lls thoughts and inquiries...

-Chas.

Compressor stays engaged in all positions except econ. or off. The system mixes cold and warm air to stabilize in car temp to what you dialed up on the wheel. When the diaphram tears in the MONO valve, the heater might produce warm air at idle, but will go colder as driven. Verify that the diaphram is intact within the valve. If not, replace it. Then report back. If intact, you might have air in the cooling system or the aux. water pump might be inoperative which would keep the heater from getting very hot.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

So, I finally replaced the monovalve. It was in pretty bad shape. The rubber was completely worn and warped. I also recharged the AC. This worked at low speeds for the first couple of days. When I'd get above 60 the AC would get warm again. Then, it just stopped getting cold all together. I checked the 134a level, and it was a little on the high side. Then I went to the mechanic and he said I was low on freon. Humph.

When I replaced the monovalve I used a rebuild kit. The copper wiring that wraps around the valve was wrapped in tape. I'm wondering if this could need to be replaced as well...

So in an interesting twist of events, just went out to start my car and smoke flew up out of the engine after this aweful squeal. Looks like the compressor is out. So....perhaps that was the problem all along? I'll write back when I know...

Edit: Does anyone know if I can just cut off the belt from the AC compressor and drive around like that?

You could do that.

Thanks. Damn, it's been a hot summer.