1990 300TE blows hot and cold air simultaneously

Hi, my 1990 300TE after some amount of normal AC operation, will suddenly start to blow hot air at the same time as the cold air. This happens at random times regardless of the setting of the temperature wheel or push
buttons. I've taken the control box out and cleaned and resoldered any suspicious connections- didn't help. Any suggestions or pointers would be very much appreciated- thanks!
Tom Sharples
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I suggest you use a can of air to blow clean the air intake sensor for climate control system... where there is a cabin temp sensor... should be on the top ceiling... near the middle... small cube with fins...
Sometime dust settle there and block passage for accurate reading.
Another source of your problem could be the climate control unit itself.
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Sounds to me like the valve that controls hot water flow into the heat radiator under the dash is leaking. This would cause exactly the symptom you describe. The valve is mounted on the inside of the firewall above the battery, on the passenger side of the car, costs around $100, and is fairly easy to swap out.
Chris
Tom Sharples wrote:

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if it has a mono like my 82SD then unscrew the 2 screws that hold the top on. pull the magnet out and look for ANY rips in the rubber.
if it has leaked at ant time it will bind it up. i just got the new valve and replaced it. i know of people that get a new valve and put the guts from the new one in the old one for fear of breaking ANY of the plastic crap under the hood.
if you take your time things will come loose <well they did on mine> if your car doesn't have this valve OOPS
the case, minus a few cans!
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I have a similar problem (turn on the heat and I get cold air out of everything except for the upper side vents on my W126 - even the center vents open to put out cold air - but replacing the monovalve and cleanign the temp sensor did nothing for me. I'd be interested in hearing the solution. Richard

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