My 1990 300E has icy cold a/c, but suddenly it's started blowing hot air after the a/c has been on for awhile. I've been looking for a pattern to this, because sometimes I can drive for 20 minutes or so with nice cool air, and other times the a/c blows hot right away. With the temp. select wheel set to 65, fans on auto, and climate control button set to EC, the fans seem to run really fast, regardless. If the normal button is depressed, the fans quiet down. Changing the temp. select wheel doesn't seem to have any effect.

I've taken it to a shop thinking that it just needed more freon, and they gave it back saying the freon level is fine, and they didn't have a clue why it was blowing hot air. I noticed that when it blows hot, it's sometimes a lot hotter than the outside temp, as if the heater is kicking in after the car has cooled down. I confess to not being sure exactly how the a/c works, so I'm not sure if this is a thermostat issue - like the once the car is cool, the heat's coming on to raise the temp again? Is the a/c & heat dependent upon or reacting to the outside temp , or only inside temp? Any suggestions appreciated.

sounds like a monovalve. millions of post on that, and it's pretty cheap and easy to fix. this is from the diy page.

http://www.peachparts.com/shopforum/tech-help/138522-monovalve-problems.html#post1026761

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Eric Zacharias

'88 300 SEL
'64 Ford Falcon
'99 Infiniti QX4

May be as simple as a loose connection on the monovalve.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html

Could also be a bad cabin temp sensor blower motor (behind the right dash vent) or a hose fallen off, so that the interior air temp sensor doesn't have air blowing past.

Other possiblities are a bad wire so that the monovalve opens when it shouldn't, and someone pointing the center dash vents up -- this blows cold air right from the vent onto the temp sensor (it's beside the sunroof switch) and will turn the heat on. Keep them pointed down, and if it's too cold, close the vent down, don't point it up. You could also have a defective pushbutton unit.

Most likely is a bad monovalve or wire to monovalve.

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks for the replies! I checked the monovalve by pulling, cleaning and reconnecting the wire, then tapped on the top of the unit to see if it was simply stuck. The heat hasn't been coming on since then, so I'm thinking y'all were right. I also noticed that the center vents don't seem to have air blowing out of them, regardless of climate or fan settings, so think the blower might be gone as well. I want to go ahead and rebuild the monovalve when I get into that blower motor, but haven't found a repair kit as described. Any leads on where that can be found?

Thanks again for the help, y'all rock!

There are available for other models like the 126's. May try a used one...

Good luck!

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J.H.
'86 300E