Well,

After having ALMOST everything replaced on the A/C of my car, they seem to have missed something. I figured I would go to the real pro's so when I chew my mechanic's ass until it's sore, I will seem like I know what I am talking about.

The A/C works for about 10 to 15 minutes blowing out of the left and right vents. Then for some reason, the center vents come on and the air becomes HOT. Not ambient....HOT!!! As if the heater was put on full blast.

If anybody has any ideas of what they missed, I would be glad to hear them.

If it's blowing hot with the temp set to cold, it's probably the monovalve (heater control valve) opening when it shouldn't. The two most likely causes are the valve itself failing (this is common, and the valve can be rebuilt for about $20), of the climate control failing (these also fail, but not usually like you're describing). Either way, it may not be related to the AC work.

Thanks Craig,


I have been reading up on the monovalve going bad and at first, it didn't seem like it might be the cause. I have seen monovalve rebuild kits online - is the monovalve pretty easy to get to or is it quite involved? I am mechanically inclined -I do "most" of the work on my car. Oh, where would the valve be located?

Thanks again!!

It's easy, the monovalve is the electric valve in the heater line on the firewall, right next to the battery. It gets 12V to close and 0V to open. If it fails to close, it will give you heat.

If you remove the 4 screws on the top, the internals can be removed. It looks like this:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41U40YBX4S&year=1983&make=MB&model=300-DT-001&category=All&part=ACC+Mono+Valve

This is the repair kit for $56:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1CQ0J3JZ41U40YBX4S&year=1983&make=MB&model=300-DT-001&category=All&part=Mono+Valve+Repair+Kit

I thought they were available cheaper, so check around.

Thanks again.

I guess the first thing I should do before ordering a repair kit would be to check the signal to make sure the valve is getting the 12 volts required to close. Any idea on where that signal originates?

I think I saw the repair kit for about the same price, but I will do a bit of shopping.

The signal comes from the push button control unit in the dash. When the temp is set to cold, you should get 12V at the valve. Some people have tried blocking the heater hose to verify the problem (carefully using vice grips, etc.). You can also check to see if the AC compressor is running and the AC lines under the hood are getting cold. If the AC is OK, it's probably just being overpowered by the heat.

The monovalve is powered through fuze 14 and grounded directly through the CCU in the OFF mode. In the remaining modes except DEFROST, it is grounded through the temperature control unit.