Yet Another Hot Air Issue, 1985 300D - PeachParts Mercedes-Benz Forum

neatokino · #1 06-27-2006, 08:22 PM

Hi all--

I'm having trouble getting the forum search to work, but I don't think this is a monovalve problem, so I'm going to just go ahead and present my problem and see if anyone can help. My 1985 300D (just got it, runs great, 175K miles, etc) had the not-unusual problem of hot air blowing through the vents when the climate control was in the 'EC' position. Initially, the climate control blew hot air in both AC and EC positions.

We changed the monovalve insert, but that didn't help. Then we changed the whole monovalve assembly, and that helped for a few minutes, then didn't help. Then we replaced the monovalve insert again, and it seemed to work fine, at least for a while. In any event, the climate control now blows cold air when set to AC.

Unfortunately, I'm still getting hot air in the EC position. Given all the work on the monovalve, and given that the AC is working, I'm assuming the monovalve is no longer the problem. Can anyone suggest what the next likely culprit(s) would be? I normally drive with climate set to EC, so having hot air blow through the vents this way is no fun at all. Any ideas?

TIA
Michael

tangofox007 · #2 06-27-2006, 08:48 PM

Could be the electrical connector at the monovalve. If the connection is broken (like when the connection is bad) the monovalve opens to the full heat position.

Jim Holland · #3 06-27-2006, 09:01 PM

Michael,

I just installed a "rebuilt" monovalve (monovalve repair kit) last night and, while the electrical connector was disconnected, sprayed both the male and female ends with electrical contact cleaner, then applied dielectric grease. I've gotten into the habit of doing this with all electrical contacts when I have a reason to disconnect them.

Maybe, as the previous poster suggested, there is something wrong with the contacts and cleaning/increasing conductivity is worth a shot.

Good luck with this. My AC is now blowing nice, cold air.

Jeremy5848 · #4 06-27-2006, 10:38 PM

I recall reading somewhere that, once the engine warms up, all of the air intakes and hoses and such get warm too, so that, especially if the car has been sitting in the sun, the climate control will blow warm to hot air in EC mode. I have found that, on a sunny day, even if the air temp is low, the car gets warm enough so that a/c is necessary.

However, if you are talking about genuine HOT air, then something is amiss with the mono valve. If the mono valve doesn't get a reliable 12 Volts, it will not stay closed (0 Volts = mono valve open). You can experiment by running an external wire that will allow you to put 12 Volts on the mono valve by turning a switch. If you can turn the hot air on and off by cycling that switch, then the mono valve is working. The problem is then the connector, the wiring to the ACC, or the ACC itself.

Jeremy