The electrical connection to the mono valve was not floowing properly. So, of course it was blowing hot air on all settings. I rigged a connection so that the mono valve is activated whenever the key is turned on. Activating the mono valve stops the flow of hot coolant into the heater core. Now that we'll be getting cooler weather, I'll want to be able to turn on the heat without opening the hood to manually disconnect the mono valve.

What are the common reasons for the failure of the electrical connection to the mono valve? Also, I've heard that the switch is to the ground connection rather to the positive electrical connection. Is this true?

I read long ago,some add a fuse to it,some how you car can catch on fire if the valve gets stuck.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

What car and year? It is true that the switching is done on the chassis/ground side. Why it matters which side it switches if all electronics is up to par? It could be a fuse to the climate control box. I would suggest fixing the electronics, most likely the climate control. There is nothing wrong to put a 12V to the mono valve to hold it close. You can wire a switch into the cabin. I have done it before until I fixed the climate control.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

Do you mean that the original factory connector to the mono-valve was bad and you connected 12 V directly via another connector (home-rigged or junkyard?)?

Yes, one monovalve pin gets 12 V all the time (w/ key on) and the other pin is shorted to ground to turn it on. Several things can short it to ground. Per my car's schematic, the push-button box does that when the rotating wheel is clicked to the "lowest temperature" setpoint (blue). Otherwise, the TemperatureRegular box controls it, and I don't know if just on/off or proportionally.

My 1985 300D was blowing hot air sometimes this summer, even w/ the wheel at "max cool". I know the AC worked since the clutch was engaged and the return tube cold. The heater was just over-powering it. I rigged ring terminals to measure the monovalve dV. I also tapped the Regular box pin to the monovalve and shorted it to gnd in the cabin. Either with or without the later, I measured 12 V across the monovalve, yet it blew hot. I finally swapped in the innards from a junkyard monovalve and no more "blowd hot", though I only drove that car a few more times last summer. While out, I tested both servos and the junkyard one seemed to push out the stem more aggressively with 12 V applied. The former one looked OK, but was perhaps too wimpy to overcome coolant pressure and close the valve. I recall it was an MTC replacement. But, another variable is that I use Evan's Waterless Coolant and sometimes run with the cap loose so no pressure builds up, and didn't do a with and without on both valves.

Worst-case, if the W123 mono-valve bothers you or is $$$ to replace, people have wired in a later W124 mono-valve (cheaper, maybe better) where the "aux water pump" sits. That is a useless device you can do without. Also, that is the culprit in "dash on fire" mentioned in post 2, so fuse it if you keep it.

Oops. I am assuming again. Greg didn't tell us WTF car he has. Why do they do that? His might be a W124 already.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

On mine the pins in the connector were so corroded they snapped. I went to get a spare solenoid switch from someone tearing an engine out of a w123 to put in a Toyota truck. When he went to unplug the connector, both pins broke off and were stuck in the plug. The next one I got, I had the fellow cut the wires leading to the plug and solenoid. I'm not going to try to unplug it, ever.

I was able to get mine with the two broken pins to work properly for a couple of months by putting a very short piece of folded copper wire into each of the broken and hollow pin halves in the solenoid that then went into the hollow pins left in the plug. The connection was good enough. Messing with one of the bad ones I have, it looks like once the broken pins are removed from the plug, they could be replaced with 10 gauge solid wire.

1985 300TD

Sorry, it's a 1984 300CD.

Similar happened on my '83 300CD. I too had to rebuild the monovalve connector. Has been fine now for about 4 years.