Disconnect Monovalve = Heat? Why?
 

Here in So Cal, we don't need the heats so much. So as part of my fenagling with the new SD, I swapped monovalve with the 300D which had itself been recently replaced (innards only). Both magnets tested good (.015 ohms). But both valves were also intact - the rubber was solid, no evident damage. From the manual, it appears that the monovalve is de-energized during vent operations at min temps. To test, I disconnected the electrical connection to the valve. I _still_ had great gobs of heat coming from the vents.

So what does monovalve control, if not the flow of water to the heater core?

Heater valve
 

1984 300d
the heater monovalve is on to turn off the hot engine water
off when heat is needed, pull the plug to test the valve for heat

If you disconnect the monovalve then it stays open and you have maximum flow of water! if you connect the monovalve directly to the batery it will close and you won't have the heat at the vents!

Disconnected=heat.

i was told that for liability reasons all mercedes climate control systems will always defaut to a heat mode during major component failure or total vacuum or power loss.

the example i was given was if you are in freezing temperatures and the system fails and cannot provide heat or the ability to clear the windshield etc.. the customer could be killed or injured in an accident. they could also freeze if the system is not providing heat.

on the other hand it was said that if you were in the desert and the system failed, no a/c but if the heat were on it would be uncomfortable but would not under normal cicumstances cause an unsafe of fatal condition.

this is what i was told, i have never actually seen it in print

so by unplugging the monovalve you are removing the power and it opens allowing coolant to enter the heater core

Not really liability, just a "fail-safe" design that Engineers prefer.

Exactly....and also a very good way for it to default to for those of use in Michigan! :D :D :eek:

Makes sense to me, but this doesn't....
 

So I wired to solenoid to "energized" and voila, no more heat! Thanks for the explanations.

Then I took out the CCU, resoldered the connections, and voila, no more heat! Did I mention that I reinstalled the original connection? And still no heat!

Time for a new thread....

Thanks for your help. Nothing more to see here. Move along.

True. My CC vacuum connections are all unhooked, I only get heat through the defroster and side vents(All I need this time of year)