Hello.


Can anyone tell me under what conditions I should see 12v at the solenoid connection to the monovalve? I would assume that I would see it when the heat is turned on. What configuration of the CCU would you use if you wanted to verify 12V to the solenoid? This is a '90 300SE

Y'know, I think it's pretty ironic that MB uses vacuum in places where every other manufacturer would use a solenoid. In the one spot where every other manufacturer uses vacuum (heater valve) they chose to use a solenoid.

I am having heating issues and I think the CCU might be suspect. I just replaced the monovalve solenoid.


Thanks!


-tp

It'll click on and off every few seconds during normal operation. You can warm the engine and set the climate control to heat, say 80 degrees, and it should turn on and off every 3-5 seconds or so. If it doesn't come on at all, turn the heat all the way up. If that doesn't cause the voltage to cycle, then the climate control head is not operating properly.
In a properly operating momovalve, you can just put your finger on the top of the plunger and feel it click every few seconds.

DG

Panzer, I just read your other thread, with several comments by my Dallas co-conspirator - Hit Man.
Yeah, it's gotta be the monovalve, and the plugged screen sure wouldn't help.
Sometimes you can't really see a defect in the valve or diaphragm, but it just doesn't open properly. It will really thump when it's popping open and closed properly.
There is also a small auxillary water pump next to the monovalve, that actually will turn on to pump more hot water during prolonged idling in very cold temps. I unplugged mine years ago when it started to mis-behave, and have not noticed any difference here in Dallas. I doubt that is your issue.
If you are getting air from weird vents, it could be your vacuum dashpots leaking.
I'd try the monovalve kit first before messing with those. Let us know how it works out - I've replaced all that stuff over the last 18 years, so we can walk you thru it if the valve doesn't fix you up.

DG

Wow, I really appreciate such expert help. Thank you all very much.

I installed the new monovalve solenoid this afternoon. This evening's drive home will reveal how successful I was.

That's interesting that it opens/closes so often. Most cars I've worked on it's either open or shut. I guess that's why they went for a solenoid instead of a vacuum pod.

As for the air coming from weird vents, it's intermittent and difficult to put a finger on. I am betting either the vacuum pods, or the CCU. I've seen other posts where people talk about bad solder joints inside the CCU. Does that apply to my car, or is that a different version of the CCU? I am quite handy with a soldering iron and good at getting small things apart.

If this was your problem to fix, which would you check first? Which is less of a pain to get at?


Thanks again for all the great help.


-tp

SOLVED -

Yes, it was the solenoid. I just had to know now, so I went outside and fired up the engine.

At first I was discouraged, as I couldn't feel the solenoid moving from putting my hand on it. I thought I was SOL with a bad ccu. But by the time I went back inside the car, the heat was blasting out of the vents. The dash temp guage hadn't even moved yet. This is 1000% better than what I had. It was taking many miles of driving to even get slight warmth. Now I am getting heat very quickly.

It's gonna be below freezing tonight!

Thanks so much for all the help!!


-tp