12V to Monovalve--AC or Heat--Bad CCU? - PeachParts Mercedes-Benz Forum

Rooster300SD · #1 10-09-2009, 02:06 PM

I have no heat except through the defrost vent on my 84 300SD.
I'm getting 12-13V to the monovalve plug if I turn the temp wheel
to heat or cold.

do I have a bad CCU unit?

Thanks,
Jeff

tangofox007 · #2 10-09-2009, 05:46 PM

Quote:

Originally Posted by Rooster300SD

I'm getting 12-13V to the monovalve plug if I turn the temp wheel
to heat or cold.

That is perfectly normal.

Cr from Texas · #3 10-09-2009, 06:22 PM

Sounds like a vacuum issue. The default mode on the dampers is defrost. Unplugging the electrical connector at the mono valve gives you full heat.

Jeremy5848 · #4 10-09-2009, 06:25 PM

The monovalve always has system voltage (12 Volts) at one of its two pins regardless of the temp wheel setting. It's the ground pin that the CCU modulates to regulate the flow of hot coolant through the heater core. Heat at the defrost vent is the default setting. It means that the CCU is effectively "off."

Rooster300SD · #5 10-09-2009, 07:00 PM

I'm getting air flowing through all my vents.
The pods and switchover valves are all good.
My AC works fine I'm just not getting heat.

I took out the monovalve and it looks just fine.

I had read somewhere here that the voltage to the monovalve
is 0 for heat and 12 for AC. I believe that the logic for that was
that you would always have heat for defrost even if you lost
power to the monovalve.

I still need to check the aux pump to see if it's working.

Jeff

Jeremy5848 · #6 10-09-2009, 07:11 PM

I'm confused. In your first post, you said "I have no heat except through the defrost vent." In post #5 you said "I'm just not getting heat." Which is it?

Heat through defrost only means the pod/flap system is not working. No heat at all could mean a stuck-closed monovalve or a clogged heater core. If the cooling works then it sounds like the pods and flaps are working, at least in that mode.

tangofox007 · #7 10-09-2009, 07:19 PM

Quote:

Originally Posted by Rooster300SD

I had read somewhere here that the voltage to the monovalve
is 0 for heat and 12 for AC. I believe that the logic for that was
that you would always have heat for defrost even if you lost
power to the monovalve.

When analyzing the operation of electrical equipment, a schematic will serve you much better than logic and hearsay. The schematic will tell you that the monovalve is controlled via a switched ground. A loss of power would have precisely the same affect as an open ground circuit.

Rooster300SD · #8 10-09-2009, 07:29 PM

Yes, as I said in my first post, I only get heat through
the defrost vent when I set the temp wheel to max heat.
Only ambient air blows through all the other vents, not hot air.

When I unplugged the monovalve nothing changed--
defrost heat and no heat through other vents.

The AC is working great through all vents.

Jeff

Jeremy5848 · #9 10-09-2009, 10:27 PM

Check the wiring to the inside temp sensor. I don't know if it's in the center of the dash like the 123 (access through glove compartment) or in the overhead with the dome light like the 124. If one of the wires is broken, the CCU won't work correctly.