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Hi! My name is ***** ***** I can assist you with this. I may need some additional information from you first in order to give a complete answer.
Which thermostat do you have? What have you checked thus far and do you have a small digital multimeter for some testing? About $20 or so at most hardware stores and easy to use. I can direct you in this and we an sort this out. All follow ups are included in our fee.
White covers that snap on and off? Or gray cover?
Are these in a motorhome or a park model?
Did you check the main fuse in your load center and it may be labeled "furnace" as that is how most of these got power to the thermostat and control board? And if that checks out, which make and model furnace(s) do you have? I can tell you that these controls are obsolete and if you have to replace that thermostat, it will be an upgrade to the whole system.
The furnaces are controlled from those thermostats are they not? And those should have a fuse on the control board and that would be on the roof under the shroud.
The power for these comes though the furnace circuit on most all installations. That was my reason for asking about which brands you have.
Do you have a fuse marked for "A/C" in the load center? If not, I really don't know how they would have wired up the power for these. And yes, they are 12 VDC powered.
If you take one of these off the wall...........what, if any wiring comes into the thermostat except the ribbon wire? And I would try unplugging both ends just to see if the connections were the problem.
Take a look at this thermostat in this link and blog posting and tell me if this is the one you have. http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/20620892/print/true.cfm
Or look at this installation instruction for these A/Cs and tell me if the thermostat shown is the one you have? If you can pull the cover off and give me the part numbe that would help. http://www.bdub.net/manuals/DuoTherm_Penguin_610115_Install-Operation.pdf
Did you unplug the cables at the A/C end and try that?
The first link photo or the second?
Scroll down and look at the photo of the one with the cover off. Do you have red and blue wiring at the lower right hand corner as shown?
But, with the cover off do they still have the red and blue leads at the lower right hand corner?
And what is the part number on them up at the top left hand corner?
No red and black there incoming? Any wiring at all coming into these except the ribbon wiring?
And what is the part number?
That isn't the Dometic number.......it should start with "310" and some additional numbers.
The one in that photo was "3105356.004" and it should be similar.
I will see what I can find, but IF there are no other incoming wires except the ribbon cable, then these get their 12 VAC power from the A/C end. That would be where you need to check. They may even have a small transformer like some of the older Coleman units have.
I am adding a link to the upgrade kit if that ends up being what you need here. Not cheap! https://www.amazon.com/Dometic-Conditioner-Thermostat-3101***-***-****625-014/dp/B00BDEQUMO
Go to this link for a better price........http://www.adventurerv.net/dometic-duo-therm-comfort-control-center-thermostat-upgrade-kit-3307713010-p-20795.html
Let me know what you find at the A/C side. I might be able to get more information from Dometic Technical on Monday when I get into my shop.
See the attached parts list and part numbers for those older A/C controls. http://beamalarm.com/foretravel-links/PDF/dometic_penguin_manual.pdf
Not yet............sometimes it takes them a few days to get back to you. I might try them again in the morning and see if they have an answer.