Testing 3 wire Dometic Thermostat

Just went through this and not too bad if yours is the same as mine in a 2015 Thor ACE. Go to your board and pull it out and unplug the six wire connector. Turn thermostat so the heat should be coming on, have your multi-meter set to check DCV. Place black test lead on black wire and the red lead on the blue and you should see 12 volts.
If you have that it should be the board, look at the bottom with all the little solder joints. If any of these are discolored this shows a short and needs replaced.
Any board I have found bad I have replaced with "Dinasour Electronics." I recommend them because they are a good product with great customer service but that is my only connection with the company. Give them a call and a technician will troubleshoot over the phone and if it is a bad board will recommend a replacement. If you do need a new board they do not sell direct so Amazon is your friend. Make sure you send in the warranty card, I just had the replacement board go bad and they said to just send it in they would look up the warranty. They replaced it even though it was caused by water damage with a note they only warranty once for water damage. To me that is a very good warranty and it is a three year warranty. Jim.

Perfect, thank you Jim.

Your Roof AC fan comes ON because you have the Tstat set to Auto. This is normal. Set Fan on Tstat to Low or High. Depending on HOW the OEM wired the AC and furnace 12 volt will determine what your problem is. DOES the roof AC function in all modes? It may be as simple as the furnace 12 volt fuse is blown. You have 2 blue wires at the upper AC control board. BOTH are related to the furnace Tstat operation. rv's will have 12 volts on one of those blue wires. Makes no difference which one. When you activate the furnace the AC board on top closes the furnace relay on that board and the 2nd BLUE wire now has 12 volts. Going back to the furnace to engage the furnace controls. IF you have NO 12 volts on either Blue wire the problem is between the Furnace and the upper AC control board and is NOT the wall tstat or AC control board. SOME Oems do NOT use the furnace tstat wires(furnace has 2, 12 volt OUT and then 12 volt IN from the AC board) for the 12 volt power to the upper AC control. Very rare but some do this. It would be fused at the main interior 12 volt fuse board. Doug

thanks Doug. The roof AC fan functions as it did before when on Furnace. If in auto, it comes on when it is requesting the furnace. if in low or high, it runs constantly at the appropriate speed.

ticmjpm wrote:
thanks Doug. The roof AC fan functions as it did before when on Furnace. If in auto, it comes on when it is requesting the furnace. if in low or high, it runs constantly at the appropriate speed.

Sorry, had it backwards. Auto is for NO AC fan on furnace. Low or High if you want the AC fan to run when furnace is on. Doug

IF your AC fan runs when furnace is selected and fan set to Auto, you have a problem other than just the furnace not operating. I have never tested this, but I don't know what happens on AUTO if the AC control connections for furnace have NO 12 volt supply on the blue furnace wires .

C. “Furnace” - Furnace Mode
1. There are 3 fan speeds in Furnace Mode.
a. “Low”: Fan operates continuously at low speed.
b. “High”: Fan operates continuously at high speed.
c. “Auto”: Fan is Off.
?If additional indoor air circulation provided by the air conditioner is NOT desired during
Furnace Mode of operation, select “Auto” in the Fan Mode to shut the air conditioner
fan off. If “Low” or “High” is selected, the air conditioner fan will continue to operate at
selected speed.
2. In Furnace Mode system will cycle RV furnace On and Off based on room air temperature
and temperature set-point on CT thermostat. CT thermostat can be configured to operate using an On / Off differential of either 1ºF or 2ºF. This feature is programmed during the system
initialization. See “B. System Initialization” on page (3).
3. To set temperature differential, system must be Off. Press button and simultaneously
press and hold / Mode button for three seconds. Press button to toggle between “d1”
and “d2”, “d1” for 1ºF differential and “d2” for 2ºF differential.