Thermostat not getting 12v

Sounds like you have a poor 12V connection from the fuse box to the furnace.

When you ran the jumper wire to test the thermostat and A/C, did you also connect the 12V wire from the furnace up there at the thermostat to the jumper? If there is a poor supply wire to the furnace that would send power from the jumper back down to the furnace to override the original connection. If that works you need to find the bad connection from the fuse box to the furnace.

Did you try running a jumper wire from the fusebox to the furnace instead of using the one that pulls down to 1.5V?

For the wire nuts you tightened, did you remove them and inspect the wires? They should never need tightened and I suspect the wires may be damaged. Make sure all of the strands are intact, particularly where the enter the insulation. If you pull the wire nuts you may need to cut the ends and strip them back a bit further to ensure a good connection. The wire nuts do not make the connection. Twisting the wires together is supposed to do that and the wire nuts only hold the twist in place.

Hey guys, So I still haven't figured it out. I tightened some wire nuts and got the 12v supply to give 12v. However whenever I go to test at the wire nut junction on the furnace RED, T-RED, +BLUE and 12v+ wire nut it is reading 1.5v until i remove the 12v+ wire then it reads 13.6v on the 12v+ wire. So there is an immediate voltage drop whenever I join all of the wires together. I tried jumping the 2 blue wires together but the furnace does not kick on. I also tried putting a jumper wire from my fuse box to supply 12v (or more accurately 13.7) to my thermostat. Whenever I did that My ac kicked on when set to cool, but my Furnace would not kick on. So to me that shows that the Thermostat and rooftop AC unit are both functioning. Which leads me to be believe that there is a wiring issue or connection issue or something in the furnace since the 12v is starting there and moving from the furnace to the thermostat and the ac. I am sort of at my wits end.

Have you checked your meter on some other 12V source to make sure it's reading 12V accurately? Bad battery in meter or wrong setting on dial can screw you up.

So you are going from 13.7v at the fuse box to 2.5v on an orange/white wire in a drawer to 1.67v at the furnace? that's some serious voltage drop.

If you pull the furnace fuse what else goes dead in the trailer?

When you test 13.7v are you getting that 13.7v on both sides of the fuse? I'm assuming you are testing in the little windows on the back of the fuse while it's plugged into the panel?


can you get behind the fuse panel? On my trailer I can remove a drawer just above the panel and access the wires right behind. If you can get behind the panel is the orange/white wire back there?

I traced the power source for furnace to a wire nut union beneath the a drawer that is orange and white and coming out of the wall. It reads 2.5v, I cannot seem to find a orange and white set of wires on the fuse box anywhere so i am not sure where these wires are coming from.

I Tested all of the Fuses, they all read 13.7v. I will do the test with the wires next.

Did you do any additions to the trailer that involved screwing or nailing into anything? If so you probably hit a wire.

First thing I would do in your situation is identify and make note of your power. Since no battery start at the fuse box. The furnace is 12v so ignore the breakers and concentrate on the fuses. Take a reading from positive to ground at the fuse box and write it down and let us know what it is. Is it 12.7, is it 13.6, is it 12.1, is it 11.2 what is the voltage it does make a difference.

After that I would take that wire nut off the red wires and separate the wires.

The small red wire is obviously the thermostat.
The blue wire is the furnace.
I'm assuming the large red wire goes to the blower.
So that leaves the white wire as your suspected 12v positive.

Make a temporary connection to test. Under the wire nut add
the small red wire
the blue wire
the wire that goes to the blower (assuming the red one but if it's the white then use the white)
Your jumper wire back to known good power
Leave out the wire that is reading 1.8v, just tape it off.

Now does the system work?

With only 1.8V from the 12V supply you need find out why. The options are bad connection, cut wire or bad fuse including fuse contacts.

Again a wire tracer can be very helpful. Buy, beg, borrow or steal one. OK maybe not the last option.:B

Tried running the jumper still getting 1.8v from the red wire on furnace and a good ground. All of the fuses I’m seeing are good. I’ll try jumping the two blue wires and see if it starts.

bottom furnace picture.
The RED(large gauge) and RED small gauge are connected to the 12 volt positive supply as well as ONE of the BLUE. THAT connection supplies 12 volt positive to the FURNACE and also supplies 12 volts to the wall Tstat. IF NO 12 VOLTS AT ALL, THEN YOUR 12 VOLT SUPPLY FROM THE FUSE CENTER IS THE PROBLEM. Take a voltage reading of ALL the fuses at the fuse holder. ALL should have 12 volts on both sides of fuse. Doug