Dometic Ac keeps blowing 12 volt fuse

[Tjpries1]

I have a roof top dometic ac unit and the 12 volt 15 amp fuse keeps blowing immediately upon inserting a new one, it has the duo-therm t stat, any one have any experience with this issue? Or advice? Thank you

[dougrainer]

Tjpries1 wrote:
Thanks, I have determined it is one of the 2 white/orange wires that are wire nutted together, going to the 12- , while I had everything disconnected and the box out I thought I would twist the two wires together and it popped, with the box sitting on the counter. Do you know where those 2 wires go/come from and are they 12 volt negative as the 12- indicates? Thank you

Without the schematic I would not be positive. There should be a schematic ON the Galvanized box showing what the various wires are for. Doug

[dougrainer]

Tjpries1 wrote:
Thank you dougrainer for your help. I was able to get the ac and the furnace both working. I simply hooked it back up but left the one white/orange ,the one that must be shorting out, disconnected . Since I had two of the 12volt - wires . I checked and had 12 volt, not really sure why they have 2 of the same wires, you were right the control box was good.thanks again for walking me thru the process !

There are EXTRA wires that are NOT used coming out of the control box. You have Red Pos IN from the fuse supply. You then have a Pos output to the tstat. You found that someone had combined 2 wires and should not have and should have had a wire nut installed to prevent shorting. SOMEONE had to have combined those wires or you would have had this problem from when the unit was new. Doug

[Tjpries1]

Thank you dougrainer for your help. I was able to get the ac and the furnace both working. I simply hooked it back up but left the one white/orange ,the one that must be shorting out, disconnected . Since I had two of the 12volt - wires . I checked and had 12 volt, not really sure why they have 2 of the same wires, you were right the control box was good.thanks again for walking me thru the process !

[Tjpries1]

It still doesn’t work and it’s HOT today

[Tjpries1]

Thanks, I have determined it is one of the 2 white/orange wires that are wire nutted together, going to the 12- , while I had everything disconnected and the box out I thought I would twist the two wires together and it popped, with the box sitting on the counter. Do you know where those 2 wires go/come from and are they 12 volt negative as the 12- indicates? Thank you

[dougrainer]

Tjpries1 wrote:
I disconnected the blue wires for the furnace, still pops. I disconnected all the wires except the 12volt + and -, it still pops. Ac 6 pin is unhooked also. When I disconnect the 12v + from the control box the lp warning works fine and the fuse doesn’t pop

The 6 pin is ALL 120 volt. The control box is just a galvanized box held in place with 1 or 2 screws. Drop the box and remove the 1/4 inch case cover screws. This will expose the Circuit board. Look for a shorted inside wire connection or melted connection. Turn the 120 breaker OFF for the AC unit when doing this. You have isolated the problem to the control box. The Dometic P/n you are looking at is for the complete Galvanized box and board, once you remove the cover you will see a Dometic P/n ON the control board. Usually starts with 31. Doug

[Tjpries1]

There are 2 orange wire, 1 orange and 1 orange/white for 12+ Which are wire nutted to the red 12+ coming out of the box. There are 2 wires white and white/ orange wire nutted to the black 12- coming out of the box. With everything disconnected it doesn’t pop. But I did discover when I twist the 12- white and white/orange together (12v-) but with out the black wire to the box hooked up it pops, and the white/orange 12v- looks a bit discolored so it looks like the white/orange is the problem. Do you know why there would be 2 12v- wired together and where these wires come from/ go to?

[Tjpries1]

I disconnected the blue wires for the furnace, still pops. I disconnected all the wires except the 12volt + and -, it still pops. Ac 6 pin is unhooked also. When I disconnect the 12v + from the control box the lp warning works fine and the fuse doesn’t pop

[dougrainer]

Tjpries1 wrote:
Thank you for your reply and information sorry for lack of details. It is 2008 Heartland Bighorn model 3400RL. The AC is Dometic model 59516.331. I see 6 wires on the t stat, not phone style. The control box is 3107541009. Also furnace does not work. The furnace has a 20 amp fuse in the fuse box, but it’s not blown. One odd thing I noticed is when I put a new fuse in(which blows immediately) the CCI LP gas alarm beeps and the lights on it go out. Any help is greatly appreciated, thank you

The clicking is the 12 volt relay on the control board. Normal. ODD that the LP detector is on the 20 amp circuit for the AC. Most OEM's install the LP detector on a 5 amp fuse at the main control panel. The short could be in the LP detector wiring. Doug

[dougrainer]

Label the wires at the control box. Disconnect the 2 BLUE wires at the AC control box and then see if the fuse blows. If it does , then disconnect the remaining wires and see if the fuse blows. DISCONNECT them at the control box. See if the fuse blows. Report back.
DO NOT REPLACE THE CONTROL BOX. This is a common error when novices work on a RV system. I have NEVER replaced a control box for a Dometic AC for a fuse blowing. This is with hundreds if not thousands of Dometic AC's over 42 years as an RV tech. Doug

[Tjpries1]

I think I need a new control box 3107541.009 but can’t seem to find one, looks like they are obsolete

[Tjpries1]

Also I can hear and feel something clicking in the control box when I put a new fuse in

[Tjpries1]

Thank you for your reply and information sorry for lack of details. It is 2008 Heartland Bighorn model 3400RL. The AC is Dometic model 59516.331. I see 6 wires on the t stat, not phone style. The control box is 3107541009. Also furnace does not work. The furnace has a 20 amp fuse in the fuse box, but it’s not blown. One odd thing I noticed is when I put a new fuse in(which blows immediately) the CCI LP gas alarm beeps and the lights on it go out. Any help is greatly appreciated, thank you

[dougrainer]

Depending on how your OEM wired the system. The AC system is protected by a 3 amp fuse ON the upper AC control board. That means YOUR fuse is shorted by the 12 volt wire run from the Roof top to the wall tstat. That said, you need to state WHICH Dometic control system you have. There are up to 6 or 7 different ones. Also, your Furnace may be on that 15 amp fuse and there is a short between the Furnace and the upper AC control board. You need to state if you have Phone type wire connections or standard 7 wire multi strand AC/Furnace wiring. Year/Brand/Model of RV will also help. Doug

PS. you just joined. ALWAYS list Year/Brand/Model of RV and Brand and MODEL of the appliance when asking for help.