[SOLVED] 3 way fridge won't run on 12v

j-d wrote:
When I looked at the wiring diagram in the manual I linked (different manual but seems to cover many of the same models), looks to me like Thermostat is in fact two-wire and the circuit relies on that multi-contact switch to provide ONLY 12VDC OR 120VAC. Does this by making both sides of the selected power source and breaking both sides of the de-selected source. Suggests to me that an internal switch failure, like a contact falling out of its insulated carrier, could result in crossing/backfeeding those circuits. I don't like that, but it sure seems that's what Dometic did.
Reminds me of the selector switch on boats that connects the boat to either shore tie cable or onboard generator, and locks the other one out. You have to move a little slider out of the way to move the switch, then the slider drops in place to prevent the switch from being moved back.

Bingo. It's hard for most people to get this, as they would assume that the 12v coil would just be chassis grounded and therefore have 120v running through it and start a fire! But it's not, the ground is isolated all the way back to the switch, and is cut off at the same time as the positive 12v by the switch. It's definitely not the way I would wire anything but it probably saved a TON of money in manufacturing costs!

I realized something that makes this electrical design a bit more dodgy. The 12v ground on the heating element contacts the steel guide tube by design, which contacts the chassis of the fridge. This means that when the fridge is in 120vac mode, that 120v neutral is in fact electrically connected to ground before the breaker panel. I *think* this means that the RV chassis becomes a 120vac current carrying conductor which is what separate grounding is meant to prevent. I may try to isolate that connection from it's tube as I believe that wouldn't be to code these days.