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

dougrainer wrote:
Finally found the RM 361. There are only 2 possibilities.
1. The selector switch is wired wrong for the 12 volt in and out. That is easy to check, verify 12 volt IN and 12 volt OUT when selected to 12 volt.
2. The tstat is defective on the 12 volt contac side. I would bet on the tstat as being burnt/corroded due to the hi 12 volt amp draw over the years. There HAS to be 4 wires on the tstat. The 120 and 12 volt side cannot share the same contac points. There has to be a contac point for completing the 120 current and a contac point for completing the 12 volt current.
3. IF you are not the original owner, you cannot be positive that someone did not change out the original tstat for a 120 only tstat. Doug

http://www.laurelhurstdistributors.com/parts/refrigerators/dometic/RM361-461-661-761.html

Looking at the 12 volt path. Notice that the ground(neg) for the heater goes straight to a ground source. But notice the 12 volt positive flow goes FIRST thru the selector switch, then thru the tstat and then BACK thru the selector switch instead of direct to the 12 volt element.

Hey Doug, thanks for that. I have now solved the problem and it was actually not an electrical problem at all. After I did the switch replacement yesterday (old switch had continuity issues across 12v section) and it still wasn't working I assumed problem was still electrical. What had happened in reality was that the spring mounted plunger (connects the heat source dial at the front to the actual switch at the back) didn't have adequate force to engage with the switch stem when the control assembly was rotated back into place. I tested on 120, all was well. I then turned the dial to 12v, disconnected 120v and connected 12v and experienced a no-12v-operation condition. Of course as the dial wasn't properly engaged with the switch, I was in reality testing for 12v voltage whilst in 120v mode.

That said, about your points 2 & 3; they're incorrect. As shown in the wiring diagram, the 120v hot and the 12v positive are BOTH wired to one contact on the t-stat and the 120v neutral and the 12v GND are BOTH wired to the other contact on the t-stat. The reason this is possible (I know it seems weird electrically) is because the switch can NEVER pass 120v and 12v at the same time, so the systems can never combine. I visually confirmed this. The 120v hot and 12z positive both run to the same terminal on the t-stat, but the switch makes it impossible to ever see both voltages at the same time.

As for the coils, they are ONLY connected to their respective neutral and ground through the switch, so even though technically the 12v coil recieves 120v on one side because of the common t-stat terminal, as it's not grounded no current can flow through it. Same principle applies to the 120v coil. No voltage potential = no current = no heat and the coils don't suffer damage.

It's actually quite ingenious!