Edit: SolvedThe switch, which I had previously determined was the problem and replaced, was in fact the problem. Replacing the switch with a new one had fixed the original problem. However, the issue I had after installing the new switch was a re-assembly problem. When you spin the control section back in to mesh with the switch stem, t-stat stem, the pilot stem and and piezo stem, the switch stem sprung section had been pushed back and stuck there. It didn't engage the switch stem. When I turned the dial to 12v (after testing 120v successfully), the switch hadn't actually been rotated to pass 12v - it was still in 120v mode. The fridge was actually set to run on 120v but wasn't plugged in to 120v; and as I believed it was set to run on 12v at that time, it *appeared* to still be broken. So the 12v was fixed after the switch replacement, but the dial not engaging the switch left the fridge unable to change heat sources.Original message:Have an old dometic RM361. Here's what I've ruled out.- The cooling unit is in tip top shape and runs great on 120v and propane.- The 12v heater works fine when removed and connected to a 12v battery- The switch is brand new today (thought it was the problem... it wasn't)- There's no 12v relay in this model, and no circuit board- The thermostat is shared by 120v and 12v so I know it works as well.- There is 12v at the terminal block.I've tested literally everything in the 12v path and each component functions find on it's own; yet when all is assembled, the 12v heater does not come on. There's no power going to it. I'm at the end of my wits now.. if anyone has a suggestion other than ghosts, I would LOVE to hear it.

So the cooling unit is fine as is the 12v heating element.Time to break out the multimeter and find where the connection isn't making it thru.

Chris Bryant has a lot of Service Manuals HERE. The one called RM77 might help you. There's a wiring diagram Fig 66 Page 38 that might help you. That Switch does a lot. Are you sure it's connected correctly? Also shows a Fuse on the 12V side. Have you found it and is it OK?

According to the wiring Schematic, there is a FUSE between the 12 volt element and the tstat 12 volt output. You need to check for 12 volts at the OUTPUT of the tstat. If you have 12 volts, then that fuse is the problem. Since it is so old, I have no idea where Dometic located that fuse, but it is between the tstat and the 12 volt element. DougIt is a 25 amp dc fuse.

I’ve only had one 3 way Dometic. It was in a small motor home and would only work on 12 volts if the engine was running. So it had a separate 12 volt input from the engine 12 volts to allow that. Does that apply to you?

Thanks everyone for the thoughts!Valhalla: I did this as best as possible; only the 12v and the coil 12v out are measureable when the fridge is assembled and installed. There's 12v going in to the switch, but no 12v coming out to the coil (mode set to 12v and tstat maxed out). I checked continuity across the switch when it was out and there was no issue. When it's assembled, meter probes can't reach the switch or the t-stat :(D-d and Doug; I had noticed that fuse in a couple wiring diagrams, though the diagram on the fridge itself does not show a fuse. The only place I can't see is the wire inside the cable loom to the heat stack, but I've checked continuity over both wires with a meter and they're fine. I think I can safely rule out a fuse.Neschultz; there's 12v at all time. My domestic battery is actually completely isolated from my vehicle battery; the alternator does not charge the domestic battery.

[SOLVED] 3 way fridge won't run on 12v | Good Sam Community

obscenic
Explorer
Mar 21, 2018
neschultz wrote:
obscenic wrote:
Thanks everyone for the thoughts!

Neschultz; there's 12v at all time. My domestic battery is actually completely isolated from my vehicle battery; the alternator does not charge the domestic battery.

I wasn’t clear enough. The 12 volts from the alternator to the refer did not operate the refer or charge it. It was a logic voltage that allowed the use of the TT 12 volts to operate the refer. It probably operated a relay. The refer would also not run on 12 volts automatically...you had to manually select it. When the engine stopped, it would revert back to AC or LP automatically.

Definitely not, the only low voltage wires into the fridge are +12v and gnd. Those lines will run a 120vac inverter which I can run the fridge off of (my workaround last year due to no 12v operation) whether I'm running the engine or not. They have 12vdc at all times.
Old-Biscuit
Explorer III
Mar 21, 2018
wa8yxm wrote:
There is a relay and circuit board and FUSE somewhere.. I just can not tell you where.

NOT on the OPs old piezo ignition fridge :S
wa8yxm
Explorer III
Mar 21, 2018
There is a relay and circuit board and FUSE somewhere.. I just can not tell you where.
dougrainer
Nomad
Mar 21, 2018
The 3 way Dometic, had a 12 volt Ignition wire connection on the Dometic refer. The OEM ran that 12 volt wire from the Ignition side of the Motorhome to that connection. The PROBLEM most OEM's caused was, they rarely ran the required 8 gauge pos and neg wire from the battery to the refer. On Auto or on the models that had a 12 volt switch option, when the Ign terminal on the rear of the Dometic refer was HOT, the refer ran on 12 volt element in transit. When you turned the Engine OFF, the 12 volt refer element also went off. They did this to keep the refer from draining the battery if you forgot it was on 12 volt. They have 2 models of a 3 way. OLDER models did run thru the tstat. Newer models run direct and bypass the tstat and are advertised as "maintainers of cold and not designed to cool the refer. Just will maintain the cool while in transit. The OP's refer is an OLD model that does have tstat that will control the 12 volt. Doug
neschultz
Explorer
Mar 21, 2018
obscenic wrote:
Thanks everyone for the thoughts!

Neschultz; there's 12v at all time. My domestic battery is actually completely isolated from my vehicle battery; the alternator does not charge the domestic battery.

I wasn’t clear enough. The 12 volts from the alternator to the refer did not operate the refer or charge it. It was a logic voltage that allowed the use of the TT 12 volts to operate the refer. It probably operated a relay. The refer would also not run on 12 volts automatically...you had to manually select it. When the engine stopped, it would revert back to AC or LP automatically.
Old-Biscuit
Explorer III
Mar 21, 2018
Clip to t-stat S/B a 'fuse holder'???

See Fig 20 pg 13 Item 17

LINK
obscenic
Explorer
Mar 21, 2018
I have a boatload of amp hours battery, and a boat load of solar, and I'm rarely off grid for more than a few hours. I can't run on LP because I decommissioned my onboard tank due to 35 years of rust, so 20lb tanks travel inside with me and get hooked up when I'm camped (they sit on the ground beside the RV).
MrWizard, that's all I'm looking for. A few hours on the road on occasion. And yes, testing at a toasty 22 degrees celcius. The t-stat comes on on 120v no prob.
MrWizard
Moderator
Mar 21, 2018
obsenic

you do realize , that 12v mode was only intended as a keep cool mode while traveling, and not for primary cooling

my first choice would just be to run on LP even while driving, and forget the 12v mode

my second choice would be to rewire the 12v heating element with a 30amp toggle switch and connect directly to the 12v coming to fridge
its the sane thing as switching manually using the built in switch you just replaced, eliminates all that hidden wiring problems

bypassing the T-stat won't really change the cooling factor, as the 12v pretty much stays on and hardly ever switches off,

one question
you are testing this in a warm above 60* environment correct ?
i have doubts about testing 12v mode inside a cold garage or RV
Harvard
Explorer
Mar 21, 2018
Seems to me running the fridge on 12VDC is not a good idea. If you stop somewhere for lunch you are apt to have a dead battery UNLESS you have lots of AMP Hours built into the 12V system to handle the fridge heating element. JMO.
j-d
Explorer II
Mar 21, 2018
neschultz wrote:
I’ve only had one 3 way Dometic. It was in a small motor home and would only work on 12 volts if the engine was running. So it had a separate 12 volt input from the engine 12 volts to allow that. Does that apply to you?

We had a 3 way Dometic also. It was RM663, with a logic board, but the 12 VDC was powered only with engine running. It had a Relay. A 200 watt element isn't much amperage at 120 VAC but it's a bunch at 12 VDC. 3 way fell out of favor because of that. Motorhome has so many current draws that even with an upgraded alternator, the coach charging system was coming up short.