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

[obscenic]

Edit: Solved

The 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.

[dougrainer]

obscenic wrote:

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!

The reason I stated about the tstat was, LOOK at the Schematic in the link I sent. It shows the LOAD(Black Hot) of the 120 line going thru the Tstat, NOT the Neutral. Neutral can be thought of as a ground in this appliance. I thought it was weird that the tstat would have 4 spades as I never remember a tstat having separate inputs and outputs for 2 sources. Remember, YOU had stated you had completely checked out everything, and wanted ideas:B I was throwing ideas. Doug

[j-d]

Wonderful!

And Congratulations! You looked carefully for adjoining issues and found the problem! I'm not very good at complex wiring diagrams, and far worse at anything electronic. But I can sometimes find something that just doesn't feel or look right and that turns out to be the problem.

So this is a Rotary Switch? We had an old Dometic, RM763, and it had a very complex rotary switch. As I recall, the Knob was part of the Panel that covered the switch, so it could have been easy to not have it connect mechanically or be cracked and not turn the switch far enough.

Our 763 had an independent circuit for 12VDC. A lead from Engine Ignition closed a Bosch Relay that powered the 12VDC element. It was billed as "keep cold" and not able to actually chill the fridge and maintain cold for camping. No thermostat involved. Not even sure the logic board would prevent 12VDC operation if the the Fridge was actually running on LPG or 120VAC.

It's a good thing you didn't get zapped by 120 while you were in there looking for 12! Nice thing about a Fridge is we can usually unplug it right where we're working and know the 120 is OFF. Hard to say what might happen when a Switch like yours is OFF. I think the 763 switch was still made up in OFF. I do remember that some of its many terminals were jumpered.

[obscenic]

For anyone who found this thread looking for a no-12v-operation condition, check your switch. Mine was the ORIGINAL problem which correcting lead to the problem that lead to this thread.

In my case, the switch plastic housing, brittle with age, had cracked so when 12v was selected, the electrical contacts had an air gap between them. 120v worked fine as the crack in the switch housing was at the 12v end so only 12v was interrupted.

[obscenic]

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.

[obscenic]

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!

[j-d]

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.

[dougrainer]

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.

[obscenic]

dougrainer wrote:
The fuse is there. It is incorporated into the side of the tstat assbly. Look at figure 20 in the link biscuit posted. Number 17 is the fuse. Figure 22 Item 3. Doug

I did look, it's not there as drawn. The figure is a different model of fridge as well. Thanks though...

[dougrainer]

The fuse is there. It is incorporated into the side of the tstat assbly. Look at figure 20 in the link biscuit posted. Number 17 is the fuse. Figure 22 Item 3. Doug

[obscenic]

Old-Biscuit wrote:

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

Yes, definitely no PCB and definitely no relay. I've had it completely apart. I think I'm going to have to find some method of firing it up when it's out of the RV and in pieces to find out where the 12v is stopping.

[obscenic]

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]

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]

There is a relay and circuit board and FUSE somewhere.. I just can not tell you where.

[dougrainer]

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