Need Help with Dometic Refrigerator Not Switching from AC to LP

[gibsito]

I have a Dometic RM 3804 refrigerator in my 5th wheel that will not consistently switch from AC to LP operation. This fridge was installed by the previous owner right before I bought it in 2009. Since then it has sat on my property and never moved, so the fridge has always been running on AC only. But now I'm getting ready to take it on the road, and have discovered this problem running it on LP.

When I started looking at this earlier today, I unplugged the AC cord and the LP burner would not light, or even attempt to light. It did nothing. So I pulled the rear cover off and just looked inside at the ignitor, the ignitor wire, etc., and could not see any visible problem. So I tried it again, and this time it switched over to LP normally. I repeated that several times and each time it worked as it should. I then put the covers back on and went about other business. Just before finishing up for the day, I decided to try it one more time. This time it failed to work again - with no attempt to light.

Any idea what the issue could be?

 

[TheLooks]

It could be that the unit is not calling for cooling at the time you try to test it on propane. The thermistor can be unplugged to force constant operation to test the system, but you need to know how to do it.

 

[gibsito]

It could be that the unit is not calling for cooling at the time you try to test it on propane. The thermistor can be unplugged to force constant operation to test the system, but you need to know how to do it.

Good point, I didn't think of that. I'm used to the way the very old Dometic fridge works in my truck camper, where the pilot is burning constantly. It's one of those old ones that has no circuit board. I somehow assumed this one would work the same way.

I saw a video somewhere that showed how to disconnect the thermistor so I'll look for that. Thank you!

 

[USRoofer]

I have a Dometic RM 3804 refrigerator in my 5th wheel that will not consistently switch from AC to LP operation. This fridge was installed by the previous owner right before I bought it in 2009. Since then it has sat on my property and never moved, so the fridge has always been running on AC only. But now I'm getting ready to take it on the road, and have discovered this problem running it on LP.

When I started looking at this earlier today, I unplugged the AC cord and the LP burner would not light, or even attempt to light. It did nothing. So I pulled the rear cover off and just looked inside at the ignitor, the ignitor wire, etc., and could not see any visible problem. So I tried it again, and this time it switched over to LP normally. I repeated that several times and each time it worked as it should. I then put the covers back on and went about other business. Just before finishing up for the day, I decided to try it one more time. This time it failed to work again - with no attempt to light.

Any idea what the issue could be?

Thank you for such a wonderful contribution.

 

[gibsito]

So I just returned from over a month in the 5th wheel and for all but a few days of that we had no working fridge. Initially the lp would light up like it should, but then it would randomly shut down and generate an error on the status panel. I pulled the thermocouple off of it and cleaned it, because it looked quite dirty. After that it lit up again. But before too long the error occurred again and it would not relight. It went back and forth for a few days but eventually shut down for good. We ran it on AC via the inverter for awhile, which worked, but it pulled too much juice to be able to run it that way all of the time. We resorted to filling the crispers with ice every other day, and limped along like that for the rest of the trip.

Soon I am going to replace the thermocouple, ignitor and control board. I'll post my results here after that is completed.

 

[TheLooks]

Intermittent operation is the hardest to nail down. If I were called to test such a problem I would repeatedly turn the unit on and off to try and track the point of failure. That means tracing the 12 v path as it is what makes the thing turn on. Unless this is an old unit, there is no thermocouple in it. It is called a flame sense and it is done by the same ignitor wire. When the flame lights it increases conductivity between the ignitor rod and ground and the board senses this. Very hard to test. But this is where I would start as boards are expensive comparatively.

 

[sanda]

I had the same problem a couple of years ago. Seems the gas valves can stick closed after sitting for long periods. A few light taps on the valve solved my problem. Make sure it is calling to cool. It's worked ever since with heavy usuage.

 

[gibsito]

Intermittent operation is the hardest to nail down. If I were called to test such a problem I would repeatedly turn the unit on and off to try and track the point of failure. That means tracing the 12 v path as it is what makes the thing turn on. Unless this is an old unit, there is no thermocouple in it. It is called a flame sense and it is done by the same ignitor wire. When the flame lights it increases conductivity between the ignitor rod and ground and the board senses this. Very hard to test. But this is where I would start as boards are expensive comparatively.

The rig is a 1994 model but the fridge was replaced just before I bought it in 2009. But I believe it's still a fridge of that early '90s era that had been refurbished - Dometic RM3804.

 

[TheLooks]

You are correct about the age of the fridge. Most of my documentation is for newer models. However, they still work in a similar fashion.

Dave's Place - Dometic AES System

Check this out and tell me what kind of interface you have with the fridge. Maybe I can fix it over the internet. But there are several things that prevent the gas from firing.

 

[gibsito]

You are correct about the age of the fridge. Most of my documentation is for newer models. However, they still work in a similar fashion.

Dave's Place - Dometic AES System

Check this out and tell me what kind of interface you have with the fridge. Maybe I can fix it over the internet. But there are several things that prevent the gas from firing.

Wow - that site is full of great information. Thanks so much for that!

The interface is an eyebrow board. It has an on/off switch, push buttons and lights to indicate if it's in AES mode and whether it's running on AC or LP. I haven't seen any way to manually select 12v mode. I'm not even sure if it's a 3-way fridge as I have heard that some of these models were 2-way. It does say 12v on the eyebrow board but I can't tell if there is a light there that would illuminate if it were in that mode.

I should mention that while I was trying to troubleshoot this while camping out in the desert I inadvertently shorted out something while trying to remove the thermocouple to clean it. That burned up a fusible link that was in there. I couldn't find any fusible links where we were camped so ended up replacing it with an inline, slow-burn fuse. Before working on it I had turned off the 12v switch inside the rig and presumed that would cut power to the fridge too. Obviously I was wrong. Should have tested for voltage before messing with it.

But many days after that, after we'd just been using ice to cool the fridge, I switched it on again just to see what would happen. There was an audible noise that sounded like propane igniting, but when I went out to look the burner was not lit. That was the last time we tried to light it.

 

[TheLooks]

You absolutely must replace that fusible link, it is a safety feature for flame coming out of the burn chamber. If you follow the wire over to the chimney, it will go to a thermal disc that also trips on high temp. you might have blown that also. It should be closed when cold, so use a multimeter set on ohms. Or a continuity tester. The next step back is the power supply block. I am or would be surprised that 12vdc would have been an option to run the compressor. 12 vdc is the control voltage for all the functions.

 

[gibsito]

Just wanted to provide an update for anyone else coming along with the same problem. I believe I've got it fixed now. The electrode was not holding still and could easily rotate down to contact the burner, so I replaced that. I also removed the wire and inline slow-burn fuse that I'd previously used to replace the burned up fusible link, and installed a new fusible link. That was yesterday, and when I tried to light it up I got the same familiar fault light on the eyebrow board with no attempt to spark the new electrode.

Today I noticed that the electrode's wire (which has not been replaced) was resting up against the flue, and I wondered if it could be shorting out against it, which would prevent the igniter from initiating a spark. I also noticed that the blade connectors at each end of the wire were somewhat loose. So I very carefully tightened those up with a pair of needle-nose pliers. When I reconnected it I routed it in such a way that it was not near any grounded metal parts. That did the trick. I turned the switch on and it fired right up. I let it burn for about 20 minutes and then shut it down. Later I tried it again and it worked fine. So it's looking like this problem is headed for the history books, finally. I will test it a few more times over the coming weeks and then give it a real test on a trip I have planned in April.

Thanks to everyone who responded, and special thanks to TheLooks, for providing me with that link to Dave's Place. That helped me so much in understanding exactly how these things work.

 

[TheLooks]

The ignition wire is one of the trickiest ones to find. You can't see the spark in the chamber and you can still hear it ticking. Congrats on solving the problem.