Dometic frig works on AC but not on propane

When we bought the Southwind in March, the previous owner told us that the Dometic RM-3862 frig worked on AC, but he didn’t think it worked on propane. Before buying the Southwind, we verified the frig worked on AC, but assumed it wouldn’t work on propane. The fact that the freezer section cooled down on AC was an indication that the big dollar item, the cooling unit, was probably OK.

When the weather finally warmed up a couple months later, I checked to see if the frig would automatically switch to propane when it lost electricity. With the frig ON and set to AUTO, I removed shore power.

I heard a distinctive click, but several seconds later, the “CHECK” light came on signifying it had detected a problem and did not switch to propane.

From the outside, I removed the frig panels and covers from the boiler area to look for any problems. I didn’t see any obvious problems like bird nests, rusted or plugged burner tube, etc, These are the normal reasons why an older frig quits working on propane. I disassembled the burner section and used #200 sandpaper and a small wire brush to clean the burner tube, igniter electrode, and the thermocouple. The igniter wire needs to be shiny clean or it will not spark against the burner tube and thermocouple. The tiny openings one the burner tube need to be clear, and the thermocouple needs to make sure there is no carbon buildup on it.

Cleaned burner, igniter, and thermocouple with small wire brush and #200 sandpaper. The igniter wire needs to be nice and shiny

I reassembled and tried again. There didn’t appear to be any sign that the propane solenoid valve even attempted to open or the igniter sparking.

CHECK GAS VALVE AND IGNITER OPERATION

To verify that I had gas and the valve and igniter worked, I’d bypass the controller board and energize the solenoid and igniter directly. To do this, I removed the 4 pin connector, P3, from the controller board. Using a jumper wire, I applied 12 volt (positive) directly to the plugs yellow wire. The gas valve immediately energized, the igniter started sparking, and the burner lite. Excellent, this says the burner will light if the controller tells it to.

The gas solenoid and the igniter are wired together. Anytime 12 volts is applied, both are energized.

When energized, the igniter will spark once a second or so until the gas is ignited. The flame changes the resistance in the gap and the igniters spark cannot jump across the flame, so it stops sparking, even though its still energized. If the flame goes out, the igniter will start sparking again. This is the reason why the spark wire needs to be reasonably clean or the igniter spark cannot jump across the gap.

Connect 12 volts to yellow wire to energize propane solenoid and turn on igniter

One reason the controller may not supply 12 volts to the propane valve would be if it detected a problem with the thermocouple. I checked the thermocouple with an ohm meter, It read shorted, so its probably OK. If it read an open, it would definitely be bad. Also – the thermocouple can set up very small microvolts to ground (ground bounce), so if there were some weird grounding issues, the Dometic controller could interpret this as bad thermocouple. The Dinosaur board solved this particular problem.

It was appearing like it was a bad controller board. Dinosaur makes a replacement that has improved circuits that can deal with weird thermocouple grounding issues.. Part number, MICRO P-711, is available from Amazon for about $120. This would solve my the problem, but before I commit, I want to make sure there isn’t any other issues with the frig.

TESTING AC PORTION OF FRIG WHILE ON ROAD TRIP

We had a 10 day road trip coming up where we’d be traveling and would be connected to shore power whenever we stopped. I decided that before I committed to buying a control board, we’d verify the frig operation and make sure there wasn’t any other problems.

The frig worked OK during the 10 day trip. For the 6-8 hours that we were on the road, the frig freezer kept cold. When we got home, I ordered the new Dinosaur Micro P-711 from Amazon.

INSTALLING DINOSAUR BOARD

The P-711 is almost a perfect, plug for plug, wire for wire replacement. The only minor difference is an additional spade connector that Dinosaur uses for the thermocouple. Dinosaur added a connector to their board on the right side. Instead of the thermocouple connecting to the GND terminal, it connects to this new terminal. This is clearly shown in their connection drawing.