Before working with electricity, visit the electrical safety page.
All energy sources are controlled by the AES printed circuit board on refrigerators that use it. It is the brain of the refrigerator and requires 12 volt power to function. If 12 volt is not available, the refrigerator will not operate on any heat source. The graphic below is not a specific circuit board, but a generalization of all of them. The graphic also does not contain substantial detail, because we're not going there.
Inspect the circuit board for corrosion or burned spots. You may have to remove the circuit board to inspect it thoroughly. Be sure to disconnect all power before removing the board. If you find severe corrosion on the board, you might try cleaning it up, but you'll probably have to replace it. Also, check the modular plugs to the circuit board for corrosion. Corrosion is usually caused from water, and often times water gets to the circuit board by being squirted through the louvered access door when washing the outside of the RV.
If you find a burned component or spot on the circuit board, it is very possible that some other component in the AES system may have been the cause of the burn. The circuit board will have to be replaced, but before replacing the board, check the library for service bulletins diagraming burned spots and their probable causes for your model. Many times the replacement circuit board will come with a copy of the "burn" diagram. Also, inspect wire harnesses and ribbons for shorts.
One area of the circuit board that is relatively easy to check with a volt meter is the AC portion. Both 12 volt and 120 volt power will have to be present, so be careful not to create any shorts when testing for, safety reasons and to not cause damage to the circuit board. With the refrigerator turned on ( or set to AES or AC mode on later models) and 120 volt available, test for input voltage at the board. The graphic below does not show which is the input and output connections for the AC, because the input and output are not consistent between boards. You'll need to look at a wire diagram or inspect the wiring to see which connections are the input. If there is 120 volt at the input connections and everything else is working right, there should also be 120 volt at the output connections. If there isn't, the board is bad. If power is present, but the heat element doesn't get hot, then the heat element or the connections to it are bad.
Another problem you might run into is a chattering (cutting on and off) AC relay. You can usually hear a chattering relay and, if you put your finger on it, feel it chatter. If this is the case, the circuit board needs to be replaced. The cause for this is simply a defective relay and normally not an external cause.
Since power to the 120 volt heat element passes through the circuit board, it is not uncommon for a bad heat element to burn out the board. A heat element simply going bad (open circuit) won't cause the burn out, but one that shorts to ground or has an internal short and therefore pulls more amperage than it should will burn out the board. Any burn spot between the AC input and output connections and the AC relay on the circuit board is a sure sign of a shorted element. Unplug the 120 volt element and, with an ohm meter, test for continuity between element leads and ground. If there is continuity between either lead and ground, replace the element before testing the new board.
An internal short in the heat element will become apparent by testing the resistance between the two element leads when disconnected from the circuit board. The highest wattage element designed to be used with the circuit board is a 325 watt element. A good 325 watt, 120 volt heat element will have a resistance reading of 42 ohms across the two element leads. A reading much lower than this (ohms go down as wattage goes up), is an indication of an internal short. Here again, the element should be replaced before testing the new board. Some after market boards come with fuses built onto the board and in-line with the AC circuitry so that only a fuse is blown when an element shorts out, instead of ruining the board.
Sometimes there is nothing visible wrong with the board, but still you strongly suspect it is the problem. If you can find an RV repair shop near you with a board tester, take the board to them and they will probably test it for free.
When changing a circuit board be very careful not to create new problems by pinching or cutting wires in the various harnesses going to the board.
RV Mobile Inc. 11715 HWY 99, Everett, WA 98204
The owner of RV Mobile Inc. apparently suffered a heart attack and the original website was shut down.
It has been reposted here to preserve this wealth of information RV refridgerator information.