Before working with electricity, visit the Electrical Safety page.
Although the second generation of Dometic electronic refrigerators requires 12 volt to power the circuit board and therefore to work on any heat source, what we are discussing here is the use of 12 volt DC as a heat source to heat the cooling unit. Most 3 way models of this generation have a button on the eyebrow to turn the DC (12 volt) on. When on, DC overrides all other modes, which means the DC button would have to be turned off for the refrigerator to operate on AC or gas. Since DC operation pulls a lot of amperage (almost 18 amps on some models), it should only be used when the vehicle is running and charging the batteries. Using 12 volt as a heat source without the batteries being charged can drain your batteries in only a few hours. Models RM3607, RM3807, and RM4873 have a different way of handling the DC heat, which is discussed in detail below.
The 12 volt system is relatively simple. Power (12 volt +) comes into the circuit board via connection J4 (the same 12 volt connection that provides power to operate the electronics). When DC heat is called for, 12 volt is provided to the 12 volt heat element by the circuit board via connection J1. The circuit is completed by the ground (12 volt -) wire from the heat element going to the common ground terminal on the refrigerator.
Troubleshooting The first thing to do, if you suspect a problem with DC as a heat source, is to make sure the refrigerator works on another heat source (AC or gas). This lets you know that a lot of common parts, such as the thermistor, are working. If the refrigerator works on other heat sources, there is 12 volt present at connection J4 since 12 volt is required for the refrigerator to operate on any heat source. To test DC operation, be sure the DC button is depressed and the refrigerator is on. Turn the temperature control all the way up to be sure cooling is called for. Using a meter or 12 volt test light, check for 12 volt at J1. If 12 volt is not present at J1, the 35 amp fuse (the one to the left on a three fuse board) on the circuit board could be bad. You can test for voltage through the fuse, or you can pull the fuse and test it for continuity with an ohm meter. If voltage is coming in through J4 and the 35 amp fuse is good, and there is no voltage output at J1, the main circuit board is bad. This, of course, assumes that everything else is working, as verified by operating the refrigerator on another heat source.
If 12 volt is measured at J4, the heat element should get hot. You should also check the ground wire connections at the heat element and the ground terminal. After 15 minutes or so, feel the insulation pack (right above the burner) for heat. If the heat element does not get hot, it is bad and needs to be replaced.
AES RM3607, RM3807, and RM4873 On these models when AES (Automatic Energy Selector) is selected, DC becomes the second choice as a heat source, behind AC. AES has a priority system for choosing heat sources with AC first, DC is second if AC is not available, and gas is last if AC is not available and the DC voltage starts to get low. Note the AES system for the second generation refrigerators is different than the first generation Dometic AES refrigerators. Also, the three models listed above are the only second generation models with AES, possibly a sign they weren't all that successful.
The key to the 12 volt operation on these three models is a battery protection system. If AC is not available, the system will switch to DC as a heat source. However, if the input DC voltage at the terminal block is less than 12.8 volts, the system will by pass DC as a heat source and attempt to light on gas. The input voltage must rise above 12.8 volts for 25 minutes before DC operation can continue. As soon as the input voltage rises above 12.8 the DC light will come on at the eyebrow, but the unit will continue to operate on gas for the 25 minute delay, then switch to DC. The delay is to allow time for the RV battery to recharge. If AC (120 volt) ever becomes available during these times, the system will automatically switch to it.
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