If it were a "fridge on 120VAC" problem, then the very first thing which should have blown is the 3A fuse within the Fridge (rather than the 20A breaker in the TM). The 120VAC "hot" wire doesn't even reach the switch before going through this fuse! If the fuse has been bypassed - restore the fuse first. Then test to find problems, using a voltmeter measuring resistance - with the selector switch set to "Off". 20A *120V is 2400 WATTS, that can kill you almost instantly!
Nope. DC operation has nothing to do with 120 VAC power at all. (120VAC operation, OTOH does depend on 12VDC to run the control board and work with the thermostat.) I might have confused "AC Breaker" with "DC Fuse".
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Lets see if Lizard can clarify. He did talk about "replacing the breaker" - but Breakers are reset, not replaced. At the same time, "Fuse" is a lot shorter to type, so I'm not sure what to think.
If a former owner bypassed the 3A fuse, that is a very bad thing (TM), it must be replaced FIRST. Tthen the downstream problem problem needs to be fixed. Wiring diagram shown in the photo. There are 4 possible points of failure:
#1, a short circuit in the heater coil (the 120VAC heater should have DC resistance of about 140 ohms). Disconnect both leads and test this between the two terminals. Also test that resistance from EACH terminal to the fridge frame is infinity.
#2 a short circuit within the Selector dial, leaking 120V "hot" to Frame "Ground", instead of connecting only the black wire into the thermostat. This can be tested by disabling the TM circuit breaker, VERIFYING the 120V terminals at the fridge are "cold" - and then testing resistance between the 120V input wire to the selector switch and the body of the Fridge. It should be infinity. Now proceed to turn the mode switch to 120V. Test that resistance again - it should still be infinity. If it isn't, then your fault lies within either the Selector Dial or the Thermostat. Turn the "mode switch back to 'Off', and test the Thermostat in isolation.
#3, It could be a be short circuit within the thermostat, where 120V is connected to the fridge frame (leaked to "Ground", rather than connecting only to the downstream 120VAC "hot wire" (Color = Brown) when the switch is enabled (and closes the circuit). The "thermostat" runs propane mode by setting the flame orifice, directly from the dial position. 120VAC is only present when the "selector dial" (part #17 in diagram) has been turned to "120VAC" operating mode.
This can be tested without actually presenting 120V power: With the "temperature" dial is set to 1, the resistance between the 120V Black wire "hot terminal" (WHICH MUST BE COLD IN THIS TEST!) and the metal body of the thermostat must be infinity. The resistance between the Black and Brown wire terminals must also be infinity, AND the resistance between brown terminal and thermostat body must also be infinity.
Then, turn the selector dial to "12V mode", and turn the "thermostat" dial to 6. While turning the dial, a tiny "click" should be heard when the thermostat closes the circuit between black and brown. The resistance between black and brown should now be zero -but most important, the resistance between the brown wire terminal and the metal body of the thermostat must also be INFINITY.
#4 a fault in the Brown wire (leaking to ground).