trouble with 12 volt compressor fridge

[RogerDat]

I have a very old 12 volt refrigerator. Runs off of 12 volt and 110 volt. Has a compressor. When plugged into 110 a relay switches to run compressor off of a fairly large transformer, about size of a large grapefruit. Unplug the unit and the relay switches to 12 volt battery supply.

Only draws about 5 amps when running, runs intermittently rather than continuous so doesn't run down the battery the way the heating element of an absorption refrigerator does.

I ran it for several days in my garage mostly off of 110 but also for several hours off of 12 volt. Then took it to a friends cabin and hooked into his solar battery bank. Ran for about 20 hours and just stopped.

Still runs off of 110 volt, doesn't run off of 12 volt. Compressor is always 12 (or 24 volt running on 12) so thermostat and compressor are both working or it wouldn't work on 110 power.

I'm looking for a couple of answers.

Has anyone any experience with these old style 12 volt compressor refrigerators with an idea of what could cause the 12 volt side to stop working. Just out of the blue. I did find a ground wire that had come unsoldered and was just barely attached. That has been attached now. Still no joy.

What sort of inverter would be required to run this off of AC inverter? I think the motor always draws close to 5 amps with the AC power going through a transformer to convert to 12 volt to run the compressor I was thinking modified sine wave would still work but have seen some stuff about modified sine wave and transformers that makes me unsure.

I am really hoping I can figure out what suddenly failed so that the straight 12 volt power didn't work while the AC to transformer power does still work. If I can't then running off of an inverter is still viable. I think the power draw is much less than a typical home fridge.

I'm also unsure of the load the inverter will put on the solar system. It is a 285 watt PV set up with 4 batteries and running on 12 volt directly the refrigerator didn't do much to draw the battery voltage down over night. Inverter will up that load some but by how much?

We had ice for a single night at the camp, now we are wanting to make that a permanent feature. :-)

[Raz]

WAG The 110 energizes the relay to direct the output of a DC power supply made up of the transformer, some diodes and a filter circuit, to the compressor. When the relay is not energized, it feeds the 12 DC to the compressor. Fault - bad relay or bad connection feeding the relay on the 12 volt side. Is there a fuse there? If you can identify the compressor side of the relay (output) you could hard wire for 12DC operation only. Do get the polarity right!

[RogerDat]

Originally Posted by Raz

WAG The 110 energizes the relay to direct the output of a DC power supply made up of the transformer, some diodes and a filter circuit, to the compressor. When the relay is not energized, it feeds the 12 DC to the compressor. Fault - bad relay or bad connection feeding the relay on the 12 volt side. Is there a fuse there? If you can identify the compressor side of the relay (output) you could hard wire for 12DC operation only. Do get the polarity right!

I do not see a fuse but the transformer and relay are in a small space under the refrigerator. The compressor is on the back. Not much room underneath to work on it. I have scrapes on my arm that are just now healing from attaching the new 12 volt wires after I removed it from my school bus camper where it was wired in.

The idea of hard wiring to be 12 volt battery power may be a good one. There is a small indicator light underneath that indicates the unit has power. That light comes on. I'm guessing that is not part of the main power lines so may well be ahead of the relay.

Thing that gets me is it was just running along all happy and then quit. It was even at a time the air temperature was getting cooler so probably running a bit less with motor not as hot.

I had turned it cooler by about 1/3 of a number (0 -7 scale running at 4). Freezer was at 10* F and refrigerated compartment below it was at about 41* F so I turned the thermostat down a touch more. Had been doing that in small amounts for a while to adjust to getting the temperature where it should be.

I may look at the possibility of removing the framework under the refrigerator that has all the hardware on it. It all plugs into a wiring harness so it unplugging and moving to a bench would provide a much more accessible way to test.