I think I have FINALLY WON the battle to fix my Norcold DE441 fridge in our Sportsmobile! Last year I discovered that the fridge was working, but would not begin cycling until the box got to 25°F inside and froze everything. I tried several different thermistors, no luck, factory parts are discontinued, etc.
So I got a $13 Chinese 12VDC PID controller off ebay, cut the traces on the circuit so instead of switching on and off 12VDC to it's output, I made it into just a simple switch, open/closed, temperature controlled by the controller. I wired it across the terminals of the On/Off switch for the fridge, and it is cycling the compressor properly and has about a 6° to 8° swing between cycles. The fridge still has it's dual voltage capabilities, it just uses the PID controller as a thermostat now which seems to be working great! I will install the fridge back in the van tomorrow!
This has been a major PITA but I didn't want to give up on the fridge, the compressors for these are ingenious in their operation, all it took was some efficient method of controlling the compressor. This photo shows the PID controller installed in the grille at the bottom of the unit, the knob to the right is a variable speed controller for a fan that I mounted inside the fridge to hopefully circulate air and keep temps more even instead of colder at the top and warmer at the bottom by 15° or so.
Red arrows show the terminals for the On/Off side of the factory rotary switch/temperature control. This is where I connected the switching part of the PID controller, and it simply turns the unit on and off via a termperature controlled relay.