I noticed my old fridge was unable to stay cold. At first I assumed that constant opening had caused it to freeze up so I unplugged it and let it defrost completely as there was a layer of ice on the coils in the freezer. After plugging it back in ice is slowly forming on the inbound end of the evaporator coil. The evaporator fan works fine. I tested both the thermostat and the heater...
First testing of thermostat after removal showed it was open - no voltage across it. I placed it in a working freezer and after 10 minutes it had continuity, so it appears to be working. I tested the resistance of the heater element and it showed 30 Ohms.
I reattached the thermostat and plugged the fridge in. The voltage across the terminals to the heater was 100v AC (Which I found odd, as I expected 120?). So the thermostat was closed and sending voltage to the heater. The voltage directly to the thermostat was 40v AC.
Even though at least 100v AC is getting to the heating element, I still feel no heat coming from it.
I don't think this fridge has a defrost timer. I believe it is all controlled by electronics in the fridge side. Is it possible that the control board is the issue? Is there any way I can further test it?
Any other possible tests?
Edit: Over night last night I had it shut down for 5 minutes every hour via an outlet timer. After ~8 hours the freezer (empty, set at the manufacturer recommended 50% cold) was still only 39° F (measured via a wifi thermometer) and there was still ice on the first few inches of the input line after a 55m run, fridge was 62°F. The fridge side has drinks in it, but is set as warm as possible. If all were fine I would expect an empty freezer to cool down much faster than that right (even with the fridge being full)? Seems either... a) 5 minutes isn't long enough to actually defrost and the line is remaining frozen. b) 5m/hr is too often and the unit just can't keep up or c) there is still some unresolved issue.
Is it possible this is a refrigerant issue? Is there a more accurate test for that?
Edit: Looking at this answer: https://diy.stackexchange.com/a/141659/77713 I am inclined to think this is related to the compressor system / refrigerant. My coils look exactly like those shown in the last pic - iced at the input but no frost whatsoever below. So how do I diagnose this further? Or if that system is something I can't repair myself then is it even worth continuing to diagnose?
Edit: Based on other reading I'm just going to roll up my sleeves and install a valve on the compressor suction line and test the pressure there. A bullet piercing valve is easy to install and given that this fridge is probably 25 years old I'm not worried about voiding any sort of warranty. Although I haven't completely confirmed that the defrost heater works, I feel certain that the evaporator coils are not cooling properly (regardless of whether the heater works). So I'll need to address that issue first. Will update again soon.