I have a 23 year old Maytag MTB2156BEB that has been a great refrigerator.
Currently it has been having trouble maintaining cold in the freezer, it will fluctuate between 5deg and 25deg. Refrigerator will hold 40deg. It is also frosting up and when I've removed the rear freezer panel, I observe the heat exchanged was completely frosted over.
So far I have:
1. Tested the defrost timer by manually advancing it and it clicks and enters defrost cycle. I've also timed it and it is cycling as appropriate (10 hours cooling, 21 minutes defrost -- roughly)
2. Removed defrost timer and tested with DMM, it correctly shows open/closed on relevant pins when in cooling/defrost cycle
3. Removed freezer back panel and tested continuity of defrost heating element, tested OK for continuity
4. Removed freezer thermostat and tested for continuity at 0deg (cooled in my basement freezer) and at >50deg (using hair dryer). Continuity checks OK on open/closed as appropriate on both tests.
5. Evaporator coils are free of major dust
I don't want to give up on this unit, but I'm not sure where to look next. Any suggestions?