Given that it eventually works, I think you could be on target with a slow to react thermistor. Thermistors do have time constants, though I have no way of knowing the proper value for those in your fridge. Especially telling is the fact that you have tested the thermistors outside the fridge and found them to react slowly. Might be worth replacing one or both. I tend to suspect the Fresh Food Temp thermistor if you go that route.
Another thought, your fridge uses a TDM (Time Divided Multiple Evaporator) refrigerant cycle as far as I can tell. This means that a 3-way valve supplies the fresh food evaporator with liquid refrigerant when called for, and this refrigerant partially evaporates and then flows into the freezer evaporator and fully evaporates. (Called position A on the valve). The other position of the valve supplies the freezer only (Position B). It uses Position D (all off) to minimize gurgling refrigerant sounds when the compressor is off. (It doesn't use position C,all on) I am wondering if your 3-way valve is slow to react to calls for cooling in the fridge. You might want to replace the coil on the 3-way valve since maybe it is slow or sticks so it doesn't go from the all-off position (Position D) to the Fresh Food Evaporator (Position A) or hangs at the freezer only setting (Position B).
The defrost is an adaptive defrost that monitors operating conditions and schedules defrosts accordingly based on compressor run time and door open time. I do not think there is a discrete defrost timer, it is on the main control board.
Your exact manufacture date would be handy as some significant changes were made in October 2014.
It sounds like you have tech literature, but here's a link to the service manual
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I reset the fridge this morning and watched and listened to everything. Compressor, Condenser Fan, Ice Box fan, Frozen Food Evap fan all turn on. The fridge has risen to 53 degrees and the fresh food evap fan is not running. As soon as it turns on, all is well. The fan is okay though as I have tested it. I tested fresh food and fresh food evap thermistor and they showed the correct ohms but they react slowly. Could their slow reaction be causing the issue? The temp display never shows what the thermistor value is when I put it in diagnostic mode and the temp probe I have in the fridge never matches either. Sometimes is only 2 degrees off, sometimes 10.
There is water on the back where the fresh food coils drain, so I know the heater is working. What could cause the fresh food evap defrost to turn on too frequently and last too long? The evap thermistor checks out with ohm readings, just slow to react.
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