Hi Keith,
You were right, the defrost cycle controls by the main control board on the back of the fridge.
Unplug the refrigerator, remove the blue wire from the "defrost" terminal J9 on the control board and measure the resistance between this wire and the neutral orange wire (pin 9 on the J7 connector). The freezer must be cold at this time.
If the reading is about 22 Ohms then the main control board has to be replaced. It is recommended to replace the board together with both freezer thermistors.
If the circuit is open then you have to remove the rear panel in the freezer and check the defrost heater (#230 on the
break down diagram)and the defrost thermostat (#240 on the
break down diagram) for continuity.
If any one of them is open - it has to be replaced.
- The part number for the main control board is
AP3950129
- The part number for the defrost heater is
AP4355467
- The part number for the defrost thermostat is
AP3884317
- The part number for the freezer thermistors (they are identical) is
AP3185407
Gene.