Your model has a motherboard in the back that controls the defrost. It also has a thermistor that mounts above the evaporator coil that tells that board when to shut down the defrost cycle. Thermistor is # XXXXX in the picture. And there is also a defrost thermostat that acts as a safety so it wont overheat. It is # XXXXX in the picture. The thermostat should have continuity when it is cold and the thermistors resistance changes as the tempature changes. See chart for testing. If those test good then you have a bad motherboard.