The first thing I would look at is the area around the compressor.
I want the fan to be running down there. I also need the coils to be cleaned so they don't restrict the air flow. I want the compressor to be warm to the touch but not hot and not cold. If the fan is running and the coils are clean I want to take a closer look at the compressor. The relay on these will simply fall apart over time.
I want to make sure I am getting power to the compressor. No power down there? We go upstairs to the control. I want to first take a look at the defrost timer or electronic defrost control. I want to make sure the timer is in compressor run mode and is not hung in defrost mode.
I want to check an automatic defrost control for corrosion at the connection. Sometimes yanking it out of its socket will make a better connection and all will be well again.
If all of that is fine and the compressor is running I want to take a look at the evaporation coils. If they are iced over then I know I either have a bad defrost heater or defrost heater termination thermostat.
Why do we need a termination thermostat? It activates the heater when the temp in the freezer is below 25f AND the defrost timer or control calls for defrost. You see the defrost heater and terminator are still same whether you have the old tried and true defrost timer or the fancy smancy electronics. The defrost termination thermostat kills the power to the heater when the temp reachs 50f. Why is this important? Because the defrost mode stay on for 20 minutes or longer. If we leave that heater on that long the ice box would be damaged or destroyed. The ice buildup resulted from a failed defrost takes a week or longer I have more Info HereI have service manuals and more good stuff hereand Here
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