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A defrost timer is nothing more than a timed switch that controls the flow of current to one side of the compressor or defrost heater.

Checking your defrost timer
Manually place the defrost timer in the defrost cycle and wait approximately thirty minutes. If the timer motor is working properly and the thermostat inside your refrigerator is calling for cooling at the time the defrost timer switches back to the cooling cycle, the refrigerator compressor should start running.

A simple way to test your defrost heater without professional equipment
This is a simple test to help you determine if the defrost heater element is working. This test should not be relied upon to diagnoses a system.
Assuming that your refrigerator has completed a full refrigeration cycle (Note #1) and assuming the termination thermostat (Note #2) is working properly, manually place your defrost timer in the defrost cycle and allow it to advance on its own for approximately five minutes. With your kitchen lighting dimmed, manually pull the refrigerator plug from the wall receptacle. If the defrost heater is working, you should see a pronounced arc when you remove the plug. Other than the small defrost timer motor and some small door heaters, the defrost heater is the only device that would produce a pronounced arc while your refrigerator is in the defrost cycle.

Note #1:
This is to make sure the evaporator coil has a sufficient buildup of ice crystals and the defrost termination thermostat is in the closed position.
Note #2:
Defrost termination thermostat (nonadjustable):
The defrost termination thermostat is usually located next to the evaporator coil and is inaccessible to the average homeowner. When the coil temperature rises above approximately 60 degrees, the defrost termination thermostat will open and switch off the defrost heater. When the coil temperature drops below approximately 15 degrees, the defrost termination thermostat will close, thus allowing the defrost heater to engage during the next defrost cycle.

Disclaimer: The information contained on these pages should never be relied upon as a repair guide or be used to make repair decisions.

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