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When the refrigerator is running ( cooling mode ) the defrost timer contacts prevents any power from getting to the defrost heater. When the defrost timer turns off the power to the compressor and fan motor(s) the power is then redirected to the defrost system. The defrost timer motor
During a defrost cycle, the defrost heater causes the metal alloy in the switch to warm and as it does it curls back and breaks the circuit. As the metal cools, it makes a closed circuit again. A defective thermostat can prevent the defrost heater from coming on or allow it to overheat which could result in heat damage or fire.
The defrost thermostat is located near the defrost heater and is wired in series. It is usually located at the back of a side by side freezer, behind the freezer back wall of a top freezer or under the floor. It will be necessary to remove obstructions such as the contents of the freezer, freezer shelves, icemaker and the inside rear or bottom panel of the freezer. At room temperature the defrost thermostat is an open circuit ( no continuity ) and when cooled down the defrost thermostat closes the circuit ( has full continuity ) to allow the defrost heater to come on.
The defrost timer is sometimes found behind the front grill of the refrigerator. It may also be found behind a cover plate inside the refrigerator or freezer, in the temperature control console, or behind the refrigerator near the compressor.
The timer is usually held in place with one or more screws. Remove the screws and gently pull the timer out far enough to disconnect the wiring connector.
Locate the timer switch and turn it clockwise until you hear it click. One click and the refrigerator shuts off = defrost mode, second click the compressor and fans come back on = run/cooling mode.
*Some defrost timers are a constant or continuous run timer, which means when ever the refrigerator is plugged in the defrost timer is powered up and running. Some defrost timers are an accumulative run timer, which means when the refrigerator is off the defrost timer is off and not running, when the refrigerator is running the defrost timer is running. This is also sometimes called a demand defrost. The power for the defrost timer goes through the cold control first on an accumulative defrost system. The timers are often the same but will be wired differently to get the different operations.
Test the heating element for continuity using a multimeter. Set the multimeter to the ohms setting X1. Place a probe on each terminal. The multimeter should display a reading somewhere between zero and infinity. Because of the number of different elements we cannot tell what your reading should be, but we can be certain of what it shouldn't be. If the reading is zero or infinity the heating element is definitely bad and should be replaced. Proper power ( careful!! ) to the heating element ( 110-120 volts AC ) and the defrost heater does not come on = bad defrost heater.
Some newer refrigerators have been using an electronic board instead of an mechanical defrost timer. The adaptive defrost control does the same job as the defrost timer ( shuts off the refrigerator cooling items and redirects the power to the defrost heater for the defrosting cycle, then redirects the power to the compressor and fans to come back on when the defrost cycle is over ). This adaptive defrost system is a bit different when testing, normally speaking if the defrost heater and defrost thermostat ohm ok, we replace the adaptive defrost board.