A defrost thermostat is the temperature-controlling device within the automatic defrost system of a refrigerator. There are three components to the defrost system: a timer, a thermostat, and a heater. When the coils within a refrigerator become too cold, the defrost timer cues the heater to click on and work to melt any excess ice buildup. The function of the thermostat is to prompt the heater to turn off when the coils return to the correct temperature.
When a defrost cycle begins, the metal alloy switches of the defrost heater are triggered by the timer to switch to warm. When the circuit closes again, it will then cool off the metal. As the thermostat detects this new temperature, it prompts the heater to turn off. A defective thermostat can cause the heater to either overheat or fail to operate at all.
The defrost thermostat differs from the refrigerator thermostat, which is the temperature gauge used to regulate the cooling system of the refrigerator. This device works to measure internal temperatures and prevents the refrigerator from becoming too hot or too cold. If temperatures do become extreme, the thermostat prompts the refrigerator's compressor to turn on or off.
Typically located underneath the refrigerator, the defrost thermostat may alternately be found in the back of a side-by-side refrigerator/freezer. It is attached to wire plugs near the coils of the defrost system, and connected to a wiring harness that will allow for its removal. In some cases, it is wired directly to the harness and will need to be removed by cutting the wires.
Inside the thermostat is a small vile of mercury used to measure the temperature of the refrigerator's coils. The thermostat should read 0°F (-17°C) at its coldest. Readings for this can be done with an Ohm meter, or multimeter, in cases where the thermostat needs to be tested for operation. Often, the vial can come out of alignment and will need to be reset. If it still does not read within limits, it is most likely defective.
Many newer-model refrigerators feature adaptive defrost control boards. These digital boards replace the traditional mechanical defrost timers and thermostats, and plug directly into the refrigerator. The defrost thermostat in these boards works in the same manner, but the entire board can detect failures in the thermostat, compressor, or heater features. The adaptive defrost control board will automatically reset itself after repairs are made.