How to Up-Keep Your Own Older Appliances

How To Test A Refrigerator Defrost Thermostat

If your refrigerator isn't cooling, test the defrost thermostat. The defrost thermostat is a bi-metal switch that prevents the unit from overheating. As the defrost cycle runs,it warms the metal alloy in the switch, which closes circuits to the heating elements.

It triggers another circuit as the metal cools, then the defrost heater restarts. A malfunctioned defrost thermostat prevents the defrost heater from restarting. Here are some tips to test a refrigerator defrost thermostat.

Prepare to Work

For this project, you need:

  • work gloves
  • warm towel
  • Philips screwdriver or flat-head screwdriver
  • digital tester or continuity tester
  • small metal pan
  • thermometer

Disconnect the power cord to the refrigerator, which is commonly located on the wall behind it. You may need to move the refrigerator to unplug the cord. For extra safety, turn off the power to the room from the breaker box.

Find the defrost thermostat. On models with freezers on top, check under the front floor plate of the refrigerator, or the back of the freezer. On model that have freezers on the bottom, look under the roof plate of the freezer compartment. The defrost thermostat is commonly located on the rear of the freezer side on side-by-side models.

Remove the Defrost Thermostat 

The defrost thermostat is hidden behind a panel. Use the screwdriver to loosen the screws on the panel. Press down on the retaining clip with the screwdriver to free the defrost thermostat.

If your refrigerator is old, you may need to remove plastic molding before you access the freezer floor. Molding breaks easily, so remove it slowly. 

Detach the thermostat from the retainer clip by squeezing the clip, and unhook the two wires by unscrewing the wire nuts. If needed, use needle-nose pliers to remove wire nuts.

Some models may use screws to attach wires. Remove the screws by rotating the screwdriver left. Wipe ice from the defrost thermostat with a warm towel.

Test the Thermostat

Set the number of OHMs on the multitester to X1, and insert the red test lead into in the red jack marked VmA, or volts millampres. Attach the black lead into the black jack labeled COM. If your defrost thermostat is working properly, the multitester will give a reading of zero.

A defrost thermostat should be closed at a zero reading, and it should open at forty to ninety degrees Fahrenheit, or infinity. Test to ensure it opens at the correct temperature by placing it in a metal pan filled with water. Drop a thermometer in the pan, heat the water, and check the temperature.

Defrost thermostats that don't open and close at the right temperature, or give readings other than zero or infinity,need replacing. If the defrost thermostat still works properly, but the refrigerator still isn't cooling, contact an appliance repair service. If you don't feel comfortable doing these types of jobs on your own, contact a business such as Prompt Appliance Services, Inc.