Dear cookdd,

( Caution ) Unplug the refrigerator.No power is needed for these tests.

The defrost system consists of three parts.
The timer, heater and thermostat.
Rule out the two before replacing the timer because the timer is a little more complex to advise on.
If you have an ohm meter set it at the lowest setting ( RX1 ) to check continuity on both the heater and the thermostat.
The heater should have continuity.
The thermostat will also have continuity when its frozen or at its coldest.
Be sure to isolate the parts by disconnecting the wires for accurate testing.
The heater and thermostat are located behind the back wall in the freezer section near the evaporator.
The timer is either in the control housing with the control that adjusts the temperature in the fresh food section or down on the bottom behind the kick plate.