Jess,

In most cases the defrost thermostat is a closed circuit below a certain temperature(usually some where in the 35 degree range).

When the refrigerator is in a cooling mode, it is normal to have an even coat of frost on all the coils of the evaporator.

After a certain amount of accumulated compressor run time(usually 8 hours).

The defrost timer or ADC control board cycles the defrost cycle on, and with the thermostat closed the heater comes on and stays on until the defrost thermostat opens and turns the heater off.

There's usually a 10 to 15 minute delay, after the heater turns off, before the control turns the power back on to the compressor and the cooling cycle begins, again.

Hope this was helpful,

:cool: :cool: :cool: