help me understand this fridge auto-defrost circuit
It seems &quot;DEF TERM&quot; might mean &quot;defrost terminator.&quot; If you can locate this component it may shed some light on what it does -- I wonder if it might be an auto-resetting thermal switch. It seems that a well-placed switch would experience temperature of 32F (or lower) until the moment when the ice surrounding it has turned to water, and then the temperature of the switch would rise. A control could use that behavior to terminate the defrost cycle at the right moment. A time-based defrost would run the risk of being inadequate by letting ice remain or inefficient by heating the freezer compartment excessively.
&quot;D-SENS&quot; might be an abbreviation for &quot;defrost sense.&quot; If the guess about the terminator is correct, this sense line would provide a way for the controller to detect that the defrost terminator had activated. That could have been done by sensing the current flowing through the defrost heater, but maybe in their circuit design they decided a voltage-based sense was better than a current-based sense.
There's definitely some experimentation in your future. I'll guess you could use the NC relay to break the connection between the terminator and the heating element, leaving the sense connected to the heating element. That'll probably interrupt the power to the heater and hopefully still provide the sense feedback so the controller can decide to end the defrost cycle. But you may possibly upset the controller -- it could possibly do some kind of self-test to confirm the heating element is functional and this tampering may cause such a test to fail.