When you work on a heat pump system and you want to test defrost there are many different test procedures to follow to test the board and sensors.
Most involve “forcing” a defrost by shorting out pins on the board or advancing the time of the defrost initiation and installing a factory provided pin jumper.
Lots of pins and jumping involved.
But one thing to need to be able to distinguish is whether the system uses sensors or thermostats to initiate and terminate defrost.
A thermostat is an open and closed switch, they are usually round in shape like the one shown above and they open within a set temp range and they close within a set temp range. The one shown above is a Carrier Defrost Thermostat and it closes at 30 degrees +/- 3 degrees and it opens at 65 degrees +/- 5 degrees. In this case, because this particular sensor closes in colder than 32-degree temps you can’t even use a (freshwater) ice bath to test it.
If it is below 32 outside it is easy to test (duh) otherwise you can just run it in heat mode with the outdoor fan off and see when it closes by using an Ohmmeter and testing against a line temperature clamp in the same location.
On a defrost thermostat you can also easily jump it out to test the board since it is just open and closed.
A defrost “sensor” is generally a thermistor. A thermistor changes resistance based on the temperature it is exposed to. In order to test you can measure the ambient temperature, make the sensor is removed and acclimated, measure the Ohms of resistance and compare to the manufacturer chart.
You CANNOT jump out a thermistor with a typical jumper to test.
P.S. – A podcast about Heat pumps is available HERE