Hi again.

This fridge is equipped with a test plug.

According to the fridge's wiring diagram, measuring the resistance between the orange wire of the test plug and the blue wire going to the defrost timer will measure the resistance across the defrost heater. I did that and measured 38.7 ohms. The factory spec is between 34.6 and 37.5, so it's a little out of range, but at least I know that the defrost heater is in one piece.

According to the fridge's wiring diagram, measuring for continuity between the orange wire of the test plug and the brown wire going to the defrost timer will determine whether the defrost thermostat is making the circuit to the defrost heater or breaking the circuit to the defrost heater. I did that and got no continuity through the defrost thermostat. So, the defrost thermostat is breaking the continuity to the defrost heater.

That means that the defrost heater is good, but the defrost thermostat isn't allowing any current through that heater, so I still could have a block of ice around the evaporator coil.

In this case, a good assessment would be to unplug the fridge for a few days to allow that ice to melt, and then fire up the fridge again. If it freezes water initially, but then the freezer compartment warms up, then I've got good evidence to believe the defrost thermostat is on the fritz and preventing the defrost heater from working.

Thanks for everyone's input.