RESULT: I checked it as you noted. Thank you for the response.
There is a valve near battery, two prong connector. Unpowered it is open - water can pass. Powered with 12V directly, it closes properly. There is also a small pump further forward in engine compartment. It does pump water through the valve quite well when powered.
My problem seems to me electronic in nature, not with an electromechanical valve. The problem is intermittent.
When the AC decides to trow out heat, i can check the power at the valve connector and sure enough, it is zero. at the same time, the power at the little pump is 12V.
Is there a temperature probe in the car somewhere that may tell the controller that it suddenly got way too cold in the cabin? Where are connections that I might check for this. Is there a procedure that you know of to detect which electronics may be bad?