The problem with the cooling fan control turned out to be an easy fix. The electric thermostat switch (on right in photo) I added to control the fans had unscrewed slightly so it wasn't making good contact with the engine. With it screwed out, heat from the engine wasn't transferring to the switch. The switch screws into the head of the bolt that holds the thermostat housing down. I probably hadn't screwed it in very tight because it is only a 1/8" diameter screw made of brass and I didn't want to take a chance on breaking it off. The cooling fans are now working as intended again..