So I understand.. the manual switch should work...
1) OFF meaning... let the thermal switch do its job.
2) ON meaning.. turn it on NOW and override the thermal switch
right? So this way, there is no chance the fan will miss a cycle and somehow lose its ability to cool the engine.
So, ON, if left on by mistake would just run the fan (whenever the Ignition switch is ON) forever. But if OFF, it would run the fan according to the Thermistors setting. I like this. There is no chance of ruining the engine due to no fan at all that way.
I can see using a dual thermistor circuit in a perfect world, where you have one go off at the standard 185 degrees, and the other starting at 190 or so. Ok, that is also a new idea. Remember, if I do two Thermistor circuits, it would be two thermistors in the same lug cavity, set 'differently'. Meaning YOU set the temperature of one, and YOU set the temperature of the other via a small potentiometer on the circuit board for each. So this is configurable by you on both counts. Yes?
I can also add a 10 second delay circuit, but that is not really exactly what you are going for. You are going for optimal engine cooling. Keepem Coming Folks! Great Post Seattle!
