Main Motorcycle: '07 Dyna Low Rider
Join Date: Apr 2012
Location: New Hampshire
Other Motorcycle: '99 Legend TT
Extra Motorcycle: Thunderbolt, T150V
I forget, did you test the temperature sensor? How? The light should come on when the sensor reaches about 50 ohms. The water should be very close to boiling at that point. The factory test is at 60*C, expect 255-310 Ohms.
At this point you have two courses of action available. The best thing to do would be to use an IR temperature gauge to monitor the temperature of the elbow at the top of the cylinder head. Since you said the fan switch is new, and you've tested the fan and it works, it seems likely that the temperature has not reached the set point for the fan and the light is giving a false indication.
If you don't have a way to measure the temperature your only other course of action is to continue testing it to see if it boils over before the fan comes on. Since you've got good circulation and the fan works, I think that's a fairly safe way to proceed. To be more certain, test the fan switch in boiling water. If it closes before the water boils you're good to go.