do you have a thermo-bob or other aftermarket thermostat??
According to Clymer, the thermal fan switch activates at 201-212 degrees F. (Generation 1).
Thus, any thermostat opening at any temperature below 201 degrees F. (including the Thermo-Bob's) would have no significant effect on fan activation, seems to me.
After the thermosat opens, Themo-Bob (or other radiator bypass) or not, engine cooling appears at the mercy of the water pump coolant circulation and radiator air flow alone, seems to me. An operational thermal switch kicks in regardless when the coolant around it reaches 201 degrees F., or thereabouts.
My perception only; clarifications and corrections welcomed.
Coolant temperature correlation (lineraly across arc, from 150 to 270 degrees F.), from Wattman''s (Thermo-Bob) website:
These coolant temperatures, of course, are from coolant about the cylinder head, where the temperature sensor is located. The temperature at the bottom of the radiator, where the thermal switch rests, is likely lower, especially at idle.
I think the coolant temperature is highest at the cylinder head, near combustion, and lowest at the bottom of the radiator, having exchanged heat as the coolant descends through the radiator passages; YMMV!
Should have mentioned; a paper temperature gauge background, calibrated in degrees F., is available from Thermo-Bob, who provides a detailed installation procedure. The new background can be pasted behind the temperature gauge glass, and the coolant temperature read in degrees, instead of red markers.