it should come on low with the ac all of the time and when the pressures in the ac system gets high enough the fan kicks into high, the other time it can come on is when the sensor on the right side (cool side) tank of the radiator gets high enough the fan will come on low and if it gets hotter it will kick into high, this only happens when something is seriously wrong and you are minutes away from engine dammage if it can't regain control of coolent temp. the fan's first function is to make the ac work at low road speeds and keep pressures in check, cooling the engine is secondary
all america wants is cold beer warm cat and a place to take a poop with a door on it