Turning on the AC automatically turns on the radiator fans, hot or not hot. In order for the AC Condensor to do its job it must stay as cool as it can. As for the engine coolant temp sensor, if the coolant is not hot enough to make the fan come on, then there is no need for the fan yet and there is no problem. Simply the engine is not hot enough to turn on the cooling fan, its not hot enough outside, you are driving getting plenty of air flow through the grille to keep things cool without the assistance of the fan. Its possible that you have no problem at all.