Why isn't the fan by the radiator running

the fans will come on instantly when the A/C is turned on,and will also come on when the engine reaches the temperature required for them to come on to cool down when the computer sends a signal. when you are driving it,say on the interstate and outside temperature is 60 degrees,then you probably have enough air flowing through the grill to keep it cool and the fan(s) may not need to come on.