[misterfixit]

The radiator fan comes on HIGH speed when AC is on,the low speed comes on when engine temp reaches a limit set by the engine temp. sensor, You won't get over heating if the coolant is full but your air won't be coldest at idle and the engine will bog down when the compressor kicks on and cycles through the high limit. WITH THE AIR OFF the high speed comes on only if the engine temp. exceeds 140 deg., Like after you slow down from freeway trip and the hotter coolant rushes from the radiator. It's normal to do that to stabilize the coolant temp..The previous post explain this.