The aux fan only runs when the key in the ignition is in the on position. and then only when one of two conditions are met. There is a thermo relay on top of your engine, in the water jacket, when it senses temps of approx. 100 degrees C, it will cut on the aux fan. Also there is a pressure switch mounted on the receiver drier for your ac. when ac pressure builds, it turns on the aux fan. IF you motor is running hot, it would help to determine when, as in only when sitting at idle, or when moving down the rode at speed. If it's hot at speed, I would check that t-stat. If it is hot at idle, with the ac off, check that temp sensor, it may not be turning on the aux fan early enough. Or of course it could be both(lol).