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The idea is to suck the cold air through the radiator, thereby cooling the water within the radiator. It tries to simulate driving conditions, when air is naturally forced through the radiator, thereby cooling the water. (Im assuming that youre talking about the radiator fans?)
Are you sure? When the fans come on, put a small plastic bag on the radiator front, but dont cover the whole radiator, and if the bag gets sucked on, then its working OK, and if blown out then not. I cant see how the fans can be reversed, as the electrical plugs should only fit on one way. What car are you experiencing this problem on?