Need to know what make model an year vehicle to say for sure . Most radiator fans are controlled by a relay . Do you know what a relay is ? The relay is energized by the engine computer / ECM/PCM . On the engine somewhere is a ECT - engine coolant temperature sensor , this tells the engine computer how hot the engine is . When the engines temp gets to around 201 degrees the computer turns on low speed cooling , then when it gets to around 221 degrees high speed kicks in ! Some vehicles have up too three relays controlling the fans .
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