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4a. The high-tech fan controller takes its cues from this water temperature sending unit that I spliced into the upper radiator hose. The controller will also turn the fans on anytime the A/C compressor is activated via a wire run to the fan clutch. Fine-tuning of the on-and-off cycle of the fans is made by adjusting a small potentiometer on the controller, resulting in the coolant temperature always staying between 180 and 195 degrees.