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Just a "Wild Donkey Guess" but sometimes one fan terminal (HIGH-side) is wired directly to +12VDC and the other terminal (LOW or GND-side) goes to the "switching-to-ground" circuitry, either a grounding thermal switch or electronic-control from the engine ECU/ECM that provides the electrical path to ground. Hence, if he has connected to the "high-side" terminal, it would appear that the fan is ON all the time.
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