Here I am testing the fan speed sensor on one of several fan clutch units. I supply the hall effect sensor with 5 volts DC using a power tool battery and a voltage regulator set to output 5 volts. I have connected a multimeter to read continuity between the hall effect signal output and ground. As the fan clutch rotates the sensor grounds out its' output terminal and the meter beeps for continuity indicating a closed circuit. There are 6 of these pulses per revolution of the fan.

On a fully assembled fan clutch in a vehicle you can do a similar test by backprobing the hall effect sensor wire at the connector on the fan shroud and test for DC voltage, not continuity, to ground. With a working sensor you will see about a 5 volt value in six places as you very slowly rotate the fan blade.

Why 5 volts output while in the vehicle and not while bench testing? In reality the sensor is not sending out anything on the signal terminal. The sensor is grounding that terminal. The PCM pulls that circuit weakly high to 5 volts and the sensor grounds it out 6 times per revolution. The PCM sees the pulsing of the voltage drop as the sensor grounds out the 5 volts and this is how the PCM knows the fan RPM.

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