rurwin wrote:Did you measure it with an oscilloscope? because a meter will just tell you lies.
It's almost certainly a short pulse every revolution. A meter will just give you the average voltage, whereas what you need is the peak voltage.
I was hoping someone would mention that. It is indeed a pulse train, related to the rotation speed. Some fans seem to have a transistor pulling the line to ground, with a pull-up resistor to the 5V rail, so you will see an "average" voltage of between 0v and 5v if you are reading it with a Digital/Analog volt meter. It is most certainly NOT the voltage you are reading!