All the RPM sensing is is a small contact inside the fan with one part on the spinning part and the other on the box part. The contact is connected to the +voltage inside the fan. As the fan spins it connects and disconnects the contact and it gives you a pulse.
If you hook that wire to ground POOF no more fan.
If you have a voltage tester take and hook the ground to your case and the +voltage to the RPM sensing line on the fan. Now turn on the fan. If your not getting anything then the contact inside the fan is bad, if you are then it might be your motherboard...
Make sure you run the fan at 12v or it most likely wont get a reading. It will be real jumpy because of the pulsing...
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