Thank's to all...
PBROUWER: Can you explain this better ? "If you use PWM to control the speed of the fan, the tachio output can be a bit noisy, you may need some filtering / debouncing."
I Have resolved theproblem with a 4n35 optoisolator and a TIP137 PNP.
If you break the negative, on the tachio cable ther is a 10V and is dangerous for the aurdiono's pin.