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.
Nevermind, I got confused with a four pin fan (has a seperate PWM pin), and you are using a three pin fan. If you use high side switching you should be fine, it keeps the tacho sensor's ground pin tied to ground.