Fan yellow wire.

[Marduk]

hello all.. i was looking to find some info about the yellow wire on PC fan or 3 wire fans other then the Pos and gnd. what is the 3 wire use for temp sensor or speed control?

[Kremmen]

Speed control. You get a pulse train from the wire when the fan is turning.

[mariush]

sense = fan reports number of rotations to motherboard by sending 2 pulses for every rotation
control = motherboard sends signal used by fan to adjust its speed

3 wire fans don't have the control wire.

The colors are sometimes different, for example I saw the fourth wire colored green. But if the fan is to be used with computers, the order is this one all the time.

[TerminalJack505]

If it is a three wire fan then most likely the third wire is a tachometer output.  The output will be wired as an open collector.  To use the output you'll have pull it up with a resistor to Vcc.  The open collector allows Vcc to be (within reason) any arbitrary positive voltage. 

Most fans will pulse twice per revolution.  You can use the rate of the pulses to determine the fan's speed.

[ivan747]

This pin serves to measure the speed of the fan in RPM. If my memory serves me well, you connect a pull-up resistor on this pin and the output should give you 2 pulses per revolution. If you are not familiar with pull-up resistors:
http://en.wikipedia.org/wiki/Pull-up_resistor