Small fans keep your desktop computer's internal temperature cool and stable. The fan's label indicates the amount of air flow, which is expressed in cubic feet per minute (CFM). Larger cases will have both an intake fan and an exhaust case fan. Color coded wiring makes it easy to troubleshoot defective fans.
There are standards that determine the pin-outs for interconnected components in PCs. There are only two standards for connecting the fans. The three-wire connection provides positive DC voltage, ground and fan speed. On the four-wire version, the fourth wire is for a speed control signal to the fan.
The three-wire configuration includes a Molex connector to accommodate three wires. They are: black for ground, red for positive DC and yellow for the output tach from the fan. The tach line provides RPM information back to the computer.
Most computers use the 4-wire configuration. This includes a fourth wire that provides a pulse width modulated (PWM) signal to the fan to control its speed. The fourth wire is usually blue.