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... the voltages on the various wires running to that Molex, they are as follows:
Yellow: +12v
Black: Ground (earth)
Black: Ground (earth)
Red: +5v

Note that there are two black wires, both of them are ground, or earth, wires. My fans have one red wire and one black wire. You might expect that you would connect red to red and black to black right? Wrong, at least in this case it would be wrong. These fans, as are most case fans, designed to run at 12v, so if you connected it in that way, the fan motor would only receive 5 volts and would either run very slowly or more likely not run at all. To connect these fans for 12v operation, you would connect the red wire on the fan to the yellow wire on the Molex and the black to black.

Another option is to hook up the fans using the so-called "7-volt trick" so they run more quietly. This involves connecting the red wire on the fan to the yellow wire (+12v) on the Molex and the black wire to the red wire (+5v). Sounds kinda scary when you first hear of it, like a recipe for acrid blue smoke, but it really isn't. The main thing to remember is that voltage is all about the "difference" across a circuit. In this case the difference is 12v - 5v = 7v. Now some people predict that all sorts of doom and gloom will befall you if you try this, but I've being hooking fans up this way for years and the planet hasn't spun off it's axis yet. If you are really adventurous, you over-volt a 12v fan to 17v...

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