I am sure there has probably been a thread about this long, long ago, but i recently installed a tachometer in my PC, and I think it is really cool. Sorry if this is old news.

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I didn't order the preassembled tach, just the software. and used a tach i bought from a local autoparts store(exact same one that they use), along with a serial cable, a 4pin MOLEX fan connector, and some misc parts from radio shack. (side note, it would have been cheaper just buying the preassembled one, but not nearly as entertaining)

basically, you wire up the tach, and install the software.(only about 3MB zipped) the software has to be running for the tach to work, so it loads at startup. It is a rather pointless mod that gives you no performance gain, but it is a great way to show the world that you are truly a computer geek.

the only problem i came accross is that they forgot to send the wiring schematics along with the software, but after a few emails and some trial and error, it is working.

the tach has 4 wires: red, white, green, and black. the red and white wires connect to the 12v+ lead on the molex connector, the black goes to the black on the molex connector. the green wire connects to Pin3 on the serial cable.

the only other problem is mounting it. it is too large to fit in a 5.25" bay. The one i bought has a mounting bracket that could be used to mount it to the steering column in your car. i am thinking of mounting it to my monitor instead of in my case. either way, you have to run a wire through the back of the case.

if it is mounted in the case, you will have to run the serial cable through the back, if you mount it outside, then you have to run the power wires through the back.

i made a little modification so i didnt have to do either. i bought a 5pin DIN jack and mounted it in the back of my case. it is connected to the molex connector inside. this gives me an external power jack. i then connected the 5pin DIN plug to the power wires on the tach. I used some insulated spade connectors, so i can remove the DIN connectors and connect the tach directly to the molex connector, in case i want to mount it in side my case at some other point in time. and since i am only using 2 of the pins on the DIN jack, i could always connect a third one for the serial connection and just have a little patch cable running from the DiN jack to the serial port ouside.

i used some of that spiral wire wrap to keep the wires all nice and pretty. I'll take pics when i get home tonight and post them.