Putting a cooling fan onto a 3D printer is pretty necessary. If you don't, then the layers on small objects will not have time to cool, and the print head will mush them around and ruin your crisp lines. You can avoid this in Slic3r by telling the tool head to go off and draw random circles if the time it takes to do a layer is under a set amount of seconds. This can add a lot of time to your print, especially if you've got a tall, thing tower.
Fans fix all of that. I got a small (1.5" x 1.5") fan from Makergear when I purchased my hot end from them, which was a nice little surprise. The wires were only 3" long, but a quick and dirty soldering job fixed that. After that, it's just plug and play, right?
First I had to figure out what to plug the fan into! A quick Google revealed the connection should be at D9 on the RAMPS 1.4 board. Plug the wires into the screw terminals, and we were off!
Not really. I wanted to test the fan to make sure it would work while printing. I typed M106 S100 (M106 activates the fan, S### controls the speed, from 0-255) into Pronterface.
I read on the Reprap forums that sometimes the fan had to be all on, or it wouldn't work. M106 S255.
At this point I was a little worried, since people were talking about possibly having shorts and replacing MOSFETs and all that. That was way too much trouble for a little fan. But I saw that my D9 LED was lighting up when I activated M106, so I figured the board must be fine. But what about my connections? Remember when I said that the wires were too short, and that the soldering was quick and dirty? Well, I wasn't exaggerating! I thought that maybe my solder joint was so bad that no signal could get through.
I took the fan off the RAMPS and hooked up each wire to the terminals of a battery. Lo and behold, the fan spins! So the problem, obviously, must be on the board.
I went to plug it back in when I noticed something obvious. There was a small "+" next to one of the screw terminals. And, wouldn't you know it, it was next to the screw terminal that I had connected the ground wire to.
I plugged the red wire into the "+" terminal, and the black to the other, then loaded up Pronterface. M106 S255. A small whirr greets me! I tested it with other values, before finally typing M106 S0 to turn off the fan.
So, to make a long story short:
To use a 2 wire cooling fan on a RAMPS 1.4 board, plug the fan into D9, making sure to plug the positive wire into the positive socket. If M106 doesn't do anything, try switching the wires.