http://open3dp.me.washington.edu/2012/06/jumping-a-power-supply-for-12-volts-dc/

Jumping a Power Supply for 12 Volts DC

Use a 18-20 gauge wire to jump the circuit (they fit well without slipping).

For those of you who have never jumped a power supply the article below is written for you. For those of you who have, just use the images for reference.

The two wires that will produce voltage are green and purple. Depending on where the circuit is jumped you can get the power supply to produce 12 volts. If the fan goes one, you have done it correctly. Always double check the voltage readings with a multimeter.

The double jump is shown since it is not as simple, The single jump works the same, without the wire on the left.

Didn’t work?… I will be more specific.

For a 24 pin it is usually a Single Jump green to black. Sometimes, different black pins work with green. For a 20 pin it is common to require a Double Jump green to black and purple to yellow. You can get a permutation of 4.8-5v and 7.6-8.4v combined to giving out ~ 11.4-12.8volts on the four pin plug used for the Printrboard. If the plug is not required for the board, you can use the black and yellow wires (single or paired) directly into the source. Remember to unplug the power supply, and let the capacitor power down, if you intend to cut the wires. (This means wait until the fan dies, and/or the LED lights go dim)

Wrap clear tape around the wires so they don’t fall out. This preserves the Power supply for other uses. The plastic sheet is taped down near the fan to show: the fan is on and producing suction, so the plastic is flush to the fan intake. (This power supply has an on switch, but no LED light)

12v DC should be produced here.

Note: Most motors in cars are 12V DC.

Junkyards are a great place to be.

This is useful for powering the boards for 3D printers. And, it can be used run any 12V DC motor, as long as the amperage is not ridiculous ie really high, or really low. A great example,  the WOOF group using a (windshield) wiper motor to test out a large extruder driver.

Hint: high torque stepper motors are usually inside the doors driving the power windows. Think about is high surface area with heavy drag, these ones have serious push.

Have fun, and see what you can hack.