Help using a computer power supply to operate a stepper controller

Hi,
I would like to know if anyone knows how to set up an atx computer
power supply to run a stepper controller...
I have figured out all the power supply wiring , But as soon as the
12 volts/8.5 amp lines are connected to the stepper controller, the
power supply turns off.If I connect a small 12 volt fan to the power
supply, it works with the fan only.
I am running a 5 volt line to the stepper controller to power up the
board... when any set of wires are connected , the powersupply shuts
off
There is a wire coming from the power supply called " computer Good".
I assume this wire sends a signal to the powersupply which informs
the power supply that the mother board is good..and to allow the
power supply to work..
What I don't know is how to get the power supply to stay on...
I am using this as a test before buying/ building a larger power
supply to handle all three motors...

The motor connected to the controller is a 3.6 volt 4 amp/phase (
which I believe is 8 amps) unipolar motor
Any help would be appreciated.