I have connected the green wire (ps-on) and a black wire (ground) together to get this psu working but it powers on for a second (the fan spins) then it stops. Whats causing this? How can I fix it?

your board is being grounded out by something.

First, unplug all the case power wires from down there and try using a screwdriver to jump off the board (just stick it between the two 'PWR ON' prongs just like the jumper would do). If that doesn't work, I am guessing that the wiring is right, so the fastest way to resolve this is to take out all of your hardware, even the mobo, then put it all back in piece by piece, taking notice that nothing is touching where it shouldn't be. Once you have put it back together, try jumping it off with a screwdriver again. If it stays on, you are good to go. Just turn it back off, hook the case wires back up, and have fun.

If you're talking about the psu alone meaning it's already out and not working with the green and black wire trick then you might have a short inside the ps, could be just the fan or it could be more serious but there's definitely a short