Precision t3600 power supply upgrade not working

I got a t3600 and tried upgrading the power supply from the 425w one it came with to a 685w power supply but it won't turn on with it. When the new power supply is not in the computer and I plug it in and press the button on the back the light on it turns solid green, but it stays completely off when I do the same with it in the computer. I switched back to the 425w one and it runs fine. I checked where I bought the 685w one and it says that it should be compatible. Help would be much appreciated