24 pin power supply wont work unless i remove part of my 24 pin

[destroyer6295]

i swapped out my mobo and cpu and when i went to turn it on nothing happened i checked cable connections and used my psu's self test feature to find out that if the 10 pin part of my 24 pin was unplugged (smaller psu side connection) the self test would work and everything would light up but as soon as i plugged it in and used the self test button nothing happened its a corsair ax1500i psu

[mariushm]

Your psu uses Type 3 cables, as you can see written on the cables. 

You can find the pinout online, in fact I posted it a while ago in another thread ... the first two are TYPE 4, look below them at TYPE 3

 

So the 10 pin gives 5v stand-by to the motherboard (chipset, bios) and has the power ok  (PWR OK) wire, which tells the motherboard that the power supply is ready to turn on the other voltages and start. When you press the power button on your pc, the motherboard (the chipset / bios ) checks that a signal is present on power ok pin and if so, it can tell the psu to start through the PS ON pin (it connects it to ground through a small value resistor) and then the power supply starts completely sending voltages (3.3v , 5v , 12v ) through the other wires. 

 

The 10 pin also has a 3.3v sense .. basically it's a wire that's used only to measure the voltage arriving at the end of the cable, on the 24 pin connector. If there's a lot of current consumed on 3.3v, there's gonna be losses on the wires between power supply and 24pin connector, so at the connector you may see 3.2v or 3.1v - With that 3.3v sense wire, the power supply can measure that drop in voltage and counteract it by sending slightly more that 3.3v through the wires, knowing that due to losses, it's gonna be 3.3v by the time it arrives at the 24 pin connector .

 

I would start by checking if there's a PWR OK signal using a multimeter, when the 24 pin connector is plugged in. For example, it could be there's something taking too much power on 5v stand-by or there's some short circuit on 12v or 5v on motherboard which causes the power supply to be unable to turn on the other voltages, so the power on pin says power supply can't start. 

 

 

[destroyer6295]

i wasnt sure how to test what you said i have a small voltage tester never used but no multi meter however since you said it could be something on the motherboard trying to draw too much power i tried un-plugging all power from the board (all i had was some sata power plugged in) and use the psu test button but still no power so not the motherboard drawing too much