What's that square 4-slot power plug for on motherboard

I am trying to trouble shoot a possibly bad power supply.

What is the smaller 4-slot power plug on the motherboard for? (the reason for my asking is below)
slot A in this photo.

My PC was not powering up this morning.
the power supply has a flashing green light on the back.
when I removed the power supply and disconnected everything from the MB and repowered the PS, the light goes steady green.

figuring, what the heck, I plugged the longer power male back into the MB (I think it is called an ATX power supply) And a light went on and I was able to power up (sort of) at least the fan fire up and it made all the right poweringup sounds.

if I then disconnected though the power supply box from the mains power and reconnected, I could NOT get the unit to power up. (at this point I still had not connected the smaller square 4female slot plug).

So, the only sequence that allows me to sort of power up is:
- Disconnect ATX plug.
- leave the small 4slot plug DISconnected
- power up the Power Supply box
- plug in the ATX plug to the MB.

hit the ON button and it seems to power up. (though with the HDD attached it does not boot to Windows XP)

When however I then plug in the small 4-slot male plug it all just dies.

What does the square 4-slot plug power?
8 answers Last reply
More about what square slot power plug motherboard
  1. Well, Slot A seems like a regular PCIE power port. I guess that's how it gets power.

    Have you made sure you put caps on the right motherboard pins? Those things to the far-left of the board, near the bottom. Very easy to not put a cap on right or to forget it.

    I had a similar problem once I installed a new CPU. Had to turn off my compy and turn it back on around 12 times to get it to boot (And once it booted, 'Twas fine). The remedy was to overclock my ram (Stock 667 MHz) by 8 MHz (675 MHz).
  2. A = 24-pin Main Power
    B = 4-pin CPU Power

    Both are required to boot.
  3. That 4 pin plug on the motherboard provides additional 12 volt power to the cpu
  4. So, when the 4pin plug kills all power? what's happening?
  5. On my power supply box I have a steady green light when it is just externally powered. But as soon as I plug into the motherboard, it goes to flashing green.
    does that signify something (he asks naively)?
  6. pjstock said:
    So, when the 4pin plug kills all power? what's happening?

    Than it wont boot : ), but the mobo is still under power from the 20/24 pin power cable.
  7. faulty psu or motherboard most likely motherboard its irrelevant that when you d/c that 4-pin it "works" its either its all plugged in and working or its NOT

    new motherboard
  8. MajinCry said:

    Have you made sure you put caps on the right motherboard pins? Those things to the far-left of the board, near the bottom. Very easy to not put a cap on right or to forget it.


    Thank you.
    but the photo was not of my actual motherboard. (the photo I snapped of mine was too dark.) Rather it was a photo I found online that showed the same key bits that I was inquiring about.
    Peter
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