Computer turns on when i plug it in

hi. i am in the process of rebuilding a compouter after the motherboard stopped working. i bought a second hand motherboard and have fitted it succesfully. however there is a problem. after i plugged all of the wires in for the power and L.E.Ds, i plugged the power leaad into the PSU and it started without me poushing any bottons or anything. i have unplugged everything except the power going into the emotherboard, and the cpu fan, and tried it like that and it still turns on as soon as i plug it into the power.
i plugged my monitor in to see if there was a bios screen so that i could go onto that and see if there was anything on ther that would be startiing it automatically but there is no post or anything on screen.
if anyone has any ideas as to what the problem could be, that would be great.
my motherboard is a fujitsu siemens d1451 with a pentium 4 processor.
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  1. I had the same situation with my old PSU. For me the PSUs start up circuit was bad - for a while exactly what you had happened - after a few days the PC refused to POST and I later found some MoBO damage.

    That is one possible issue - the other(albeit simpler) is that your power on line (2 pins) which connect to your case's power switch is shorted.

    I would really check your PSU first - not just a short test, as the PSU is obviously providing power but its +5v rail's power on circuit could be iffy.

  2. hi. no its not the psu. i put another psu in. it is also not the power switch as it is unplugged.
  3. What you need to check then is the connector on the motherboard to which the power on which (which is the wire FROM the case's power switch). It could be shorted or poorly seated.

    A quick test would be simply to unplug that connector from the mother board and see if the PC boots (it shouldnt) . It will be a 2 pin prong not unlike a large jumper. All it does is complete the circuit that starts your pc.

  4. PS - you wont see a POST unless you add some RAM to teh bare bones boot.
  5. hey. thanks for the quick reply. no, as i have previously said, the power switch wires are not connected to the motherboard.
    thanks for the Ram part tho :)
  6. Wow, so the mobo is powering up the system WITHOUT a short in the power switch connector? ?That is first for me. Sounds like the something is wrong with the board then, likely a short somewhere.
  7. i was fearing it would be that. thanks though :)
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