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New PSU - Fans stay on when computer off

I have a weird problem i haven't ran into for the 13 years ive been working on computers! I replaced a power supply for a customer with a 350watt PSU and hooked it all up into the computer. When you plug in the PSU, the fans and everything turn on but the computer wont post until you push the power button, then it will boot up normally. When you hold the power button down, the computer shuts down but the fans keep spinning.
2 Solutions
Dr. KlahnPrincipal Software EngineerCommented:
If the fans are running, this indicates that the power supply +12V line is active.  There is no reason I can think of for the +12 line to be on when the motherboard is not running.  If fact, I don't think it should be possible if everything is working correctly.  The power supply should not produce anything except the 3.3V lines until the start pin is pulled to ground by the power switch.

First thing I'd try is to swap the offending power supply for a known good one, and see if the problem goes away.  If it does, the problem's in the power supply.

If the problem doesn't go away, and you have the equipment, take a voltmeter and check the voltages at the ATX power connector.  The pin out can be found on Wikipedia:


The only lines that should be live when the power supply is off are the 3.3V (pins 1, 2, 13) and the power on (pin 16).
HamStabAuthor Commented:
Have a dozen of power supplies laying around here that are brand new and they all have the same effect on the computer. Just to be sure we didnt get a bad batch, they are testing 3.3v 5.0v and 12.0v right on the money with a digital voltmeter when they are on. 12 and 5 arent getting any voltages, just the 3.3 when off.

Grounding issue on the motherboard? the motherboard has 3 swollen capacitors but ive never seen them cause this issue here. Could be possible though?
Check the BIOS settings - there may be a setting for what the system should do when the power button is pressed/held down.
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You have a failing motherboard, while it may not be dead yet but it sounds like it's close.
>>  I replaced a power supply for a customer    <<   why ?   this can show the bad part...
it can be most of the suggested :  bad (new) PS, mobo..
i'll throw in the bad capacitors : look for bulging or swollen ones in the mobo, and old PS;
a bad PS,  "may" have taken the mobo with it..
HamStabAuthor Commented:
Old power supply was dead which is why it was replaced. I've also had other techs here look at it and as a second opinion outside of the work place, we believe the motherboard to be bad.

BIOS settings are set correctly for this type of system.
HamStabAuthor Commented:
I agree that he MB is indeed bad and that the PSU might have taken the mobo with it. See it quite a bit but not usually on this particular model of computers.
tx for the feedback !

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