Why does PC power up for only 3 seconds?

By Mimentor
Jun 5, 2006
  1. Hi,
    I have a Antec Sonata Case with 550W PSU, Intel LGA 755 3.2 CPU on a Gigabyte 945P Mobo that won't start up. The PSU checked OK, the Mobo is new (Replaced an ASUS P5LD2 Deluxe that just stopped for no apparent reason). The Graphix Card is NVidia XFX 6600 and was OK. (Full Specs are in my profile)

    We moved house and PC was going fine. Went to start it up and dead - no fans but LED on P5LD2 Mobo was on. Checked PSU in a P4 PC and it was ok. Thought Mobo was cause - replaced with Gigabyte 945P and same problem.

    Reversed the power switch connector (Opposite Manual) and PC seems to start up for 3 secs then stops - Fans on PSU, Case, CPU and Intel 945 Chip fan all run then stop. It's too short to see monitor start up, POST etc.

    Any suggestions?
    This machine was supposed to be the flagship of our internet business. So far its had a new power pack, two ASUS P5LD2 Motherboards. Instead of the flagship its more like an expensive boat anchor! puke: puke: puke:
  2. howard_hopkinso

    howard_hopkinso TS Rookie Posts: 24,177   +19

    Maybe during the move, the heatsink has either fallen off or moved.

    Take off the heatsink and clean off all the old thermal pad/paste from the heatsink and cpu die.

    Apply a very thin layer of new thermal paste to the cpu die and reattach the heatsink.

    Instructions for applying thermal paste can be found HERE.

    Regards Howard :)
  3. Mimentor

    Mimentor TS Rookie Topic Starter

    Why didn't I think of that - I deduced it was a power issue - should have tried that!

    I'll let you know what happens.
  4. Mimentor

    Mimentor TS Rookie Topic Starter

    Tried Heat sink paste renewal

    Ok I removed the heatsinks for the 925 chip and CPU, cleaned them off and put new thermal paste - no change.

    I am puzzled because it seems to be a power (or powering up) problem but I don't think its long enough to actually overheat a chip. I'm guessing it's a voltage issue somewhere.
  5. iss

    iss TechSpot Chancellor Posts: 1,994

    What you are describing sounds very much like a power issue, or as howard allready noted a CPU overheating issue. if your PSU is not delivering enough power on the 12V rail you will get exactly what is happening. also the same thing can happen if the secondary PSU\mobo connector is not plugged in.

    Given that one Mobo has allready died for no apparent reason. I would check that PSU's voltages. if that PSU is going bad then your new board is at risk of stopping as well.
  6. Mimentor

    Mimentor TS Rookie Topic Starter

    Thanks - I 've come to the same conclusion. The 4 pin 12V extra plug is plugged into the Mobo so I can't think of anything else.

    I wish there was some way of pulsing a PSU, like the Mobo does, so that it starts up and I can meter the voltages. (Not a recommended practise for average enthusiast but if there is a simple way could you email me privately - - just as a saftey precaution, so we don't inadverdantly fry a few beginners).
  7. Vigilante

    Vigilante TechSpot Paladin Posts: 1,666

    Actually you can force start a power supply quite easily. Just get a paper clip and short the power on signal lead.
    Although it's best to have SOME load on the PS, like just plug in a floppy or a cd-rom to one of them, while you test the other plugs.
    I do this a lot for that purpose. It's pins 14 and 15 which is green and black and the two pins are just offset from the retainer clip. That is, the green and black are on the SAME SIDE as the retainer clip, and a little offset. Hard to miss the green wire :) and black is ground.
    Plug in the PS and short those two holes in your PS with a paper clip and it'll turn right on!

    I expect you to use your own precautions on doing this, and have a VOM ready to test the voltages, and a pinout sheet to show what pins are what.

    Good luck
  8. iss

    iss TechSpot Chancellor Posts: 1,994

    Antec's new PSU tester is very nice. it has a light (green\good Red\bad) indicators for all three rails it also has outlets in the back for metering each rail. cant beat it for 12.00.
  9. Mimentor

    Mimentor TS Rookie Topic Starter

    PC only runs for 3 seconds - Problem Solved!

    I got together with another system wiz and we stripped the machine. Outside of the case, it ran perfectly, so we disassembeld the case. When we removed the front, with all the switches and sound/USB etc connectors, we discovered one of the leads to the power switch broken. One end was earthing to the metal of the case and the other was hanging in mid air. When the switch was turned on, it would not work. When the connector was reversed, (the current travelling down the former ground wire) the current was able to ground to the case when the switch turned on, thus the PSU started.
    With this short circuit and no load, all the PSU output was grounded to the case, leaving insufficient to run the peripherals.
    When the CPU tested this in POST, a low voltage condition was detected, so the CPU shut the system down. That was why the PSU ran for a few seconds.
    It took the two of us three hours to find the cause.

    The wire was broken by accident when I replaced the front USB connectors a year ago (damaged when I knocked a memory stick that was plugged into it). I was careful to make sure the wires were clear of the facia's locating pins when I refitted the front cover. On the Antec Sonata case, there is a second front facia that covers the dust filter and front USB/Sound ect panel. When I fitted this, I must have fouled the wire with the locating dowels.

    When we moved house, last weekend, lifting the PC was all it needed to move the wire where it would short out againsta the case.

    The good news is that the old boards are all Ok and the PC runs again.

    We used your trick with the paperclip for starting the power pack - thanks everyone.
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