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PC Powers on then powers off

Last response: in Components
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December 9, 2011 12:39:56 AM

I have a custom built computer that has not given me any problem. I have a Intel D945 PSN motherboard with a Pentium D 2.8 processor, 4Gb of ram ,550 watt Antec power supply and Nvidia GeForce7 PCI-E card . I have went through all the steps of removing all the components and testing them one by one. I even replaced PSU, motherboard ,processor , memory and video card. I even left the optical drive unplugged just to see if that was it to no avail my computer just boots up for 5 seconds then power off. Could anyone help? Thanks in advanced :ouch: 

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December 15, 2011 2:44:11 AM

do anyone know what tis is I went and got another PSU to no avail computer will still not stay powered on. Could anyone help please
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December 15, 2011 3:01:29 AM

think the problem is only with ur power supply unit... try to change it...
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December 21, 2011 11:43:43 PM

Thanks Ganpati but I have tried a new power supply three times and still the computer will not stay on. I am at a loss here. I tried a thermalake 550 watt ,antec 750watt and a cosair 600watt and the computer still won't power. I know that all 3 could not be bad. I checked and re-checked all connections as well as the 4 pin motherboard plug. :o 
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December 29, 2011 12:03:59 AM

Thanks Davcon for your response but in my first post I replaced all components in the PC and still no luck. The only thing is left is the case itself maybe there might be a short there somewhere. I hope
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December 29, 2011 12:24:41 AM

Can you power up your system when it's not inside the case? Like place the motherboard on a piece of cardboard and make all the connections and see if it powers up?
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a c 156 V Motherboard
December 29, 2011 1:48:31 PM

It's called breadboarding, and if you take all of the components out of the case, it totally eliminates case problems.

Try working through this:
The following is an expansion of my troubleshooting tips in the breadboarding link in the "Cannot boot" thread.

I have tested the following beep patterns on Gigabyte, eVGA, and ECS motherboards. Other BIOS' may be different, but they all use a single short beep for a successful POST.

Breadboard - that will help isolate any kind of case problem you might have.
http://www.tomshardware.com/forum/262730-31-breadboardi...

Breadboard with just motherboard, CPU & HSF, case speaker, and PSU.

Make sure you plug the CPU power cable in. The system will not run without it.

I always breadboard a new build. It takes only a few minutes, and you know you are putting good parts in the case once you are finished.

You can turn on the PC by momentarily shorting the two pins that the case power switch goes to. You should hear a series of long, single beeps indicating memory problems. Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU. Remember, at this time, you do not have a graphics card installed so the load on your PSU will be reduced.

If no beeps:

Check for line power at the PSU input. Extension cords, power strips, and power cords do fail.

If you have power and no beeps, suspect components in likely order are PSU, motherboard, and CPU.

Running fans and drives and motherboard LED's do not necessarily indicate a good PSU. In the absence of a single short beep, they also do not indicate that the system is booting.

At this point, you can sort of check the PSU. Try to borrow a known good PSU of around 550 - 600 watts. That will power just about any system with a single GPU. If you cannot do that, use a DMM to measure the voltages. Measure between the colored wires and either chassis ground or the black wires. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.

The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.

You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.
http://www.youtube.com/watch?v=5FWXgQSokF4&feature=yout...

A way that might be easier is to use the main power plug. Working from the back of the plug where the wires come out, use a bare paperclip to short between the green wire and one of the neighboring black wires. That will do the same thing with an installed PSU. It is also an easy way to bypass a questionable case power switch.

This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.

If the system beeps:
If it looks like the PSU is good, install a memory stick. Boot. Beep pattern should change to one long and several short beeps indicating a missing graphics card.

Silence, long single beeps, or series of short beeps indicate a problem with the memory. If you get short beeps verify that the memory is in the appropriate motherboard slots.

Insert the video card and connect any necessary PCIe power connectors. Boot. At this point, the system should POST successfully (a single short beep). Notice that you do not need keyboard, mouse, monitor, or drives to successfully POST.
At this point, if the system doesn't work, it's either the video card or an inadequate PSU. Or rarely - the motherboard's PCIe interface.

Now start connecting the rest of the devices starting with the monitor, then keyboard and mouse, then the rest of the devices, testing after each step. It's possible that you can pass the POST with a defective video card. The POST routines can only check the video interface. It cannot check the internal parts of the video card.


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January 1, 2012 10:33:56 PM

Thanks for all of your responses but I still have the problem. I did everything as indicated before posting and now I am at my wits end here. :( 
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January 2, 2012 7:03:41 AM

jtcomp1 said:
Thanks Davcon for your response but in my first post I replaced all components in the PC and still no luck. The only thing is left is the case itself maybe there might be a short there somewhere. I hope

Did you changed the MOBO?
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January 4, 2012 11:19:27 PM

Yes I changed the mobo in the first posting thinking that was the problem but obviously that is not the case here.
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January 5, 2012 5:29:19 PM

jtcomp1 said:
Yes I changed the mobo in the first posting thinking that was the problem but obviously that is not the case here.

.... then you need to check that MOBO outside of the case. If you have the MOBO box, you can check it on there.
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