I have a Dell Dimension 3000 that will power on, but soon powers off. I tested the Power Supply with a multimeter and got readings of 11.5V to 11.9V --I think that's normal? I also got a reading of 4.98V on the other part of the supply. Seems very close to specs (12V & 5V).
CPU looks ok. No burned spots that I can see.
Green light on motherboard indicates that current is running through it.
I tried starting the computer with no RAM and same problem occurred. I also tried starting computer with no power to the Floppy drive, CD Drives, or Hard Drive and it still powers off.
Each time I try to power the computer on after it has just powered itself off, it powers off much more quickly than before.
Sounds like a short. Check all the component plug in, be sure the video card and RAM are seated fully. Check power/IDE/SATA connectors to hard drives and optical drives. If you try booting with no RAM and DO NOT get a beep signifying the MB recognizes NO RAM in the system, that suggests a MB problem. Try resetting the CMOS jumper. Unplug the PSU and remove the battery when resetting the jumper. Problem could be caused by many things. You would need spare parts RAM, video card, PSU, etc. to troubleshoot any further. HTH.
Check the capacitors on the Motherboard. If some look swollen and as if they're leaking that could also be the cause.They tend to swell up with age and start to leak.Might be the cause of your problem.