I have a Dell Dimension 8400 which recently shut down for no reason. The power switch had a solid amber light instead of the usual green. After disconnecting the power cord and reconnecting it some time later, the computer started up with no problems. A week later, the same thing happened only this time the amber light was blinking. I opened the tower and removed the power supply only to find the 4 pin, 12 volt connector between the motherboard and the PSU to be burned.

I tested the power supply by jumping the green wire and a black wire on the 24 pin connector of the power supply and reading the output voltage on the 4 pin connector. The fan works properly and the voltage output is a steady 11.75 vdc. It seems to be working; however, there is no load on it during this process.

Does anybody know what might be wrong with my computer? Is my power supply bad or am I in the market for a new motherboard? Any help would be greatly appreciated!

I would guess the power supply has problems if you have melted wires, but that may have also messed up the motherboard, as a motherboard is more sensitive than a power wire. Did you test any other voltages? Sometimes just one voltage goes wacky, while the others are ok. If your 4pin connector is fried though then I'd think it was putting way too much voltage out at some time.

A power supply would be about $40.
http://www.newegg.com/Product/Product.asp?Item=N82E16817104953

A motherboard would be about $110
I think the 8400 has a motherboard that's in between atx and micro atx, so you'd probably have to get a micro atx board to get it to fit. It might have to have some modification to get the power button/leds to work with the new board. Here's a micro atx board that should work.
http://www.newegg.com/Product/Product.asp?Item=N82E16813131545



I'd probably replace the power supply first, and if that fixes the problem, good, but if not then replace the motherboard as well. Hopefully it didn't also mess up the cpu or ram or anything, or it could get very expensive.

http://support.dell.com/support/edocs/systems/dim8400/SM/specs.htm#wp1052308

Read this as well concerning light colors/meanings.
http://support.dell.com/support/edocs/systems/dim8400/SM/adtshoot.htm#wp1053720

I have good news! Just on chance that Dell might fix it I called there tech support department. They are sending me a new tower in a few days in exchange for the old one due to some saftey clause they have. Thanks for the help!

i had one of the mem sticks in my gateway burn up through the connector on the mobo... mobo fried too. This was a result of lack of ventilation. i left it in a cabinet powered on for way to long. I hope this isn't how yours got burned but hopefully if it is now your next one wont get the same lack of ventilation.