So the power supply (I hope) went on one of my computers during an electrical storm recently, and the computer won't start. I have an old PSU, but unfortunately it only has a 20 pin connector, and my motherboard has a 24 pin header. I don't need the system to run, I just need it to POST. After I determine that it isn't the motherboard that has died, I can pick up an adapter. What I need to know is will it even get that far without the adapter? I'd rather not pull a PSU from one of my other computers just for testing.
Yes it is possible, but not recommended. It is better to buy a 20 to 24-pin adapter to avoid power problems with PCI-e cards that do not have their own power connector. See below diagram, make sure it's all the way to the right: