I've reached Vault 15 in Fallout 2 and most of the faqs say you can repair the generator on level 2 for xp. However, all I get is the message "You can't find anything wrong with it, it just doesn't work". This doesn't seem to be the normal repair skill failure message. Is their something special you're supposed to do? I am using the restoration patch, if that changes anything.

I wouldn't bother with it, sometimes the guides are not fully detailed about certain things.

How high is your Repair skill? Or maybe you need to use the advanced (electronic) repair tools on it?
I distantly recall repairing that generator, but it was quite a long time ago, so I may be confusing it with an other, repairable one (*SPOILER* like the one in the "abandoned" military base).
Also, it could be that the Restoration patch removed that occurrence from the game, if it was indeed ever repairable at all.

It means your repair skill isnt high enough. Either keep trying (its a random roll based on your repair skill), come back later when its higher, pop some mentants (if you're skill is very low don't bother) or use some tools (Tool, or Advanced repair kit).

Alternatively, if you have a high science, you can sometimes use science as well.

You must use the Science skill to determine what's wrong, then Repair it. It's a unique circumstance, specific to Vault 15's generator.