Since you mention having a "code," I'm going to assume you don't have RENIX, and are either OBD-I (1991-1995) or OBD-II (1996-2001.) You all know what happens when you ass-u-me, right? It's always helpful to post your year and such EVERY time you ask a tech question - not all of us are going to remember, and it makes things easier for us...
Now that I've got that out of the way...
There is/are one/two fuzes in your PDC that protect the rest of the vehicle from oversupply by the alternator - as I recall, they're 60A MAXI fuzes (they look like regular ATO plastic "blade" fuzes, but they're about four times the size,) and you can check them quickly with a test light or a cheap meter - with the engine idling, simply check for supply voltage at both sides of the fuze (that's what the little exposed metal tabs are for.) If you get about the same thing on both sides (light equally, or withing .05 volts on both sides,) than it should not be a problem.
Once you check that fuze, go down to the alternator, connect to the "output" post, and make sure that's giving with full voltage - it may be a "cable fault" of some variety (broken strands, contaminated cable, what-have-you.) While you're about it, check both ends of that cable for corrosion on the lug and under the screw.
If you've recently submerged anything electrical, you really should clean the parts that are not sealed. Blow out the alternator with contact cleaner then compressed air (or let it dry before you try to fire it up,) and take apart the connections, clean them, and reassemble. A little Ox-Gard won't go amiss either - check your hardware store for that stuff, in the electrical section.
Servicing all the connections will also provide an opportunity to visually inspect and clean them - don't waste it. Don't neglect the regulator connections on the back, since they will affect output directly as well. Clean/inspect/protect.
Since the ND alternator you use isnt' internally regulated, that reduces the amount of electronics invovled (the regulator is part of the PDC,) but there are rectifier diodes that might still get cooked - that's always going to be part of the alternator.
Those are tests I'd go through to see if I need a new alternator. However, I'm REALLY leery of getting them from chain stores - since they pay about $3 each for them, and when I worked at Kragen's I failed about 2/3 of them right out of the box. You can take your chances there (you just might need that lifetime warranty...) or you can click the link in my sig - I make cables, and I've got a site up for a shop that does great alternators and starters. You'll have to deal with Rod directly if you need anything - but you'll also be able to see if he can do anything for conditions you anticipate that are "other than normal" - but I've soaked his units pretty good without any trouble.
5-90
