Just recently purchased this 87 GTA. Started to drive it home and voltage, temp and oil pressure all looked great. After driving about 250 miles I stop to stretch. Car is sitting at idle and I smell hot antifreeze, quick pop the hood and sure enough, it is slinging coolant off the water pump because the seal went out. Trailered the car the rest of the way home. Replace the water pump (cussing GM, wishing I had the single belt system on this car.) Have car idling while I flush the cooling system out. While watching the temp, I notice that the voltage seems rather low, about 10 volts. I took the car around the block to test the cooling system repair, all seems well with that and I noticed that the voltage seems to be back up to where it should be, 13-14 volts.

Drive the car a couple times (~50 miles), all seems fine. Then I get in the car, start it up (just fine) and I notice that the voltage is down again, ~10 volts. I had to make a couple stops and after the third one or so it does not want to crank over too well but still starts and runs. Good thing my next stop is somewhere I can put a battery charger on for about an hour or two. After charging battery for a while, I drive home that night with my lights on, about 15 miles, and I do make it all the way home. I put a charger on it again when I get home. (Ok, to tell the truth, I was hoping that the alt just got a little wet when I was flushing the cooling system and I was hoping it would dry out and resolve itself. It doesn't look like that is going to happen.)

I pulled the alternator out tonight and took it to a local auto parts store and had it bench tested. (FYI: All connections on alt seemed clean and tight along with the battery connections.) Clerk said that it was only putting out 11 volts so it must be going bad. Of course they did not have one in stock. I reinstall the weak alt, and drive all the way home with my lights on because it is dark out, (again, about a 15 mile trip which takes me about 15 min, mostly on the freeway.)

I'd like to try rebuilding it myself just for the experience (and since it seems that most the reman ones you buy at the parts stores only last a year anyway.) Any suggestions to which parts are bad? Brushes, Diodes, Voltage Reg???

Thoughts on what caused this problem?
Age
Coolant leak
Water from flushing the cooling system
Seller 'power washing' the engine compartment

I know the car has a few other electrical issues, but I don't think they are related to this charging problem. This does not seem to be a drain issue:
Power mirrors and locks don't work (Fuses?)
Power windows work but are slow (I hope a good clean/lube job will fix this)
Power antenna does not go down, but this may be because of the hack job that someone did when they installed the cheap *** CD player.
Rear hatch does not open using the button on the dash. Will open with the key and believe it or not, the pull down motor still works!!!

When I saw a fist full of fuses in the map pocket, and the hack job they did installing the CD player, I knew I'd have some electrical matters to attend to.

 

Alternators just wear out sometimes, they dont last forever, kind of like batteries.

To take an alternator apart and replace indivdual parts would probably cost more time/money than just getting a new one. And i dont know where you would get parts.