1983 Supra alt issue maybe, and wont stay running?

[faaaaq]

My buddy has a 1983 Supra and is having some real problems. Seems his battery does not charge in the car but will on a battery charger. Car died AS HE WAS DRIVING lastnight. i went to jump his car because it wouldnt start. it would start off of my battery, but would slowly die when disconnected if he let off the gas. so we went home and charged the battery. went back today so he could put the battery in and drive it home till he can get it to a mechanic. now, with a fully charged battery, it will start no problem, but he cant let off the gas or it will instantly die. no puttering, just goes from running fine to dead. the battery is new and healthy, and the full pump, fuel injectors, and spark plugs were all just replaced, otherwise there have been no changes, alterations or repairs. I called a mechanic friend who first suggested maybe the IAC, but when i said the car died instantly and didnt putter and slowly die, he basically said "uh oh, thats not good..." and said it would be best to ask a Supra forum since someone might have experienced a similar problem and know what the issue is. The supra forums dont seem to be all that active...so....here i am lol
if i didnt already suspect the alt, that would be helpful, so thanks. but i cant commit to that idea since that car will run indefinitely if someone keeps it revved up with their foot on the gas, but then will die immedietly when they let off, whether the battery is dead or not. so, that almost makes me think its NOT the alt, since with a foot on the gas and a dead battery, the car will stay running. this car seems to be a lemon, with problem after problem. for a while nothing went wrong, but in the last 6 months, it seems every week or so something new is wrong. Starting to make my old Fiero look reliable

i had suspected that as well, but i dont know enough about alts to have had much confidence that my assumption was correct lol. right now, it is not in a position to have the alt removed, since its parked illegally in a parking lot downtown and will probably get towed to a mechanic before we get a good chance to remove the alt. if that is the issue in the end, thanks. Im still open to other suggestions though. Not dismissing that, but holding out for a simpler solution. always a chance its an easy fix, or maybe there is a way to keep it running long enough to get it home 10 miles away

[supranai]

I used to own an 82 Supra and currently own an 89 Supra Turbo. Sounds like the problem is not in the battery. I think the alternator might be bad. Because the supra has an in-line six cylinders engine, it could easily drain the juice of a healthy battery without the support of a working alternator. A good alternator in working order will crank out enough juice to run the engine, independent of the battery, once started. Hope this helps...good luck.

It could still be the alternator. Some alternators just plain quit while others slowly go out...meaning it still crank out juice, just not as much anymore. The faster the alternator spins the more juice you get. Most alternators have a built in capacitor to limit output at high rpm. Hence the high revving to keep the engine from stalling. It won't hurt to take the alternator out and take it to a local auto parts to get it tested. Auto zone tests them for free.