Ok. i used the search and could find anything about this, so any help would be great. Heres the deal, the other day i started my car and it ran just fine. later that day i was driving and the volt meter dropped to 12 volts. I believe this means that the battier isnt being charged. So today i took out the alt. and had it tested and it was fine. then i took the battier out to be tested and it crashed under load. which sounds right because i drove about 20miles on just the battier. Now i just changed the battier, and the volts stay at 12. We checked the wiring and didnt find anything wrong with it. Any ideas?

 

what do you have on in the car when this happens? are the lights, turn signals, windshield whipers, rear defroster, your radio and your main defroster on? The s4 alternator sucks and can only hold a limited charge. Many people are upgrading from the s4 to the s5 or even the fd's alternators.

 

nothing is on, i turned the everything off. im not sure if its an s4 alt. b/c its looks a lot newer than 20 years old. plus the i had it tested outside the car.

 

what was the AMP output when you had it tested, and did you hook it back up properly, if you search for S4 ALTERNATOR UPGRADE you should find a wire schematic on here pretty quickly that shows the pin out of the s4, s5 and s6 alts, if you are pushing under 70 i would say its s4, over 70 s5 and 100+ would be s6, not sure what the exact amps are for s4/5 but that is probably close, and make sure the belt is good and tight, and also does you car still have the air pump hooked up? if not you may have to upgrade pully setups

 

Forget the stock gauge, measure the battery voltage direcrtly at the battery with a proper meter. If you can't get at least 13.5V (preferably over 14V) at idle with all accessories off then the alternator is bad.

 

Alternator can sometimes go bad and then temporarily get good again. If it's not the battery and the problem really does exist (verify with a multimeter as said), then 99.9% it's the alternator. There's pretty much nothing else in the charging system except wires. Since your old battery was definitely bad I'm betting the alternator shorted the battery or vice versa. It's too much of a coincidence for the gauge to go screwy at the same time. When you get the new alt save the warranty info and/or get a recommendation to a good source. New alternators are a gamble. They can last a long time or they can blow in a few weeks.

Some stores will do an on-car alt test, which could help insure that the thing doesn't happen to start working again when they test it (or if it does then you'll see your volt gauge jump to 14V).

 

That is not electrically possible...

 

lol shorted battery, a battery is a huge short by default, just a plastic box filled with acidic liquid...