Charging System? Battery Drain?

I hope someone can help me....I have a 95 Chevy Lumina Van that has not had many problems and has 120,000 miles....Yesterday my AC stopped working and today the Anti brake service light came on and also the Check Engine soon light, my speedometer quit working and then I noticed the battery voltage got real low......I was trying to get home and it died...everyone thought it was the alternator because when jumped and the car would idle for a minute or two but no longer when a cable was taken of the battery it would die again..... two attempts at changing the alternator this afternoon and recharging the battery AND knowing the alternator is ok because we had it tested.....the van starts for no more than a minute or two and still when a cable is taken off the battery it dies.....when started and just seen if it will stay idoling all the power is drained and it dies....we are lost...Wires were checked and all are believed to be cleaned and attached properly does anyone have any suggestions? Thanks!

They won't run with a cable off anymore, that isn't a valid test these days, and can damage electronics, so stop doing that. ;)

Now, it seems clear that it is still not charging from what you describe, but you say the alternator was tested and found to be good? That leaves a couple of possibilities. You need to check the voltage at the charging post on the back of the alternator while the motor is running, NOT at the battery. This will tell you if the unit is charging, and the charge is not making it to the battery, or the unit is being told to cut output for some reason.

If you show ~13.5 volts or better at the large lug on the back of it, then you probably burnt a fuseable link between there and the battery. I'm not sure if it is the same as they used to be, but the big primary wire off that charge post used to go down to the starter, and there was a fuseable link there, before it went back up to the battery. That protected the primary ignition power feed from overload from the alternator or starter.

If you show low voltage, 12.5 or less and dropping, then the alternator's regulator is not being told to turn on and allow charging. This can be from a bad wire to the ECM, I've run across that once or twice. You just have to start chasing wiring, a manual is helpful, but you'd want to find the same tracer marked wiring at the ECM as what comes off the alternator, and if you see no obvious damage, start testing for continuity with a meter.

They won't run with a cable off anymore, that isn't a valid test these days, and can damage electronics, so stop doing that. ;)

you have no idea how many times a DAY i have to tell people that...

you say that the AC suddenly stopped working? did your belts start making a squealing sound? does the same belt run the AC AND the alternator? - do you see where i am going with this.... check to see if your belt is still tight, and that your AC unit hasn't locked up and that all your other pulleys and the tensioner are still turning ok.

you have no idea how many times a DAY i have to tell people that...

you say that the AC suddenly stopped working? did your belts start making a squealing sound? does the same belt run the AC AND the alternator? - do you see where i am going with this.... check to see if your belt is still tight, and that your AC unit hasn't locked up and that all your other pulleys and the tensioner are still turning ok.

That one's got a serp belt that runs it all, high amp load accessories will begin shutting off as the voltage drops, relays that require lots of power to stay engaged start opening and turning things off. Some of them with solenoid controlled trannies will even stop shifting, defaulting to second gear in a kind of "limp home" mode.