1988 Chevy Celebrity car still shutting off

Engine Performance problem
1988 Chevy Celebrity 4 cyl

Okay I have put on a new catalytic converter and the car ran a little longer it surged then 90 seconds later it shut off. In my mind I keep replaying what happened. The Serp belt was severely loose when the car first stalled. I just ran it for a few days without a problem then it kinda shut off then ran shut off than ran then stalled. I keep thinking that maybe this problem is coming from my alternator that maybe it's faulty and the car is running from the battery. I read in different places to start the car then disconnect the battery terminal if it shuts down then it's the alternator. PROBLEM IS one place says the black cable then another place says the red. One says the nuetrual cable. Which is what. The red or the black. I can't get to a shop and I don't have a voltage reader I do have a battery charger. My lights don't shut off but I do notice the voltage reader in the dash dropping when I try to restart the car and there is no spark in the spark plugs.I've replaced the plugs and wire and ignition coil NO HELP, replaced the catalytic convertoer which was bad, no help. Ready to do the crank sensor but before I do I waNted to check the alternator.