Hello everyone. My name is chris i am fairly new here.

I have a question and need some help with an issue i am helping a friend with.

My friend has a 1982 monte carlo. He has complained about his car never starting and dying all the time. I had came into the project with the car having a new battery, and alternator installed. I got the car started by jumping it and measured 12.1 volts at the alternator post to ground. i checked fuses etc etc etc and verified grounds were in place. No corroded terminals at the alternator. I tried installing a 12 gauge wire from the positive terminal at the alternator to the positive terminal at the battery to maybe get voltage with no luck. I found the v-belt was loose and as i was doing this i had my multimeter attached to the battery posts watching voltage. As i tightened the belt i watched the voltage jump from 12.1ish to mid 13s.... This is great the car is now charging.

This is where i get confused. I tightened the belt and adjustment nuts. I let the engine idle and watched my multimeter voltage slowly drop from 13.5ish to about 12 volts over the course of about 5-10 minutes. This is perplexing me. I took the alternator out and took it to Autozone and had it tested 3 times with it passing every time. Now one these vehicles are the voltage regulators integral to the alternator or external?

This specific alternator has a positive post for im guessing the ECM and and a connector with two wires that plugs into it. (im guessing source voltage for the rectifier circuit and ground). Can anybody help me with this issue on what to chase and or test next. I am a John Deere Mechanic but not to fancy with automotive applications and especially not to fancy diagnosing charging system issues with no schematics or testing proceedures.

Can anybody help me PLEASE!!!!!!