System Not Charging Battery

I suggest you pull out that alternator and have it tested before going much further. If you bought a rebuilt, it could be bad straight out of the box.

Once you KNOW that alternator is capable of working, then you can troubleshoot the circuits further.

the voltage across the battery should be about 13.2 volts when the engine is shut off. It should be about 15 volts when the engine is running. you definitely have some sort of problem. Maybe start by disconnecting the battery, charging it over night and check the voltages the next day. I think sometimes if the battery is fully discharged the alternator will not even try.

He already cleaned the terminals and charged the battery. But, I agree, now he needs to do it again before proceeding.

Whatever the OP does, he should work logically and systematically. Dont start throwing new parts at it. Test, diagnose.

If he has one of those GM original battery cables that has several positive leads going into the one side connector, he needs to be sure he has taken that apart and checked/cleaned the INSIDE of that terminal assembly. That is a common place for bad connections to occur.

The supplier where he bought the alternator should check it out for him for free. Check it, at any rate.

Go step by step.

You may have a blown fuse/fusible link between the batter and alternater. An easy test, see if you have battery voltage at the alt terminal.

That is a good point to test, but I dont believe there are any fuses or fusible links between the battery and the alternator B terminal..

At least, on my GMs there havent been. On my the cable runs directly from the side terminal of the battery to the (1) alternator, (2) junction box, and (3) starter. The individual cables are not soldered together inside the side terminal connector. And that is where corrosion and bad connections sometimes originate.