I have an 89 blazer it has a 454 engine swapped into it. Alternator went out so I bought a new one didn’t work got another one still isn’t working l. I installed a new 4ga ground wire and a 4ga with inline fuse to the battery. Getting 11.9 to 12.2 volts at alternator and battery. Pull negative while running instantly dies. Any help would be appreciated.

 

Did you have the alternators tested at the store? It seems odd that you'd get two bad ones in a row.

 

Since everything sounds custom, it would help to know what alternator you are using (link to product you purchased perhaps or let us know the manufacturer & part number), how it is wired (single wire, voltage reference, etc), and any other details you might be able to give us to allow us to help you work through the troubleshooting steps necessary to determine what's going on. Also, a few pictures of your setup would help as well.

Pulling the ground to the engine while the vehicle is running should result in the engine dying as I assume you are using an electronic ignition module for spark control. This makes sense in that regard.

Your system voltage does indicate that the alternator is not functioning to task, but without knowing how things are wired and what the alternator is expecting for input(s) it is difficult to say what is actually going on. I'm guessing that there is something that is not connected properly at the alternator that is preventing it from ramping up output to boost the system voltage.

Let us know what you have and how it is wired up!