I've got an '86 F150 v5.0 that has been problem free until recently. I had her die one night on the way home - i figured battery or alternator problems. Checked out the alternator and replaced it. No problems for a few days on short drives. Then after driving for 90m or about 60m, the same thing happened. LIghts got dim, and fuel gauge slowly went down (all other gauges were fine), and then the engine simply quit. Wouldn't turn over, but enough juice to run accessories for a bit. A jump got it started, but it would only run for a few minutes under full load. The battery would take a full charge, so appeared to be fine. But just to be certain i swapped it out. 90miles later, the same thing happened. All I can figure is a bad wire between the alternator and battery? Thoughts?

 

You seem to be on track with your diagnosis, the only real suggestion I can make is to have the new alternator checked also. I replaced an alternator on another truck I own and after a couple of drives the replacement went bad. Replaced with another alternator and all has been fine for over a year now. Good luck and keep us posted on your findings.