[82Stang]

These trucks have a 'smart charging system' type of setup, so they allow for alot of variation involving the voltage. The thought process being that the battery starts the truck, then the alternator takes over when running and the smart charging regulates when to slow down the alternator to not overcharge, and conversely charge when system components are being asked for usage. Although these are different systems, I tend to go the route of basics first. Unhook and clean all the battery cable connections top to bottom, grounds too, to make sure you have a good clean connection. Then load test that battery KOEO(key on engine off). Then test the alt KOEO. Then start the truck and test the running voltage of the battery. Then also do the alt KOER(key on engine running) test once again. Along the way, you might find something. If all remains the same and the alt reads 15.3 still or higher, I'd suspect that. If battery is original or old, consider this also. Batteries get maybe 3, 4, or 5 years, some more. Depends on type, kind. Just get back to basics and I'm sure the problem will show itself. Good luck.