OK, I’ve replaced the alternator & the V-reg. still not charging. Batt WILL hold charge and truck will run off batt for about 20 min then dies. after charging w/jumpers will start & run another 20 min
Check the resistance on the battery cables with an ohmmeter. Chances are they’re rusted inside and are not letting a charge get back to the battery.
I’ve had two different vehicles show similar symptoms; replacing both the positive and negative cables, plus the battery terminals, solved the problem.
Also, check your grounds, make sure they’re good.