79 Dodge 360 pickup.
The voltage at the freshly charged battery is about 12.7V at rest. When the engine is fired up it may go up to 12.8V but no real change (am I wrong in thinking it should be over 13.5V?, ok that's a third question).
Truck starts ok until battery voltage drops below about 12.2V (few days), you can then jump start it & it will run fine but will not restart once stopped.
Ammeter shows either neutral or very slight charge, not negative as I would expect to flatten battery.
It has a 3 wire altenator with external voltage regulator, could it be altenator is working at reduced level (I thought once they failed they failed completely) or could it be something to do with the voltage regulator?
If it is found the alternator is cream crackered, would it be advisable to change it for a single wire with internal regulator & if so what do I do about modifying the wiring.
I have done the usual checks belt tight, earth lead to block, checked for short circuits (appears no voltage drain on battery at rest).
Cheers for any help/advice (Yeah I'd buy a chevy but it's my son's).