I had this problem once and found that the amp gauge was a choke. The needle indicated correctly but the shunt inside was bad so actual power delivered to the battery was very low. My solution was to jump the studs on the gauge with a strap,effectively bypassing it. That brought everything back to normal. Having now solved the problem, it remains to either replace the gauge, or get rid of it in favor of a voltmeter.
In Del's diagram, you can see the gauge down on the bottom there.Normally all power for the car comes from the alternator, and whatever is extra goes to the battery,follow the path.
But if the alternator fails to meet the demand, then the battery goes to work.
The amp gauge tells you which one is supplying by the direction of the needle.And it tells you how well the regulator is balancing the supply and demand by the amount of deflection. But it cannot tell you if the shunt (bypass) inside it is adequate to pass the juice.
You don't have to pull the dash apart to get to it. You can bypass the ammeter anywhere in the circuit, between the #1 splice and the battery. The easiest place to test this is right at the bulkhead connector,lol
The fusible link can also be a problem. I have seen those corroded so bad that they pass just enough to energize the starter relay during starting,and no more. With the alternator disconnected, the engine would continue to run, but as soon as I turned on the heater or lights the engine stalled.
