83 mustang with its stock alternator I had rebuilt...... When its running the needle stays in the middle. Any draw I turn on, ( headlights ) and it stays right there, no up or down movement...... Is this thing working OK... Been a minute since I was behind an amp gauge.......
The amp gauge measures differential output between battery and alternator. So, you should see it budge when you turn on headlights or the A/C fan or whatever.
I know you mostly turn wrenches by yourself so do this. Get two lengths of wire (lamp cord will work) and attach one end of each to the battery posts however you can. Bring the leads into the car and connect the leads of your DVM to them. See what the battery is with the engine off and key off. Turn the key to the run position and the voltage should drop a little. Start the engine and it should read around 14+ volts. Turn the head lights on and it should drop just a little bit. This will confirm things are working as they should and your amp meter is just a typical late 70’s early 80’s piece of dash jewelry.