Ammeters are a bit different from voltmeters. They measure how much current the alternator is putting out, and that can vary widely. A voltmeter measures the system voltage, and that should tend to remain pretty steady.

You should notice that shortly after a cold start the needle goes pretty deep into the + territory when revved up a little, that's an indication that the alterntor is doing the heavy work of recharging the battery following all the cranking. later on when the battery is fully recharged, it has less of a load.

Frankly, I find ammeters to be a little less informative than voltmeters. It is more informative, but not the sort of information you need to know. Do you need to know that the alternator is putting out 25 amps? Not really, what you need to know is if the alternator is keeping up with the load, which is what the voltage meter would tell you.

Many alternators won't push a lot of current when the rpms are low. The pulley sizes are chosen to make sure the alternator doesn't get overspun when the motor is at the redline, and sometimes they just aren't spinning fast enough at low rpm to generate much electricity. If you see the ammeter venturing into negative territory a lot, even when revved up, that could be a sign of trouble.
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I'm JohnMc, and I approved this message.