The 'alternator' gage is an ammeter showing current to or from the battery.
Normally it sits at zero after the battery has recharged.
That means charging is increasing with rpm.
Possible reasons for this include
1. Voltage regulator not working.
2. Battery low combined with alternator that's producing at reduced capability. An alternator producing normally should be charging the battery at fast idle.
Sounds like the battery could use a good charge on a charger.
it will cook off the water (acid), or occassionally worse.
If the regulator isn't working, then the whole system is running at higher than 15 Volts whenever the engine rpms are increased. Every electric device given power at higher voltage will suck more current, so lots of things can get damaged.
In my opinion, the first check would be to take a handhand multimeter and measure voltage at the battery, engine running at idle rpm (650?) and then at 1250 rpm, or fast idle (1600ish). If it goes up a little, but stays under 15 V, increase rpms more just be sure.
If that checks out ok, then check voltage at idle and 1250 or 1600 at the at the alternator output (if accessible - some are covered in a boot), and at the nearest junction to the voltage regulator sense wire (back probe at ballast resistor or the blue wire conencted to the alternator).
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