Alternator gauge is smoking

All Amp meters have a shunt (a bypass resistor) across the meter to carry the high current around the meter movement. It sounds like the meter shunt is damaged. The shunt may be internal to the meter. Can you try using the old meter to see if the problem clears up?
I used to think similarly but as already explained that is not so in these cars.
It can be true, especially in test equipment. Like illustrated here in where Chrysle rcplains how the test instruments work and how to use them.
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In the car's dashboard ammeter we can call the metal carrying the current a shunt but its not technically correct so I don't anymore. It's plate creating a magnetic field that moves the needle. There is no permanent magnet and there is no coil. It would not surprise me a bit if this is the same way an ammeter works on a Jeep MB or CJ, and many other vehicles of that era.
On a remote shunted ammeter, the resistance is in the meter and the shunt is the low resistance path that carries most of the current in parallel.
1976 A-bodies got a remote shunted ammeter in the dash.