Sure they can go bad, Ive seen many where the needle is mechanically stuck (rusted) and doesn't move. However they don't often go bad such as an open circuit. As long as they are continuous they still can pass current even if the needle is stuck and cant move. An "ideal" ammeter is zero ohms.
One test is to pass a magnet across its face and see if the needle moves back n forth. Another test is an ohmmeter and it should be continuous and read very low ohms.
Par 5 of my troubleshooting procedure is an easy way to see if non charging is the fault of the genny or VR or other causes...for a Class A system
To be safe Id polarize the genny, see my procedure for how.
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