I am having alt problems myself. Others on the forum have given the following suggestions. So far it hasn't helped me but it may you.
Check the terminal block connections, if you have one.
Connect a ground wire from the alt housing to batt ground, the alt must be grounded to work.
Check the ground cable where it connects by the hood hinge.
Hope you find your problem.

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Darrell

1984 300SD "Ole Girl"
207K