Key on not running, do you have battery voltage at the blue wire on the alternator? If not you have a broken wire or poor connection in that circuit.

Engine running, use a jumper wire to ground the other terminal at the alternator (where the green wire would go). Measure voltage at the battery, should be quite high, 16 or better. Be quick about this as the overcharge can damage components.

This is called "full fielding" the alternator and tests to see if it capable of charging.

If you don't have overcharge at the battery, check the voltage at the alternator output stud "fat wire". If you have high volts there then the problem is bad wiring between the alt output and battery positive, including the bulkhead connection and ammeter.

Also won't hurt to check voltage between the alternator case and battery negative, this will show a ground problem.

Ran into this on my 89 d350. Had 21 volts at the alternator! 12 at the battery. Found a corroded connection in the charging wire.