A pegged ammeter would indicate that either the alternator itself is damaged, or one of the field wires is grounding. Even with a totally-dead battery, it shouldn't peg right out. My '81 Imperial had the big 100A Leece-Neville style alternator, and though all the wiring was OK it was internally shorted somewhere in the alternator. It burned out the digital dash, blew every bulb in the car, and made the battery look like a pillow. It was pushing out over 17V at idle.