I think the 'field circuit' includes the part of the PCM that does the voltage regulation for the alternator, and the wires from the PCM to the alernator field coil. These would be the small wires in the connector that plugs into the alternator. The large wire at the alternator is the output, and connects to the battery. I would guess you have either a melted wire or one that has rubbed against something - and is shorted out. All the PCM knows is that it isnt able to send voltage to that field coil. The field coil in the alternator is the part that spins. Putting voltage on that coil, via the field control circuit creates the spinning magnetic field that produces the heavy current output voltage.