1995 Plymouth Voyager charging system

It doesn't have a conventional voltage regulator. The PCM (computer) regulates the alternator field voltage. I was getting a trouble code for the field circuit open or shorted, and the alternator tested good off the car, but I couldn't find any wiring fault. Soon after, it did it again, and this time the alternator wasn't putting out voltage. The trouble turned out to have been worn brushes in the alternator causing an intermittant failure that set a code.