Alternator output on PCM 351W over 15 volts

[2001Sunsport]

1994 Supra Comp TS6M with the Pro Boss 351W. The voltage gauge on the dash started acting strange, sitting at 18 volts when the engine is running, so I checked it out last night. Engine off, the battery is about 12.4 volts and the gauge shows 13. Engine on, the battery is at 15.4 volts (gauge shows 18 volts). Output at the alternator is 15.7 volts.

Is there an external voltage regulator on these, or do I just need to replace the alternator?

Thanks!