from the Electrical troubleshooting sticky, does it matter if you have a 120amp Vs 150 amp?
from the sticky :do both alternators use the same volts for testing?- Voltage should be 14.2 to 14.4 volts. Anything less provides only a partial charge. If it is 14.1 or less volts, it needs to be fixed.
btw, i have the 120 amp