You probably mean volt meter, showing 15 volts.
Dash gauges are notoriously inaccurate, so a check with a digital volt meter, may give you a better idea of charging volts.
If it is charging at 15 volts, you will need to check the wiring that supplies voltage to the alternator regulator. ( one of the small wires )
If it is charging between 13 and 14 volts, all is good.
2 years on a rebuilt could be expected. Next time, if you haven't, get a genuine Cummins reman alternator.