ls1nova71, After doing some digging, You are correct with the 13.8 VDC being the default. And that is 100% duty cycle, 90% is 15.5 VDC
Chart & Info....
The generator battery control module monitors the generator performance though the generator field duty cycle signal circuit, the generator field control circuit, and the battery positive voltage circuit. The generator battery control module controls the generator through the generator field control, charge indicator control, circuit. The signal is a 5-volt pulse width modulation (PWM) signal of 128 Hz +/- 5% with a duty cycle of 0-100 percent . The duty cycle sent by the generator battery control module is limited between 36-90% . When the engine is turned OFF, the module will send 0% duty cycle, low voltage. When there is loss of class 2 communication with the powertrain control module (PCM), the module will send 100% duty cycle, 13.8 volts . The table shows the commanded duty cycle and output voltage of the generator.
The generator provides a feedback signal of the generator voltage output through the generator field duty cycle signal circuit to the generator battery control module. The signal is a 5-volt PWM signal of 128 Hz with a duty cycle of 0-100 percent . Normal duty cycle is between 5-90 percent . Between 0-5 percent and 100 percent are for diagnostic purposes.