GM Service Manual Online
For 1990-2009 cars only
Ignition 1 Signal: The scan tool displays the current voltage at the battery.

GEN L--Terminal Signal Command: The scan tool displays OK/No Output. The scan tool displays OK until malfunction is detected on the generator L terminal circuit, then it reads No Output.

GEN F--Terminal Signal: The scan tool displays 0-100%. The scan tool displays 0-5% until the engine is running, then the percentage value varies depending on electrical loads.