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.