Diagnostic Aids (Diesel)
WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 Chevrolet Silverado, and 2009 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
A scan tool can be used to command the GEN L-Terminal circuit ON to 84% and OFF to 14% while the engine is running. Typical voltage values for the charge indicator control circuit are 10.5 V at 84% and 2.2 V at 14%.