Instrument Panel Cluster (IPC) Description and Operation: Voltmeter
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2004 Oldsmobile Bravada, 2004 GMC Envoy XUV, 2004 GMC Envoy XL, 2004 GMC Envoy, and 2004 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
The IPC displays the vehicle voltage based on the information from the BCM. The IPC uses class 2 serial data information from the BCM to provide a voltage level indication on the voltmeter.