Instrument Panel Cluster (IPC) Description and Operation: Voltmeter
WARNING: This page is about a different car, the 2002 Oldsmobile Bravada, 2002 GMC Envoy XL, 2002 GMC Envoy, and 2002 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
The instrument panel cluster (IPC) displays the vehicle voltage based on the information from the body control module (BCM). The IPC uses class 2 serial data information from the BCM to provide a voltage level indication on the voltmeter.