Circuit Description
WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The ignition 3 voltage circuit is a discrete input to the HVAC control module. The HVAC control modules uses the input to determine that the ignition switch is in the RUN position. When the ignition switch is in the RUN position, ignition voltage is applied to the input.