Vehicle Interface Unit (VIU)

WARNING: This page is about a different car, the 2002 GMC Sierra, 2002 GMC Cab & Chassis Sierra, 2002 Chevrolet Silverado, and 2002 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.

The Vehicle Interface Unit (VIU) is powered by a fused battery positive voltage circuit, connected through vehicle wiring to the 3 button assembly, radio, microphone and the Vehicle Communication Unit. It is also attached by means of a coax cable to the GPS antenna. Ground for the module is accomplished by means of dedicated circuits that are routed with body wiring systems to chassis ground points.

The VIU also houses the cellular and GPS modems to process data from the GPS satellite system and VCU. The VIU communicates with the rest of the vehicle over the class 2 serial data bus. Power moding for the VIU is accomplished over the class 2 serial data bus on some vehicles on the ignition 1 voltage circuit on others. The VIU also has the capability of commanding the horn, door unlock and exterior lamps using class 2 messages. For vehicles without this capability, the VIU uses specific, discrete circuits for commanding these features. When an OnStar® key press is made, a class 2 message is sent to the audio system to mute all radio functions and transmit OnStar® originated audio. On vehicles that do not use class 2 to communication with the audio system the VIU uses the entertainment and comfort (E & C) serial data circuit. After the audio system is muted, the OnStar® audio is transmitted to the vehicle audio system on the remote audio right signal circuit, the remote audio left signal circuit, and remote audio output (-).