Tahoe 2WD V8-4.8L VIN V (2004)
Global Positioning System Module: Description and Operation
VEHICLE COMMUNICATION INTERFACE MODULE (VCIM)
The vehicle communication interface module (VCIM) is a 3-watt cellular device that allows the user to communicate data and voice signals over the
national cellular network. It is powered by a fused, battery positive, voltage circuit connected through the vehicle wiring to the 3-button assembly and the
radio, and attached by means of coax cables to the cellular and navigation antennas. Ground for the module is accomplished by means of dedicated
circuits that are routed with body wiring systems to chassis ground points. The module houses 2 modems, one to process global positioning system
(GPS) data, and the other for cellular information. Satellites orbiting earth are constantly transmitting signals of their current location, from which the
OnStar(R) system is able to pinpoint its own location. The navigation antenna receives these GPS signals and provides the data to the VCIM to be
processed. The VCIM communicates with the rest of the vehicle over the class 2 serial data bus. The ignition state is determined by the VCIM through
class 2 messaging. The module also has the capability of commanding the horn, initiating door lock/unlock and operating the exterior lamps using the
class 2 serial data circuit. When an OnStar(R) keypress is made, a class 2 message is sent to the audio system to mute all radio functions and transmit
OnStar(R) originated audio. After the audio system is muted, the OnStar(R) signals are transmitted to the audio system on the cellular telephone voice
signal circuit, and returned to the module on the cellular telephone voice low reference circuit. The cellular modem connects the OnStar(R) system to the
cellular carrier's communication system by interacting with the national cellular infrastructure. The module sends and receives all cellular
communications over the cellular antenna and cellular antenna coax.
