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® 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 or GMLAN serial data bus, depending upon the serial data architecture of the vehicle. The ignition state is determined by the VCIM through class 2 or GMLAN messaging. The module also has the capability of commanding the horn, initiating door lock/unlock, and operating the exterior lamps using the serial data circuits. When an OnStar® keypress is made, a serial data message is sent to the audio system to mute all radio functions and transmit OnStar® originated audio. After the audio system is muted, the OnStar® 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® system to the cellular carriers 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.

Schematic Reference

OnStar Schematics

Connector End View Reference

Cellular Communication Connector End Views

Description and Operation

OnStar Description and Operation

Electrical Information Reference

1. Ignition ON, radio ON, set volume to comfortable level and press OnStar Center Call button.
2. Verify Connecting to OnStar message is heard clearly through the audio system.

⇒	If OnStar message is not heard, go to Circuit/System Testing.

3. Tune radio to a known good station and verify that audio is heard clearly through all speaker channels.

⇒	If the sound is notheard through all of the speaker channels, refer to Speakers Inoperative - One or More .

4. If the radio is replaced, the new radio must have the setup procedure performed. Refer to Radio Setup .

Important: If the navigation mono audio signal low reference circuit is shorted to voltage the VCIM has been damaged and requires replacement.

1. Test for an open or high resistance or for a short to ground or voltage at the navigation mono audio signal circuit terminal 1.
2. Test for a high resistanceor an open, or for a short to voltage at the navigation mono audio signal low reference circuit terminal 2.
3. If the circuits test normal, test or replace the VCIM.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.