The vehicle communication interface module (VCIM) provides the cellular microphone with a supplied voltage on the cellular microphone signal circuit. When the microphone is in use, voice data from the user is sent back to the VCIM on the signal circuit. The VCIM provides a ground for the microphone via the low reference circuit.

Schematic Reference

OnStar Schematics

Connector End View Reference

Component Connector End Views

Description and Operation

OnStar Description and Operation

Electrical Information Reference

Scan Tool Reference

Control Module References for scan tool information

1. Ignition OFF, disconnect the harness connector at the cellular microphone.
2. Test for less than 10 Ω between the low reference circuit terminal A and ground.

⇒	If greater than the specified range, test the circuit for an open/high resistance. If the circuit tests normal, replace the VCIM.

3. Ignition ON, test for 9.5-10.5 V between the signal circuit terminal B and ground.

⇒	If less than the specified range, test the circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the VCIM.

⇒	If greater than the specified range, test the circuit for a short to voltage. If the circuit tests normal, replace the VCIM.

4. If all circuit test normal, test or replace the cellular microphone.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.