The communication interface module (CIM) 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 CIM on the signal circuit. The CIM provides a ground for the microphone via the drain wire.

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 drain circuit terminal 2 and ground.

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

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

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

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

4. If all circuits test normal, replace the cellular microphone.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.