For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Cellular Microphone Signal | B2455 | B2455 | B2455 | --
Cellular Microphone Low Reference | -- | B2455 | -- | --

### Circuit/System Description

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.

### Reference Information

#### Schematic Reference

OnStar Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

OnStar Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Testing

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.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Mobile Telephone Microphone Replacement

• | Control Module References for VCIM replacement, setup, and programming