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.

### DTC Descriptor

DTC B1397: Dimming Reference Voltage Circuit Low

### Circuit/System Description

The body control module (BCM) controls the dimming operation of the interior lamps. The BCM applies a 12-volt reference signal to the instrument panel (I/P) dimming switch. When you move the I/P dimming switch, the BCM uses this varying voltage signal in order to apply a pulse width modulation (PWM) voltage which controls the brightness of the interior illumination lamps.

### Conditions for Running the DTC

• | Voltage supplied to the module is in the normal operating range, approximately 9-16 volts.

• | The interior lamps must be ON.

### Conditions for Setting the DTC

• | The BCM detects a short to ground or an open in the dimming reference voltage circuit.

• | The condition is present for longer than 3 ms.

### Action Taken When the DTC Sets

The BCM stores DTC B1397 in memory.

### Conditions for Clearing the DTC

• | A current DTC B1397 clears when the BCM no longer detects an open or a short to ground in the dimming reference voltage circuit.

• | A history DTC B1397 clears after 100 consecutive ignition cycles.

### Diagnostic Aids

• | If the DTC B1397 is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections.

• | An intermittent short to ground or an open in the following locations may cause an intermittent malfunction to occur:

- | The dimming reference voltage circuit

- | The I/P dimming switch

- | The BCM

### Reference Information

#### Schematic Reference

Interior Lights Dimming Schematics

#### Connector End View Reference

Component Connector End Views

#### 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 headlamp switch.
2. Ignition ON, test for 12 volts between the dimming reference voltage circuit terminal 15 and ground.

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

3. Verify the scan tool Dimming Control Reference parameter is less than 1 volt.

⇒ | If greater than the specified range, test the signal circuit terminal 13 for a short to voltage. If the circuit tests normal, replace the BCM.

4. Install a 3A fused jumper wire between the signal circuit terminal 13 and the dimming reference voltage circuit terminal 15. Verify the scan tool Dimming Control Reference parameter is greater than 11 volts.

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

5. If all circuits test normal, test or replace the headlamp switch.

### Repair Instructions

Perform the after completing the diagnostic procedure.

• | Headlamp Switch Replacement

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