For 1990-2009 cars

### Diagnostic Instructions

1. Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
2. Review Strategy Based Diagnosis for an overview of the diagnostic approach.
3. Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptor

DTC B3890 02: Trailer Backup Lamps Circuit Short to Ground
DTC B3890 04: Trailer Backup Lamps Circuit Open

### Diagnostic Fault Information

Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance
--- | --- | --- | --- | ---
Trailer Backup Lamps | 3890 02 | 3890 04 | -- | --

### Circuit/System Description

When the transmission is placed in the reverse position, the engine control module (ECM) sends a serial data message to the body control module (BCM). The message indicates that the gear selector is in the reverse position. The BCM sends this message to the trailer lighting converter. The trailer lighting converter then applies a voltage to the trailer backup lamps through there supply circuit.

### Conditions for Running the DTC

#### B3890 02 or B3890 04

The appropriate trailer backup lamp signal is active

### Conditions for Setting the DTC

#### B3890 02

The trailer lighting converter detects a short to ground on the control circuit.

#### B3890 04

The trailer lighting converter detects an open control circuit.

### Conditions for Clearing the DTC

The condition for setting the DTC is not longer present.

### Action Taken When the DTC Sets

The appropriate trailer backup lamp is inoperative.

### Reference Information

#### Schematic Reference

Trailer Connector/Provision Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Exterior Lighting Systems 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 Verification

Ignition ON, perform the Trailer Backup Lamps test. The trailer backup lamp should be ON/OFF

### Circuit/System Testing

1. Ignition OFF, test for less than 5 Ω between the X88 Trailer Connector terminal 3, 11, 13 and ground.

⇒ | If greater than the specified value, test the appropriate ground circuit for an open/high resistance.

2. Connect a test lamp between the X88 Trailer Connector terminal 8 and ground.
3. Command the Trailer Backup Lamps Active / Inactive test with a scan tool. The test lamp should turn ON and OFF when changing between the commanded states.

⇒ | If the test lamp is always ON, test the signal circuit for a short to voltage. If the circuit tests normal, replace the K68 Trailer Lighting Converter.

⇒ | If the test lamp is always OFF, test the signal circuit for a short to ground or an open/high resistance. If the circuits test normal, replace the K68 Trailer Lighting Converter.

4. If all the circuits test normal, test the backup lamp circuit at the trailer.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for trailer lighting converter replacement, setup and programming