B388B

DTC B388B or B388C

Diagnostic Instructions

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

DTC Descriptors
DTC B388B 02
- Secondary Left Tail and Brake Lamp Circuit Short to Ground

DTC B388B 04
- Secondary Left Tail and Brake Lamp Circuit Open

DTC B388C 02
- Secondary Right Tail and Brake Lamp Circuit Short to Ground

DTC B388C 04
- Secondary Right Tail and Brake Lamp Circuit Open

Diagnostic Fault Information






Circuit/System Description

When the headlamp switch is placed in either the park or low beam position, ground is applied to the tail lamp signal circuit to the body control module (BCM). The brake pedal position sensor provides an analog voltage signal to the BCM. The BCM sends this messages to the trailer lighting converter. The trailer lighting converter responds by applying battery voltage to the left tail lamp and to the right tail lamp control circuits. This energizes the tail lamps.

Conditions for Running the DTC

B388B 02, B388B 04, B388C 02 and B388C 04

The appropriate brake lamp/tail lamp signal is active.

Conditions for Setting the DTC

B388B 02 or B388C 02

The control module detects a short to ground on the control circuit.

B388B 04 or B388C 04

The control module 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 brake lamp/tail lamp is inoperative.

Reference Information
Schematic Reference

Exterior Lights Schematics Exterior Lights Schematics
Connector End View Reference

Component Connector End Views Connector End Views By Name
Description and Operation

Exterior Lighting Systems Description and Operation Exterior Lighting Systems
Electrical Information Reference

* Circuit Testing Circuit Testing
* Connector Repairs Connector Repairs
* Testing for Intermittent Conditions and Poor Connections Testing for Intermittent Conditions and Poor Connections
* Wiring Repairs Wiring Repairs

Scan Tool Reference

Control Module References Control Module References for scan tool information

Circuit/System Verification

Ignition ON, perform the appropriate Auxiliary Tail and Brake Lamp test. The appropriate tail and brake lamps should be ON/OFF.

Circuit/System Testing

1. Ignition OFF, disconnect the appropriate harness connector at the tail lamp listed below:

* E5E Tail Lamp - Left
* E5F Tail Lamp - Right

2. Disconnect the negative terminal of the C1 Battery.
3. Test for less than 5 ohm between the appropriate ground circuit harness connector terminal listed below and ground:

* E5E Tail Lamp - Left harness connector terminal C
* E5F Tail Lamp - Right harness connector terminal C

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

4. Reconnect the negative terminal of the C1 Battery.
5. Connect a test lamp between the appropriate control circuit harness connector terminal listed below and ground:

* E5E Tail Lamp - Left harness connector terminal A
* E5F Tail Lamp - Right harness connector terminal A

6. Command the appropriate Auxiliary Tail and Brake Lamp 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 appropriate control circuit for a short to voltage. If the circuit tests normal, replace the K68 Trailer Lighting Control Module.
• If the test lamp is always OFF, test the appropriate control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the K68 Trailer Lighting Control Module.

7. If all circuits test normal, test or replace the appropriate tail lamp.

Repair Instructions

Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.

* Tail Lamp Replacement Service and Repair
* Control Module References Control Module References