B3138 - All Door Lock Circuit High (BCM)

The B3138 diagnostic trouble code (DTC) indicates a problem within the all door lock circuit, specifically a "high" voltage reading detected by the Body Control Module (BCM). This code suggests that the BCM is registering a voltage level in the door lock control circuit that is higher than the expected operating range. Addressing this code promptly is crucial, as malfunctioning door locks can compromise vehicle security and convenience.

Detailed Explanations

DTC Definition: All Door Lock Circuit High (BCM): This is the core definition. The B3138 code indicates that the Body Control Module (BCM) has detected a voltage reading on the all door lock circuit that is higher than the expected or specified voltage range. This usually implies a short to voltage somewhere in the circuit.

Affected Systems: The primary systems affected are the door lock system itself and the Body Control Module (BCM). The BCM is responsible for controlling various body functions, including the door locks. A fault in the door lock circuit can directly impact the BCM's operation.

Symptoms: The most common symptom is that all door locks may not function correctly. This can manifest as the locks not responding to the lock/unlock buttons, intermittent operation (sometimes working, sometimes not), or the locks cycling erratically (locking and unlocking repeatedly). Security system issues may also arise.

Voltage Readings: A "high" voltage reading typically means the BCM is seeing close to battery voltage (around 12V or higher) on the door lock control circuit when it should be at or near 0V when the locks are not being actively controlled. This abnormal voltage level triggers the B3138 code.

Common Causes: The most frequent culprits are a short to voltage in the door lock wiring (a wire accidentally touching a power source), corroded or damaged connectors that create a short, faulty door lock actuators that internally short, or, less commonly, an internal failure within the BCM itself.

Diagnostic Steps: The diagnostic process begins with a visual inspection of the wiring and connectors related to the door lock system. This is followed by voltage testing of the door lock circuits using a multimeter. Actuator testing can help determine if the actuators are functioning correctly. Finally, if other causes are ruled out, the BCM may need to be tested. Wiring harness testing specifically looks for shorts to voltage.

Tools Required: Essential tools include a multimeter for voltage and continuity testing, wiring diagrams to understand the circuit layout, a scan tool to read and clear DTCs, and a test light for basic circuit testing.

Repair Procedures: Repairing the B3138 code often involves repairing or replacing damaged wiring that is causing the short, cleaning or replacing corroded connectors, replacing faulty door lock actuators, or, in the worst-case scenario, replacing the BCM, which usually requires reprogramming.

Related DTCs: Codes like B3135, B3136, and B3137 might appear alongside B3138, especially on vehicles where each door lock has a dedicated circuit. These codes relate to individual door lock circuits. U codes indicate communication errors with the BCM or other modules.

Severity: The severity of B3138 is considered moderate to high. While it might not immediately disable the vehicle, it can compromise vehicle security by preventing the doors from locking properly and cause significant inconvenience.

Cost to Repair: The cost to repair B3138 can vary widely depending on the underlying cause. Simple wiring repairs or connector replacements can be relatively inexpensive. However, replacing a faulty BCM can be a significantly more costly repair due to the part's price and the required programming.

Technical Service Bulletins (TSBs): Always check for any Technical Service Bulletins (TSBs) issued by the vehicle manufacturer that relate to door lock issues or BCM problems for your specific vehicle make and model. TSBs often provide valuable diagnostic and repair information.

Component Locations: The Body Control Module (BCM) location varies greatly by vehicle make and model, but it's typically located under the dashboard, often near the steering column or in the center console area. Door lock actuators are located inside each door, attached to the door lock mechanism. Consult a service manual for the precise locations in your vehicle.

Actuator Resistance: The typical resistance values of the door lock actuators depend on the manufacturer and the specific actuator design. Out-of-range resistance values (either too high or too low) indicate a faulty actuator. Refer to the service manual for your vehicle to find the correct resistance specifications.

Wiring Harness Inspection: A thorough inspection of the wiring harness is crucial. Pay close attention to areas where the harness passes through door jambs or other points where it might be subject to chafing, cuts, or other damage.

Connector Inspection: Inspect all connectors related to the door lock system for signs of corrosion, bent pins, or loose connections. Clean corroded connectors with electrical contact cleaner and ensure that all connections are secure.

BCM Programming: Replacing the BCM often requires programming or initialization to match the vehicle's VIN (Vehicle Identification Number) and options. This programming ensures that the new BCM functions correctly with the rest of the vehicle's systems. Incorrect programming can lead to various electrical problems.

Power and Ground Circuits: Verify that the BCM has proper power and ground connections. A poor power or ground connection can cause the BCM to malfunction and generate false DTCs. Check the voltage at the BCM power supply and the continuity of the ground connections.

Circuit Description: The BCM controls the door lock actuators by sending voltage signals through dedicated circuits. When the BCM commands the door locks to lock or unlock, it applies voltage to the corresponding circuit. A high voltage reading on the door lock circuit when it should be low indicates a fault in the circuit.

Short to Voltage Diagnosis: Use a multimeter to check for voltage on the door lock circuit with the BCM disconnected. If voltage is present, trace the wiring harness to find the short to voltage. A common technique is to disconnect sections of the harness to isolate the area where the short is located.

BCM Internal Fault: If all other possible causes have been thoroughly investigated and ruled out, the BCM itself may be internally faulty. This is often a last resort diagnosis. BCM replacement may be necessary, followed by programming to match the vehicle's specifications.

Aftermarket Accessories: Aftermarket alarm systems or remote start systems can sometimes interfere with the door lock circuits, causing erratic behavior or false DTCs. Disconnect any aftermarket accessories to see if the problem goes away.

Environmental Factors: Exposure to moisture, extreme temperatures, or corrosive substances can damage wiring and connectors, leading to shorts or open circuits. Check for signs of water intrusion or corrosion in the door lock circuits, especially in areas prone to moisture exposure.

Intermittent Faults: Intermittent faults can be particularly challenging to diagnose. Use a scan tool to monitor the door lock circuits while operating the door locks. Look for erratic voltage readings or intermittent disconnections that might indicate a loose connection or a wiring problem.

Door Lock Switch Issues: While B3138 directly implicates the BCM, a faulty door lock switch (the switch you press to lock/unlock the doors) could indirectly contribute to the problem if it's causing a short or sending erratic signals to the BCM. Test the door lock switch for proper function and resistance.

Fuse Inspection: Always check the fuses related to the door lock system and the BCM. A blown fuse could indicate a short circuit in the door lock wiring or a problem with the BCM's internal circuitry. Replace any blown fuses with the correct amperage rating.

Ground Circuit Integrity: Ensure the BCM has a solid ground connection. A poor ground can cause erratic behavior, incorrect voltage readings, and the generation of false DTCs. Check the BCM ground wire for corrosion, damage, or loose connections.

Frequently Asked Questions

What does the B3138 code mean? The B3138 code signifies that the Body Control Module (BCM) is detecting a higher-than-expected voltage in the all door lock circuit. This usually indicates a short to voltage.

What are the symptoms of a B3138 code? Common symptoms include non-functional door locks, intermittent door lock operation, door locks cycling erratically, or security system issues.

What causes the B3138 code? Common causes are a short to voltage in door lock wiring, corroded connectors, faulty door lock actuators, or a failing BCM.

Can I fix the B3138 code myself? Depending on your mechanical skills, you may be able to fix simple wiring issues. However, diagnosing and repairing electrical faults often requires specialized tools and knowledge.

How much does it cost to fix a B3138 code? The cost varies greatly. Simple wiring repairs can be inexpensive, while BCM replacement and programming can be quite costly.

Conclusion

The B3138 DTC indicates a "All Door Lock Circuit High (BCM)" condition, often stemming from a short to voltage within the door lock system. Thorough diagnosis, including visual inspection, voltage testing, and component testing, is crucial for identifying the root cause and implementing the appropriate repair strategy.