Understanding and Fixing Mercedes-Benz U140B Fault Code

ABS

As a Mercedes-Benz electrician technician, I often come across fault codes that can be frustrating for both drivers and mechanics. One of these is U140B – “The left rear wheel speed signal from the traction system is not available on the CAN bus.” This diagnostic trouble code (DTC) indicates that the left rear wheel speed sensor is not communicating correctly with the vehicle’s traction and stability control systems.

If you’re experiencing this fault code, it’s crucial to address it promptly because it can affect multiple safety features, including ABS (Anti-lock Braking System), ESP (Electronic Stability Program), and traction control. In this post, I’ll explain what causes this fault, how to diagnose it, and steps to fix it properly.

Understanding the U140B Fault Code

What Does It Mean?

The vehicle’s traction system continuously monitors wheel speeds via sensors located on each wheel. These sensors send data to the ABS/ESP module through the Controller Area Network (CAN bus).

When the left rear wheel speed signal is missing or not available, the system cannot properly regulate braking force and traction control. This can lead to:

  • Loss of ABS functionality
  • ESP/traction control system deactivation
  • Dash warning lights (ABS, ESP, or Check Engine Light)
  • Potentially unstable handling in slippery conditions

Causes of U140B Code

Several factors can trigger this fault code, including:

1. Faulty Left Rear Wheel Speed Sensor

  • The most common cause is a defective sensor that is failing to send a signal to the control module.
  • The sensor could be damaged due to dirt, corrosion, or physical impact.

2. Wiring Issues or Connector Problems

  • A broken, corroded, or loose wire can interrupt the signal between the sensor and the ABS module.
  • Moisture or dirt inside the connector can also lead to a weak or lost signal.

3. ABS Control Module Malfunction

  • A defective ABS module might not be receiving or processing the wheel speed data correctly.
  • Software issues or internal damage within the module can cause intermittent faults.

4. CAN Bus Communication Failure

  • Since the ABS system communicates via the CAN bus, any issue within this network can prevent data transmission.
  • A short circuit, open circuit, or interference from another faulty component could be responsible.

5. Faulty Tone Ring ( Wheel)

  • The tone ring is a metal ring with teeth that the sensor reads to determine wheel speed.
  • If the ring is dirty, broken, or missing teeth, the sensor will send incorrect or no data.

Diagnosis of U140B Fault Code

1. Read the Fault Codes

  • Use a professional Mercedes-Benz STAR Diagnostic Scanner (computer) to retrieve the U140B code and check for related codes.

2. Inspect the Left Rear Wheel Speed Sensor

  • Locate the sensor near the wheel hub and visually inspect it for damage, dirt, or corrosion.
  • Clean the sensor and retest to see if the fault persists.

3. Check Wiring and Connectors

  • Examine the sensor wiring harness for any signs of damage, cuts, or corrosion.
  • Disconnect the plug and inspect for bent pins or moisture.

4. Perform Live Data Testing

  • Using the diagnostic scanner, monitor the left rear wheel speed signal in real-time.
  • If the signal is erratic or missing while driving, the sensor or wiring may be faulty.

5. Inspect the Tone Ring

  • Check for cracks, missing teeth, or excessive rust that could interfere with signal transmission.

6. Test the CAN Bus Communication

  • If no faults are found in the sensor or wiring, use an oscilloscope or diagnostic scanner to check CAN bus communication.

Fixing the U140B Fault Code

1. Replace the Left Rear Wheel Speed Sensor

  • If the sensor is defective, replacing it should restore proper function.
  • Ensure you install a genuine Mercedes-Benz sensor to avoid compatibility issues.

2. Repair or Replace Wiring

  • If damaged wiring is found, repair or replace it to restore proper signal transmission.

3. Clean or Replace the Tone Ring

  • If the tone ring is dirty, clean it with brake cleaner and a brush.
  • If it is broken or worn out, replace it.

4. Reprogram or Replace the ABS Module (if needed)

  • If the issue persists after replacing the sensor and fixing wiring, consider reprogramming or replacing the ABS control module.
  • A software update may be required in some cases.

5. Reset Fault Codes and Test Drive

  • After performing repairs, clear the fault codes using computer and take the car for a test drive to ensure the issue is resolved.

Final Thoughts

Ignoring the U140B fault code can compromise your vehicle’s braking and stability systems, which can be dangerous. If you’re dealing with this issue and need expert help, I specialize in diagnosing and repairing electrical faults in Mercedes-Benz vehicles. With years of experience and the right tools, I can accurately diagnose and fix the problem, ensuring your car remains safe and reliable.

Need Assistance? Contact Me for Professional Mercedes-Benz Electrical Repairs!

If you’re facing ABS or traction system issues, don’t hesitate to reach out. I provide expert diagnostics and repair services to get your Benz back in top shape.

Let me know if you’d like to schedule an appointment! 09015329867 Mercedes Benz engineer Abuja