Mercedes-Benz vehicles stand out for their cutting-edge features and refined design, including trunk release systems crafted for effortless access. When the trunk release button fails, however, it can quickly interrupt your day and limit the car’s convenience.
This guide unpacks the common reasons behind these issues and provides clear, step-by-step solutions to get things back on track.
1. Electrical Issues
Blown Fuse
A blown fuse is one of the primary culprits when a trunk release button fails. Electrical surges or short circuits can overload the circuit connected to the trunk release mechanism, rendering it non-functional.
This issue frequently affects models like the Mercedes-Benz C-Class (W204, 2007–2014) and E-Class (W212, 2009–2016).
Start by locating the fuse box, typically found in the trunk, engine compartment, or behind a side panel. Use the owner’s manual to identify the fuse responsible for the trunk release system.
A damaged fuse will appear burnt or broken. Replace it with one of the same amperage and test the trunk release button to confirm if the issue is resolved.
Faulty Trunk Release Switch
Over time, wear and tear or exposure to moisture can damage the trunk release switch. Corrosion or internal faults may prevent it from sending the necessary signal to open the trunk. This problem is common in models like the Mercedes-Benz S-Class (W221, 2006–2013).
To inspect the switch, carefully remove it from its housing and check for visible signs of corrosion or damage. Using a multimeter, test the switch for continuity while pressing it. If the test fails, the switch needs to be replaced.
Damaged Wiring Harness
Frequent trunk opening and closing can strain the wiring harness, leading to frayed or broken wires. In some cases, rodents or improper installations may also cause damage. The E-Class (W211, 2002–2009) is particularly prone to this problem.
Visually examine the wiring near the trunk hinge for wear or exposed wires. Damaged wires can be repaired with soldering and heat-shrink tubing. Secure the harness properly to avoid future issues.
2. Central Locking System Malfunctions
Defective Trunk Lock Actuator
The trunk lock actuator is a critical component in the central locking system. It uses a small motor, which can fail over time due to wear or electrical problems.
This issue is often reported in newer models like the Mercedes-Benz C-Class (W205, 2014–present) and GLC-Class (X253, 2015–present).
When pressing the trunk release button, listen for a clicking or humming noise from the actuator. If no sound is heard, the actuator may need replacement. Diagnostic tools can also help identify fault codes linked to the locking system.
Central Locking System Synchronization Issues
After a battery replacement or disconnection, the central locking system may lose synchronization. This glitch can disable the trunk release button in models like the Mercedes-Benz CLS-Class (W218, 2010–2018).
Resetting the system can often resolve this problem. Lock and unlock the vehicle multiple times using the key fob or central locking controls.
Alternatively, disconnect the battery for a few minutes and reconnect it to reset the system. Ensure you have the radio code before disconnecting the battery.
3. Key Fob Problems
Dead or Weak Key Fob Battery
A depleted key fob battery is a frequent reason for trunk release issues. This can affect all Mercedes-Benz models with keyless entry systems.
Replace the battery with the correct type, usually CR2025 or CR2032, and ensure it is installed with the correct polarity.
After replacement, some models may require the key fob to be re-synchronized, which can be done by following the steps in the owner’s manual.
Damaged Key Fob
Physical damage or internal circuit failures can render the key fob ineffective. If the trunk release button on the fob doesn’t work, inspect it for cracks, water damage, or loose components.
A spare key fob can help determine if the issue is with the fob itself. If damaged, the fob may require repair or replacement by an authorized dealer.
4. Mechanical Issues
Faulty Trunk Latch Mechanism
The trunk latch mechanism can wear out over time or become damaged due to improper use. Models like the Mercedes-Benz E-Class Coupe (C207, 2009–2017) are known to experience such issues.
Inspect the latch for physical damage or misalignment. Apply a silicone-based lubricant to ensure smooth operation. Replacing the latch with a new assembly is necessary if it is broken.
Obstruction in Trunk Lock
Debris, dirt, or small objects can get lodged in the latch area, preventing it from operating correctly. This issue can occur in any Mercedes-Benz model.
Examine the latch area and remove any visible obstructions. Use compressed air or a soft brush to clean the mechanism. After cleaning, test the trunk release button to confirm functionality.
5. Software and Control Module Failures
Body Control Module (BCM) Malfunction
The Body Control Module (BCM) governs various electrical functions, including the trunk release. Electrical faults or software corruption in the BCM can disrupt the system. Models like the Mercedes-Benz GLE-Class (W166, 2015–2019) are susceptible to such issues.
A diagnostic scan using specialized tools can reveal BCM-related error codes. If the module is faulty, resetting or reprogramming it at an authorized service center may resolve the issue. In severe cases, the BCM may require replacement.
Rear Signal Acquisition Module (SAM) Issues
The SAM controls signals for various vehicle systems, including the trunk release. Water ingress or corroded connections can lead to SAM failures, particularly in the Mercedes-Benz S-Class (W222, 2013–2020).
Inspect the SAM for moisture or corrosion. Clean and dry the module if necessary, ensuring all connections are secure. If the module is beyond repair, a replacement will be required.
6. Valet Mode Activated
Valet mode disables trunk access to prevent unauthorized use. This feature can be inadvertently activated in models like the Mercedes-Benz C-Class (W205, 2014–present).
Check the vehicle for an indicator showing valet mode activation. Use the mechanical key to unlock the glove compartment and toggle the valet switch to disable it.
7. Power Liftgate System Failures
Malfunctioning Power Liftgate Motor
The power liftgate motor can fail due to wear or electrical issues, especially in models like the Mercedes-Benz GLS-Class (X166, 2013–2019).
When pressing the trunk button, listen for motor sounds. If the motor is silent, it may need to be reset by holding the close button for several seconds. If it shows no response after resetting, replace it.
Liftgate Obstruction Detection Activated
Modern Mercedes-Benz models are equipped with sensors to detect obstructions in the liftgate’s path. These sensors can misfire if dirty or blocked.
Ensure no objects are in the trunk’s path, and clean the sensors with a soft cloth. If the issue persists, consult a dealer for further diagnostics.
Final Thoughts
Electrical, mechanical, or software issues can cause a non-functional trunk release button. Most problems can be resolved efficiently by identifying the root cause and following the appropriate troubleshooting steps.
Regular maintenance and attention to the vehicle’s components will help prevent similar issues in the future, ensuring the reliability of your Mercedes-Benz’s trunk system.