Replacing the auxiliary battery in your Mercedes-Benz C300 can seem daunting, but it’s a manageable DIY task. This guide provides a step-by-step approach to replacing the auxiliary battery in a 2015 W205 C300, addressing the common “auxiliary battery malfunction” message. This repair is often covered under warranty, but if a dealer visit isn’t feasible, replacing the battery yourself can save time and money.
Locating and Accessing the Auxiliary Battery
The auxiliary battery in the W205 C300 is located beneath the glove compartment. To access it, you’ll need to remove a few components:
- Glove Compartment Undercover: Start by removing the undercover panel beneath the glove compartment. This typically involves removing a single screw and releasing two retaining clips.
- Wiring Harness Connections: Two electrical connectors for the glove compartment lighting are attached to the undercover. Carefully disconnect these connectors before proceeding.
- Carpet and Foam: Gently pull back the carpeting and underlying foam. The foam is flexible and can be manipulated without damage to gain access to the battery compartment.
- Battery Compartment Cover: Once the carpet and foam are pulled back, you’ll see a plastic cover secured by three nuts. Remove these nuts to expose the voltage converter and the auxiliary battery.
Replacing the Auxiliary Battery
With the battery compartment open, you can now replace the faulty auxiliary battery:
- Voltage Converter Removal: The voltage converter is attached to a metal bracket with a clip and a screw. Carefully remove the screw and unclip the converter.
- Auxiliary Battery Disconnection: Disconnect the electrical connector from the auxiliary battery.
- Battery Replacement: Install the new auxiliary battery, ensuring it’s correctly oriented and seated in the compartment.
- Reassembly: Reconnect the electrical connector to the new battery. Reinstall the voltage converter, ensuring the clip and screw are secured. Replace the battery compartment cover and tighten the three nuts. Carefully reposition the carpet and foam, ensuring it fits snugly. Reconnect the two wiring harness connectors for the glove compartment lighting and reinstall the undercover panel, securing the clips and screw.
Important Considerations
Exercise caution when working in this area, as there are numerous delicate wiring harnesses and components beneath the glove compartment. Using the correct tools and working slowly and methodically will help prevent accidental damage. While this guide provides a general overview, always consult your vehicle’s specific repair manual for detailed instructions and torque specifications. A replacement auxiliary battery can be sourced online for around $100. The entire process should take approximately 35 minutes for someone with basic mechanical skills.
