Mercedes-Benz C / CLK Battery Replacement Guide / W203 W209

Can’t start your Mercedes-Benz C/CLK-Class due to a dead battery? Learn how to fix the problem by replacing the battery of your Mercedes-Benz C or CLK-Class.

In this guide, you will learn and become familiar with replacing the battery on Mercedes CLK-Class and W203 C-Class. The battery is installed in the same location on both these models, and the procedure is identical.

On average, this entire process should only take about an hour or less. Additionally, we would rank this task as easy, even for beginners. The most difficult part is getting the battery bracket off due to limited space.

What you need

• AGM-94 Battery
  • Group Size H8
  • Minimum Cold Cranking Amps760
• Flathead Screwdriver
• 3/8 Ratchet
• 3/8 Long Extension
• 3/8 Long 10mm Socket
• 3/8 Long 13mm Socket
• Flashlight or a Reliable Light Source
• Extendable Magnet

Procedure

1. Gather all the necessary tools and ensure the new battery is fully charged.
2. Park the vehicle on flat ground with a reliable light source and set the parking brakes.
3. Turn the ignition to the off position. Remove the key from the ignition.
4. Open the hood latch located below the steering wheel. Ensure the hood stays open. Sometimes, the hood struts get weak, and the hood may close as you work in the engine bay. Have a friend help keep the hood open if the hood doesn’t stay up.
5. Using a flathead screwdriver, carefully pop open the three clips holding the battery cover in place. 
6. Remove the cover and put it off to the side.
7. Loosen the negative battery terminal nut with a 10mm socket to unground the battery properly, followed by moving the terminal.
8. Loosen the positive battery terminal nut with a 10mm socket and move the terminal wire to the side.
9. Using the 13mm socket, remove the two nuts that hold the battery in place (one on each side of the battery). Space is tight, so a pickup magnet can help in this step. Remove the mounting bracket and move all the cables out of the way.
10. Disconnect the vent hose on the lower bottom of the battery. If your car has an AGM battery, it will not have a vent hose.
11. The battery can now be removed and lifted by the two handles. Get someone to help you lift the old battery.
12. Insert the new battery carefully inside the vehicle, remembering that Negative is on the left and Positive is on the right.
13. Reconnect the vent hose and ensure the mounting bracket is lined up.
14. Insert each 13mm nut back into the mounting points on each side and tighten them to specification.
15. Use a terminal wire brush to clean the terminals when possible.
16. Connect the positive cable back onto the positive battery terminal and tighten it with the 10mm nut.
17. Repeat the same steps for the negative cable.
18. Place the battery cover back on top of the battery and lock in the three tabs.

We do not recommend using a backup battery to keep the vehicle powered up while you replace the battery. While connecting a backup battery saves a few minutes, it introduces unnecessary risk, especially if there is an accidental short or insufficient power to keep all vehicle systems powered up.

Torque Specifications

• 10 ft/lbs. for both terminals
• 25 ft/lbs. for each mounting bracket bolt

Tips & Tricks

• The Positive terminal is always on the right if looking at the car from the front.
• The Negative terminal is always on the left looking at the car from the front.
• Always check the size, AMPs, and volts of the new battery compared to the old battery.
• After disconnecting the terminals, use a wire brush and carefully clean off the battery terminals to ensure no major corrosion.
• Do not leave the key in the ignition while you replace your Mercedes-Benz C CLK battery. This can cause problems to the EIS module, and the vehicle may not start.
• Once the vehicle starts, ESP and ABS lights may come on. This is due to the steering angle sensor needing calibration. To reset the steering angle sensor, start the engine and turn the steering wheel to the left and right several times.
• Synchronize the sunroof and windows by pressing the button and keeping it pressed for an additional five seconds after the window/sunroof reaches fully open and closed positions.

We hope you find the Mercedes-Benz C / CLK Battery Replacement Guide / W203 W209 guide helpful. Check these troubleshooting and repair guides for more help on your Mercedes-Benz.