Locate and test the battery. On many Mercedes-Benz vehicles, the battery location ranges from under the hood to in the trunk, under the rear seat (W210) or under the front passenger seat (w164/166/151). On the G-wagen (W461/463), the battery is under a cover in the floor board behind the center console. Most vehicles with the battery in the trunk will have a sticker under the hood stating "battery in trunk". For vehicles with two batteries, the systems battery will be in the trunk. The starter battery will be under the hood. For starting issues, you will want to replace the battery under the hood. It will be a smaller white battery.
Using a DVOM (voltmeter), check voltage by contacting the positive and negative terminals with the corresponding test wires from the DVOM. A good battery will read 12.1-12.9 volts when the vehicle is off. A Mercedes-Benz vehicle needs a minimum of 11.4 volts to properly operate the onboard control units.
Acquire a replacement. Once the condition of the battery is determined and you have acquired a replacement, double check the amp hours (Ah) and cold cranking amps (CCA) on the battery sticker to be sure that it matches your vehicle specifications. Part numbers on said sticker may differ if the original battery is older.
Remove the old battery. On vehicles with exposed batteries (under the hood), a 10mm wrench or socket is required to remove battery cables and battery top mounts. A 13mm socket and extension are required to remove battery bottom mounts. Additionally you may need to remove the cabin air filter box. There are three snap clips on top that hold it to the firewall. use a screwdriver and pop them upward and removed air filter box. Although some batteries may seem shoehorned into their respective place, push all cables and wires to the side and lift straight up. On vehicles with the battery in the trunk, you need to lift up the trunk floor panel and the battery should be on the right or center of the trunk under compartment. In some cases, there is a strap wrapped around the battery that can be loosened by finding the buckle, usually located on the side of the battery. On the S-Class or AMG S-class (2010 or later) there is a panel at the forward most part of the trunk that can easily come out with the removal of the trunk bottom panel. The trunk bottom panel has two T20 Torx bolts that sit inside the black plastic hooks bolted to the floor of the trunk. Once the bolts are removed and the bottom panel comes out, the forward most panel will pivot out from the left side (right side pivoting) and slide out to reveal a space between the forward-most trunk panel and the back of the seat. This is where the systems battery will be located. On vehicles with two batteries (starter battery and systems battery), usually S-class (221) and SL class (230/231) the starter battery needs to be connected first. Therefore, if you are replacing the systems battery (in the trunk) you need to disconnect the negative battery cable from the starter battery (under the hood). Once the systems battery has been replaced you can reconnect the starter battery. A battery light may appear in the instrument cluster otherwise.
Put the new battery in its place. Once the new battery is secured into the vehicle, connect the positive battery cable first. Then connect the negative battery cable. Make sure the positive cable connects to the positive post on the battery. If you connect the negative cable first then the positive, there is a chance the electricity could arc between cable and terminal which could possibly cause a control unit to short out. To reiterate, the negative battery cable comes off first and goes on last. Ensure that the new battery fits in your vehicle properly or that it's bracketed on correctly so it doesn't shift around while you're driving.
Clear your settings after installation. Three things need to be done: Resetting the time: On most newer vehicles the car will reset the time itself after being driven around and acquiring a satellite signal. Unless the "time zone" has been reset in the menu, there is no need to reset the clock manually. Window normalization: When you disconnect the battery the windows sometimes forget where they need to start and stop. To reset window normalization, simply roll the window up and hold the button for 5 seconds. There should be an audible click once the window has relearned its position. ESP: On all newer vehicles and most older ones, the ESP light will illuminate if the steering angle sensor has lost its positioning. To turn the light off, start the engine and turn the steering wheel from lock to lock until the yellow triangle (ESP light) on the instrument cluster is gone.
Congratulations! You have successfully replaced the battery in your Mercedes-Benz!
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