Download How to Clean Mercedes W123 Electric Ground Contacts – Step 1 — How to clean Mercedes W123 Electric Ground contacts The primary ground for your car’s electric system is the ground on the car’s battery. It connects to the chassis of the car next to the battery itself. Begin by disconnecting the ground or negative terminal from your battery. For guidance on removing the terminal, and for other suggested battery maintenance while it is disconnected, click here to see the battery maintenance guide. Follow the cable to the attachment point on the chassis; it will be very nearby. Loosen and remove the nut that holds the battery ground cable to the chassis, which will allow you to set aside the cable for now. Use some emory cloth, very fine sandpaper, or a soft metal brush such as brass to remove any dirt or corrosion from the area where the battery cable connects to the chassis. Then re-install the cable and reconnect it to the battery.