Mercedes-Benz W220 battery drains when parked
How do I find the current drain that empties a W220's battery?
Connect a clamp ammeter to the battery cable and pull fuses one at a time. When the measured current drops, you have found the consuming circuit. The drain does not necessarily show up as a fault code, so it has to be measured.
What are the most common current drains on the W220?
Often mentioned are the Comand system and audio amplifier, the alarm, the central locking pump, the electric seats and the microswitch of the tailgate lock. The connector of the SAM unit at the rear has also caused drain, and an air spring leak can keep the level control running.
Can a faulty control unit keep the car's electrical system awake?
Yes. A faulty ignition switch control unit or door control unit can stop the CAN bus (the communication bus between control units) from going to sleep, so current is used overnight. The culprit is located by measuring, and the repair is done by replacing the faulty part.
Can a flat battery cause many fault codes in the W220?
Yes. When the battery is weak or flat, many electrical devices stop working and a lot of fault codes are stored in memory. After the current drain is fixed, the codes can be cleared, but not all codes are real faults.