Mercedes-Benz W203: battery drains when parked
What drains a W203 battery when parked?
On a diesel the most common cause is a faulty electric auxiliary heater, located on the side of the engine under the intake manifold, which can draw a lot of current from the battery. Other causes are the power seat control units, a faulty diode in the alternator, a damp front SAM control unit and a Bluetooth device left in the OBD connector.
How is a current drain found on a W203?
Measure the quiescent current with a clamp ammeter or a multimeter and remove fuses one at a time until the reading drops. Allow time for the measurement, because the car only goes to sleep a while after being locked. Suspects are COMAND, the power seats, the alarm and the auxiliary heater.
Is there a normal value for the W203 quiescent current?
Sources vary depending on equipment, so an exact limit cannot be given. As a rule of thumb, the quiescent current should be only a few tens of milliamps, and if the meter shows amps, a control unit or the auxiliary heater has failed.