Mercedes-Benz W211 alternator and charging voltage
What is the normal charging voltage of the W211?
With the car running, the terminal voltage should stay steady at about 13.8-14.5 V. The resting voltage of a full battery is about 12.6-12.7 V. The voltage reading in the instrument cluster is not always reliable, so measure the voltage with a multimeter directly at the battery terminals.
What can cause a charging problem on the W211?
The most common causes are worn brushes and slip rings, a faulty voltage regulator, alternator bearings and the pulley's freewheel clutch. An oxidised ground or charging cable can also prevent charging. If new batteries do not help and the voltage drops after driving, the fault is most likely in the alternator.
What do the battery warning and the comfort functions switching off tell me?
When the voltage drops, the car switches comfort functions off and shows a battery warning. First measure the charging voltage at the battery terminals and check the alternator's sound and pulley. A wrong voltage regulator does not communicate properly with the control unit, so the correct part is found from the part number in the parts catalogue.
How can the alternator's freewheel clutch be checked?
The freewheel clutch is inside the alternator pulley. With the belt removed, the pulley is turned by hand: it should grip clockwise and turn lightly counterclockwise. The belt can be removed by locking the tensioner open.