Mercedes-Benz W212 alternator not charging and battery light on
W212 alternator not charging and the battery light is on, what is a common cause?
The alternator brushes usually wear out first, and replacing the brush set may be enough. The alternator can also give a fluctuating voltage, which makes the warning light come on and the voltage vary. In that case the alternator is replaced, and the job can be done by a workshop.
How are charging and battery condition checked on a W212?
The battery voltage and battery current can be read from the hidden menu in the instrument cluster; when the alternator works, the current is on the positive side. The no-load voltage of a full AGM battery is 12.8 V and of a lead-acid battery 12.6 V. If the voltage is immediately below 12 V, the battery is partly discharged. Charge the battery overnight and measure again: a low voltage then points to a battery fault.
Is too high a charging voltage dangerous?
A charging voltage above 15 volts can ruin the battery and cause fault codes; on the OM642 engine a normal value of about 14.8 V was mentioned. Replacing the alternator and the voltage regulator evened out the voltage, but fault codes that have already been stored may remain, so have them checked at a workshop.