Mercedes-Benz W124 alternator not charging: causes and checks
W124 alternator not charging or charge light does not come on: what is the most common fault?
Most often the alternator brushes are worn out. The brushes and voltage regulator are in the same unit at the back of the alternator, and it can be replaced with two screws without removing the alternator. The brushes are worn when they are about 5 mm from the edge of the holder, or the slip ring has a groove deeper than 2.5 mm.
What is the correct charging voltage on the W124?
With the engine running the voltage should be about 14 V. Below 13 V points to an alternator fault or a poor connection. A fully charged battery rests at at least 12.66 V and a flat one at about 12.3 V.
The alternator is new or the brushes were replaced, but it still does not charge. What else should I check?
Check the charge light bulb, the wires and sleeves of the alternator connector, and the cable between the battery and the alternator. A slipping belt or a broken tensioner also prevents charging. The voltage drop between the alternator + terminal and the battery + terminal, or on the negative side between the body and the battery, may be at most 0.5 V.