Mercedes-Benz W201 alternator and voltage regulator faults
Mercedes-Benz W201 what is the correct charging voltage and how do you check the alternator?
Measure the voltage at the battery terminals with the engine running. The correct value is about 13.8–14.4 V. The battery's resting voltage with the engine off tells you nothing about the alternator. If the voltage stays below 13 V, for example at 12 V, charging is faulty.
Mercedes-Benz W201 alternator not charging, what is the most common cause?
The most common causes are worn brushes, a fault in the voltage regulator or a burnt diode. The regulator is fixed to the end of the alternator with two screws and replacing it is easy and cheap, so try it first. If the slip ring has grooves, replacing the brushes alone is not enough.
Mercedes-Benz W201 warning lights come on or the voltage rises too high, what is the fault?
Too high a voltage, brightening lights and a boiling battery point to a voltage regulator fault. If the charging light is on with the ignition off, or the warning lights do not come on, a rectifier diode in the alternator may be burnt and the alternator must be replaced. Also check the alternator wire connectors for looseness and oxidation.