Mercedes-Benz W203 alternator not charging and battery warning
How do I check the charging of a W203 alternator?
Measure the voltage at the battery terminals with the engine running. Normally the voltage is about 14 V. If it is about 11-12 V or does not rise after starting, the alternator is not charging properly.
What causes the battery or alternator warning on a W203?
Common causes are worn brushes, a faulty voltage regulator, a burnt rectifier bridge and a failed freewheel pulley on the alternator. Water leaking in through the alternator's water cooling connector has also caused fluctuating charging voltage.
What else should be checked before replacing the alternator?
Check the wiring and earth connections. A thick power cable behind the starter motor may be loose or hidden from view, and an unreliable earth interrupts charging. Also measure the condition of the battery.
Is replacing the alternator easy on a W203?
No, because the alternator is in a cramped location. The work involves removing, among other things, the air filter housing and the charge air pipe, and on a water-cooled alternator the coolant connections. Disconnect the battery negative terminal first. The quality of remanufactured alternators varies.