Mercedes-Benz W204 alternator and charging voltage
What should the charging voltage be on the W204?
Charging voltage is measured with the engine running, and it should be slightly over 14 V. An AGM battery needs an even higher voltage. If the reading stays below 14 V or fluctuates, the fault is in the alternator, the regulator or the earth connection.
What causes weak or fluctuating charging on the W204?
The cause varies: the alternator, its voltage regulator, poor cable connections or a weak earth between the engine and the body. Check the alternator and battery cables and the earth points first. If the fault disappears after restarting, the cause is often a poor contact.
Why do the W204's ABS, ESP and rear light warnings come and go?
Under- or overvoltage often causes error messages like these. A voltage above 16 V while running points to the voltage regulator. A new battery does not help if charging is the cause. The brake system warnings should still be checked at a workshop.
Is it worth replacing the brushes or regulator of the W204's old alternator?
In an old alternator the slip rings are usually worn, so replacing the brushes or bearing helps only for a short time. If the alternator does not produce enough, a completely new alternator is often the most sensible choice. Also check the belt and the freewheel pulley before replacing.