Sequence of works
1. Connect the multimeter between the positive terminal of the generator (red wire) and "mass".
2. Let the engine run for two minutes at 3000–4000 min⁻¹ to allow the generator to reach operating temperature (about +80°C).
3. Turn on the parking light, radio or fan. This corresponds to a load of 3-7 A. The voltage on the regulator should be within 13.0-14.6 V.
4. If the device shows a higher voltage, the regulator is faulty and should be replaced. If the voltage is lower than the specified values, the brushes are worn out. In this case, remove the generator and have it repaired in a workshop.
Material borrowed from an online resource: MercedesMAN