Also check that the drive belt is properly tensioned with the tensioner. If you have a voltmeter and ammeter, you can perform the following tests:
- find the wires on the back of the generator. The red wire is connected to the "B+" terminal of the generator, the thin blue wire is connected to the "D+" terminal. For the first check, connect a voltmeter between the "D+" terminal and the "ground" (do not disconnect the wire, do not turn on the ignition). The device should show zero voltage;
- turn on the ignition and make sure that the voltage value is 1-3 V. If this is not the case, the voltage regulator or generator winding may be faulty;
- start the engine and set the rotation speed to 2000 min⁻¹. With the same connection of the device, the voltage value it shows should be 13.7-14.5 V. If this is not the case, the voltage regulator is faulty or the generator brushes are worn out. At high voltage values (up to 20 V) one can suspect burnout of diodes;
- connect the ammeter to the "B+" terminal and "ground". Start the engine and increase the rotation speed to 1200 min⁻¹, turn on several energy consumers. The ammeter should show at least 30 A. If it does not, the diodes may be faulty. Further checks are carried out on the removed generator.
If the tests show that the generator is faulty, you can do the following to avoid large expenses:
- take the faulty generator to a service station specializing in electrical equipment repair for inspection and repair;
- if you have experience and the necessary skills, you can try to repair the generator yourself. Replacing the regulator and brushes is not very difficult. To disassemble the removed generator, unscrew the bolts from the housing. The diode boards, rotor or winding can be replaced separately. A detailed description of the work is given in subsection 13.3.4.
(Article borrowed from the web portal: MercedesMAN)