Voltage will move up or down with RPM and load, that much is perfectly normal. Bosch style regulators are usually limited to 14 or 14.2 volts, and should go no higher, but they go lower depending on conditions. Some regulators are temperature compensated, and will drop voltage by a half volt or more based on running temperature. As long as you don't go below 12.5 at high load idle, it's ok.

But there's still something wrong with the D+ circuit. One thing to try is to clean the VR contacts. If you remove the VR from the alternator, you will see a spring contact on the alternator's rectifier bridge. This contacts the + feed on the regulator itself. Both the bridge contact and the rectifier contact should be clean and shiny. One of the screw mounts on the regulator serves as the regulator ground, it will have a molded in washer as a contact. This should be cleaned, as should the area on the alternator where it screws in. Finally, look inside the alternator at the slip rings. They should be clean and shiny. Once you're sure all the contact surfaces are clean, reassemble and see if that's better.

Up to my knowledge, a good alternator will all the time send the same voltage to the battery no matter the load from lights or any other electric or mechanical load to the engine or the car. Mercedes cars alternator are not high amperage like gasoline cars. It only generate enough to charge the battery because Diesel engines odes not use voltage from alternator for electric coil like gasoline engines does. Like I said, my alternator problem was resolved when I changed the rectifier with the brushes on it and the red light went away.

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Nyck - TX - USA

Not the case. As I said, voltage will vary depending on load, RPM, state of battery charge, and internal alternator temperature.

Diesels have low amp requirments while running, but glowing and starting place a huge load on the battery, and can draw it down very quickly. It can take some time at full charge to recharge it.