Everytime I have had the dreaded red light it turns out a diode in alternator has gone south. How old is the battery? If the battery is weak it can put more stain on the alternator causing a blown diode. The vibration from a diesel can also cause a coil winding to fatique and come loose. One of the McParts stores can test it for you.

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)