Quick check for this, on top of the GP relay, there are 2 connectors, one has 5 heavier leads going to the engine, and the other connector has 4 small wires. Unplug the small connector after starting the engine, this will disable the GP relay and see if your voltage comes back.

I would be looking at the alternator still, but first check your battery and alternator connections, the ground connection to the body at the battery, and the main ground between the engine and body as well (this is under the car).

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech