What is the Voltage of the battery without the engine running? It should be over 12.5 Volts.
Just to make sure I have this correct, once you run the engine to around 2300 rpm and then let it idle, the idle speed voltage across the battery is 13.8 Volts, right? This is a good idle speed value, so you have an issue reaching some threshold value without revving the engine it seems. The battery may be weak and initially drawing the Voltage down.
If the car is warmed up after a significant drive, say half an hour or more with an average speed that keeps the rpm indicator over 2300 rpm, and you shut it down, then immediately restart, do you get the exact same results? Or is this only after it sits for a while?
Well, good luck. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)