That 12.1 V was at idle and no appreciable load (climate fan off, lights off). I would expect at least 13.xx V.

I had the min/max function engaged so I could check the volt drop on crank but the lead fell off when the engine cranked so I didn’t get a good reading. I will try again later to pull this number.

I don’t have a scope but I did switch it to AC volts and had less than 100 mV so it’s not a blown diode.

What concerns me the most is we did not get a charge warning on the dash. They take the D+ to the dash but instead of the traditional bulb they have a resistor and a voltage input to the control logic.

I already have another alternator on order and it should be here tomorrow. I’m changing it on principle anyway (along with a new belt!) due to the age of the car and the fact that it will be at college with my son this fall.

__________________
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