12.3 v = not charging. That is normal for the 65A alternator at idle. Upgrade to the 120A alternator and it will be 13.1V at idle with blower on max.

My question on where you were measuring was in reference to "I have a new 14.1V regulator, BTW, and dont see 14.1V... "

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked