I had the same "problem" on my 85 300D. Checked the following which was found to be in good working order:

idle rpm,
alternator belt tension
electrical contacts at battery terminals, chassis, ground strap, alternator, starter motor
voltage regular brush length, tried different voltage regulator
charge lamp in cluster

This car had a battery from the PO. It looks clean and not very old (4 years) and starts the car fine but I would always have to rev the throttle after a cold start to get the charge light to go out.

I wanted to get an AGM battery and found a "take off" from an Interstate distributor for $40. Installed it and the "problem" went away immediately. Upon starting the engine, the charge light now extinguishes immediately w/o revving the throttle.

I was surprised to say the least.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked