When you had it apart did you take a good look at the teeth on the regulator? They can and do break. I just replaced the regulator on my passenger side of my 300CD due to 1/2 the teeth on it being broken off.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.