The driver's side front window on my 1980 300SD went down but wouldn't come back up. The switch worked and I could hear the motor running. I removed the door panel and found that the cast iron piece with gears that is bolted to the door is broken. Is this called a regulator? Can I replace it myself? Any tips on doing the job properly?

Appreciate your help,
Ray

Yes! The regulator needs replacing. Remove the door panel and remove the regulator. One important part is that the motor must be detached from the regulator before either part can leave the door (the window channel blocks it and is not easily removed). The removal is done by removing three ten mm head screws through three holes that should exist on your car. I say should because you have a late version 116 chassis. The early cars didn't have the holes and removal was about three hours longer until MB came out with a template and drilling instructions to make the holes that wer later built into the car.

One other point. The 116 eats regulators because (amoung other things) of its weird window guide geometry. The window basically follows a single bar up the door. Before reattaching the new regulator move the window up and down by hand to determine the ease of this activity. Bend, lube, adjust to taste.

NOTE**** Be careful; when broken the regulators spring can cause injury when you release the body. The inside of the door is also full of sharp edges/objects.

I had an identical problem with my '80 300SD. I purchased a used window regulator from a wrecked '74 450SEL. I did the job myself and I do remember it being somewhat of a pain. The motor does need to be detached from the regulator. It is done more easily by inserting your ratchet into the three small holes the other writer indicated.
I was able to successfully replace mine. However, I do remember that the replaced one never seemed to go up as smoothly, or maybe as far as the other. Good luck.