SEARCH for some help
This appears to be a very commen problem. I have a similar problem on my 1988 190 E and a SEARCH brought some helpful information. But I have not acted upon it since I don't have a garage and it's too cold to work outside.
You'll need to get the instrument cluster out and if your speedo is a cable driven unit (not electic), you'll need to disconnect that cable. Getting to the instrument cluster and popping it out seems to be the hard part. Some suggest starting at the bottom, under the dash, and popping the cluster out from behind. On some models, it appears that the side vents flanking the instrument panel can be removed and that will give access for nimble fingers to pop the instrument cluster out. I have the CD for my 190 E and it clearly shows kneedle nose pliers (with rubber pads) about to clamp down on one of the plastic vanes on the vent, with the intent to yank it out (I always hate to do stuff like that). Another method of removal is to get the Instrument Panel Removal Kit (I have no idea what the cost is) which appears to be a half dozen thin metal reeds (they look like feeler guages) with hooks on them. They have to be inserted strategically to pull the instrument panel. It looks to be one of those Catch 22 repairs - if you could take the instrument panel out to look for the proper positioning of the metal feeler gauges with hooks, then it would be no problem to place them correctly and pull the instrument cluster out. My Indie Wrench says he just grabs ahold and yanks it out.
The main problem seems to be a plastic gear on a thin metal shaft and that gear cracks and gets loose and starts to slip or fall off. Some recommend putting the gear back in place and adding a dab of contact cement to hold it in place. In a severe condition, sending it to VDO is the only alternative.
All of this may sound odd - I know about the repair and, yet, I have never performed it. I'll tackle the task soon when the weather warms up. Then I'll REALLY have something to say. Good luck with whatever you do and don't forget to tell us what you did and how you did it.