[Chadahar]

The drive gear strips teeth when the mileage is changed when vehicle is moving. New gears are available. Google Mercedes Benz Speedometer Gears. The place is in Virginia and will next day air new gears for about S65.00 USD.
Easy fix if you follow the directions given with the gears.

 

[Chadahar]

The Speedometer/odometer is electric, but the odometer drive gear(little one) strips the gears off of one of the two big gears when zeroed while moving after 20 some years of age makes the once pliable gears brittle, even if one tooth is gone the little gear will not turn the odmeter and sometimes one tooth is very hard to see. When you move the odemter gear by hand you can see one gear NOT moving. If neither speedometer or odometer are registering then it's probably the sender.

 

[Chadahar]

Yep! if the pin count is different, then you do not have the correct cluster. That may be your problem.

I believe that if the speedo IS working and odometer isn't then it is almost certainly the gears. If it were the sensor then neither would work.

I just put new gears in my 1984 Euro 500SEC speedometer cluster and that was what was wrong. the Black (I think) gear had one or two teeth sheared off and gone, but of course the small gear was setting in the gap and could not make a connection to the big gear. Really easy fix with the new gears and instructions provided. In fact I bought the whole set, but only needed one gear, as the others were in great shape.