Optimusprime is right, its most likely the gear.

What happens here is that metal gear on the right of the odometer section looses its grip on the odometer shaft.

Pics here are specific to W123, W124 is similar but a lot easier to do.



I would strongly recommend replacing the gear with a new one. They can be had for $25 or so.

Personally I would not resort to gluing it. I have not had a lot of luck with glues over the years.

The glue may fail at some point in time and you'll be here all over again. Why not fix it right while you have it out? The near gear will outlast the car pretty much!

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