Speedometer works, odometer doesn't.
 

'83 240D. The speedometer works fine, needle holds steady at any speed. The odometer doesn't work. When I zero the trip odometer it and the main odometer will work for a few miles, but then they stop working until I zero the trip odometer again.

Any ideas what's wrong and how to fix it?

Most likely a stripped plastic or pot metal gear in the odometer cluster. I had a similar issue and following this https://www.dieselgiant.com/repairyourodometer.htm fixed it.

Odometer Repair
 

It's almost never a stripped gear, the metal driven gear on a mild steel shaft is a light press fit and it often slips .

I took my speedo out, looked at all those tiny parts and had a speedo shop do it, $150, ouch .

It failed again after five years or so and a member here said he'd teach me ~ it's really simple but none of the on line articles I've read were complete, I'll go read DeieslGiant's blurb now, I'm quite sure there are many good articles, google " W123 Mercedes odometer Repair" .

It's critical to sweat the details here .

W123 Odometer Repair
 

Yep ;

That's it .

We didn't remove the drive part, just removed the brass swadge fitting (careful it doesn't fly off into space !) , slid the 7/64" drill bit in butt end first, very gently .

This isn't terribly hard once you have the instructions with pictures printed out, even if you don't get it right cheap old speedometers are every where .

When I said "stripped gear" I did not mean the gear teeth are stripped, but that the press fit has stripped (or worn).

Got it ! .

I was thinking literally .

I’ve fixed dozens of these.
The dg pics are great, but I don’t use the locatite.
I slide out the shaft, Knurl the end with pliers, and press the gear back on.
It is fairly simple. Hardest part is taking the speedo out of the cluster without breaking the plastic. Be cautious putting the screws back in. Turn them reverse direction until you feel the threads click in, BEFORE you tighten the screws. That plastic is brittle!

nelstromlinson Yes i have done a few these ,,and i did use super glue .Rough up the shaft slide nylon gear gear wheel in to its right place , then let it set .Dont use to much glue it will gum up the workings . Diesel Giant as a good repair tutirial on your W123 model

This is it . How to Repair Your Broken Odometer