W140 odometer not working
 

I am working on a 92 300SD which has the analog odometer. The speedo works but the odometer does not. Assuming it was probably a broken plastic gear I googled to see if I could find info on taking the cluster apart. I came across a video talking about repairing then and the odometer not working could be electrical or the gears. He also mentioned the cluster can be a cause of a battery drain, which I do believe I have so I am now thinking I could have electrical problems and possibly a gear problem. I have rebuilt a few cruise control amps replacing the caps and resoldering the joints, but I am not sure if I can tackle this as he mentions replacing transistors. If it was just replacing the caps I would be Ok with it but I suppose I could figure out the transistors as well. Anyway, anybody been down this road before or know of a place I can send it off to and get it repaired?

I just went through this with my 95 E320. Isn't your speedometer/odometer purely mechanical? Does it have a speedometer cable running from the transmission to the dash cluster? If so, there should be no transistors or battery drain.

W124 is mechanical, W126 and W140 are solid state. The pinion gear on the stepper motor shaft is a very common failure item on the 126, probably similar on the 140. If it fails, the odometer and trip odometer will not work but the speedo still will. While you have it apart to replace the gear set, replace the electrolytic capacitors since they're guaranteed to be way out of tolerance.