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.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]