My w123 82 240d had the full mechanical odometer drive. It had the classic slipping on the shaft issue. I recall it was easy to add on the miles the PO told me about (about 25,000). I just separated the number wheels and advanced the digits...kind of like rollback but forward instead.

My w126 300sd has another issue. When I go faster than say 55mph it doesn't click off miles reliably. It advances, but it seems to skip beats. I've verified this on my GPS. I've heard these electromechanical units have rotting gears. But mine seems to work at slow speed. I can hear a click.

One day I'll open it up. For now I get all upset when I calculate mpg and get some awful number like 18 mpg because the odometer didn't click off my 65mph road trip on the highway.

Did the 85 w123 go to an electronic pickup like my SD?

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79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles