Odometer stopped adding miles driven.
 

1980 911 SC Targa odometer recently stopped working. The speedometer still displays accurate speed. What could be the problem and is it fixable without replacing the entire unit?

You need to open it up and glue the gear back to the shaft. Its a common problem. Probably a technical article about it somewhere

It is most likely fixable. I had the same symptoms in my ‘84 3.2L and it turned out to be a problematic plastic gear in the odometer mechanism. There is one particular gear in there that basically just turns to mush, likely from a long term reaction with whatever lube was used by the factory. You can buy the gear for about $30 and replace it yourself, but it’s a bit of a delicate operation (it can be done if you’re very patient and careful). The main risks are mangling the very thin metal case or breaking the shaft that holds the speedo needle. For a few hundred bucks plus shipping, you can have a shop like Palo Alto Speedometer do the repair.

The small drive gear(s) broke internally. Some warned that resetting the Trip odometer while in motion could cause the brittle old fragile gear to break.
Speedo disassembly is tedious but plenty info on the U Tube if you’re handy.

Yep.. 95% of the time, it's the 20x30 plastic drive gear that needs to be replaced. Pelican sells them. There are several threads and videos describing the process. Let me know if you have any questions. I'm happy to help. I've done at least a hundred of these.

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Here is a video that helped me tackle this project. I think it only took me about 2 hours.. most of that time was spent carefully uncrimping and re-crimping the bezel: https://www.youtube.com/watch?v=pvm23fMcSEA

BTDT, this is the video you want to watch. Nico is a good guide here, and his M96 bearing can roller is trick.