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Can't really find anything searching and the speedometer works...
Nor would you anything like that me thinks !
Odo is purely mechanical - tacho is electronic.
I think failing same day is a coincidence.

However I ask the question have you been in behind the instrument cluster lately ?
I ask as I had mine removed and developed two problems on re-installation. First the tacho is missing a ground when the dash is fully pushed in. Easily solved by adding a ground wire as discussed in another post

However what is relevant is the 2nd problem.
I can no long ZERO my odometer !
I suspect it is the way I have screwed the speedo and accessory gauge back in to the cluster housing. I have got something misaligned.
Speedo works and odo turns over BUT the control knob doesn't.
Whilst that's not what your situation is I would suspect something of a similar nature.

Now incidentally I have just read an excellent post somewhere else about the tacho intermittent problem. I will relay that info here BUT I will also post it up as a solution as it seems a common problem.
The tacho/clock and accessory dials all get their ground from the speedo dial. When removed from the cluster housing the speedo has two small metal strips I believe either side of the dial at the bottom which in turn have screw holes in them. Here I believe is where the grounding issue begins.
I'll quote from the article now :
"....Probably the most fragile part of the tach itself is the ground connection. If you take your instrument cluster out and then take it apart, you will find a strip of metal running along the base of the middle third of the cluster (the speedometer part). This metal strip connects the right and left thirds and provides ground for the tach and the clock. [See the attached photo.] If this metal strip has gotten dirty or corroded over the years or if one of the screws at either end is missing or loose, you will have a flaky or non-working clock and tach. In many old cars the clock is already dead and the driver uses the clock in his aftermarket radio. In that case, the only indication is that the tach does not work..."

So have you had your cluster out at all ?
Maybe something is not right back there after replacement.
I know I will be checking this possible solution out very very soon !

Cheers
 

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Discussion Starter #3
Never had the cluster out. The previous owner only informed me that one day they stopped working at the same time.

My w123 had an intermittent tach, but this is plain dead.
 

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They are unrelated. As was said before, one is mechanical and the other is electric.

Usually there's a slipping gear in the odo that keeps it from working.

I'd get a replacement speedo/odo lined up and pull the cluster to check out both of them.
 
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