R129 ~ secondhand speedo sourced ~ mileage adjustment?

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New Member
Apr 20, 2013
R129 500SL
The speedo in my R129 500SL is broken. The speedo needle has a mind of it's own from pull-away and never registers a road speed. A Merc specialist and a very competent auto electrician have checked everything and confirmed the NSF ABS sensor as good therefore, it's the clock at fault. The Specialist, along with others I've spoken to, also think that possibly when a working speedo is put into my car, I will regain power roof operation, which is currently not working, as the speedo has to send a signal to the roof control module to confirm that car is stationary? The same Merc Specialist checked every other aspect of my roof and said that there is no other reason why it won't work (apart from control module), as hydraulics, electrics and switches are all fine.

So, following all these findings, I have purchased a second-hand binnacle with a known good speedo and will take this out and put it into my binnacle (replacement cluster doesn't have SRS lamp, etc). Hopefully, this will provide me with a working speedo! The question is, does anybody know of a straight-forward way for me to adjust the mileage on the replacement speedo, to match that on the faulty speedo?
I have a feeling that it's controlled by the car being in Park, rather than anything to do with the speedo?

I know that once you've started the process you can shift out of Park and move on.
Not on mine it doesnt. I can creep forward maybe but anything more than 5 mph and it will stop and an alarm will sound.
Success! I'm very happy to report that I now have a fully operational power roof and speedo! I hope this may help others.

Having been told it was the speedo head at fault, I purchased a (known-good) second-hand instrument cluster, removed the speedo from it and fitted it to my own cluster - nothing. Speedo reading still haywire and no electric soft-top action. So then I tried the entire second-hand cluster, including it's donor speedo in my car's dashboard - success! The donor speedo works perfectly and full roof operations have been restored. Lastly, I fitted my own speedo to the donor cluster and .... Hey presto! The speedo works perfectly, as does all the roof functions.

It would seem that somewhere on the cluster board must be a component that takes a signal from the ABS/speed sensor and passes this to the speedo head and roof control module. In the case of my cluster, this was not happening, hence inop speedo and roof.

I'm over the moon and have managed this fix for £60!

I hope this helps others who may be pulling their hair out - like I was!

Thanks for all the guidance though guys.
Not on mine it doesnt. I can creep forward maybe but anything more than 5 mph and it will stop and an alarm will sound.

Must be something they brought in for the facelift then. I wouldn't want to go too quick, mind, but it means you don't get caught out at the traffic lights. :thumb:

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