Speedometer question... W123 transmission in a W126
 

So I recently bought a 1984 300SD. The speedometer does not work, and the previous owner told me that he had replaced the transmission with the W123 version which has the mechanical speedometer output, not an electronic speedometer sender. I was thinking about a universal sender to hook up to the mechanical output, but I can't seem to find anything that is advertised to work with a Mercedes. (I see plenty of options for Ford, Chrysler, GM stuff) I've also thought about getting a GPS sender, but I'm not sure that the W126 speedo uses the same type of signal. Does anyone have any ideas for getting my speedometer working without changing the transmission?

I once had a different differential put in a truck and the speed was off on the speedometer due to a changed ratio... So I went to a "speedometer repair shop" (large cities have these) and they made me a "custom" reduction box (connected between cables) and that made the speedometer accurate.

I got the impression they could do anything speedometer related, even a bit of magic! So may want to see if your city has a speedometer shop or maybe in a larger city near you.

I'll look into that for sure. At worst, maybe they could adapt a GM TH350 sender to mating to the speedo output on the transmission. Does anyone know why hooking up a GPS sender wouldn't work. This looks simple and reasonable:

https://m.summitracing.com/parts/izl-s9020?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-intellitronix-corp&gclid=CMOLk9Sg-NICFRBEfgod18IPhA

Will my speedometer be compatible with the signal this thing puts out?

I am confused as to why the 126 inst cluster can't work... Was the differential swapped out too? At least on my 300SE the electronic speed sensor is mounted on top of the differential.

Any particular reason that the original W126 electronic style sender couldn't be installed in the transmission? The W123 is mechanical, the W126 is electronic.

As far as I can recall the W126 sender unit is built into the W126's transmission harness. It bolts into the tail shaft housing on the transmission. There is a special gear in there that is read by the sender. So you would have to swap tail shaft housings reverting back to the W126 unit and then plug in the sender unit that (hopefully) is still on your transmission harness.

Please disregard my previous post where I said the sensor is on the differential. Turns out I was thinking of the ABS sensor not the speed sensor. I went through all of that several years back in this thread here:

http://www.peachparts.com/shopforum/tech-help/285958-my-differential-fell-out-while-driving-w126-2.html

At any rate, that's what I was remembering not the speedo sensor. Sorry for the bad info.