w123 300D transmission speedometer output not working
 

Recently with cruise set on the highway at 77 the speedometer fell to zero I thought the speedometer cable likely broke so I bought a new one and that didn't work. Next step was to test the speedometer itself by manually spinning the speedometer cable, and it works just fine, my hand can only get it to go up to 20MPH, but it worked. My next step was to look into the spot where the cable goes on transmission to see if anything had broken off, but I cannot see any evidence of that in there, or on the old cable. Both seem to be intact. Last attempt was to get the car in the air, put it in drive and see if the speedometer cable spins when connected to the transmission. This is where I have concluded that it does not receive output from the transmission, i.e. it doesn't spin the speedo cable. Are there any suggestions on what to do next? Or how to get to the gears that makes this system work

Car details: 1982 300D Turbo 144,500 miles automatic

Unfortunately if the speedometer gearing doesn't work in your 722.315 (? <= check) there is a possibility that this could be a sign of worse things to come. The rear end areas of these transmissions seem to be an area where crud assembles causing trouble with the governor...

...internal troubles can be fixed but you really need to know what you're doing to fix it. Copies of the ATSG manual used to be floating about on the interweb

Yes it is 722.315. I have had no other issues with the transmission so its sad to hear its days may be numbered. Shifts firmly in all gears and even the kick down still works. Thanks for replying, I'll have to look into that manual to see if my mechanical ability can prove itself worthy