I did a search and couldnt find specifics.
Yesterday my speedometer started the bouncing bit. From a stand still, starting off, it would immediately start bouncing from 0 to 25+, up and down, up and down. Then once i hit 25, it would settle down and read fine all the way up. When slowing down, same thing...fine until i reached 25 and it would bounce until i came to a complete stop.
1. did the speedometer die, or did the cable go? or something else.
2. why did the cruise stop working as well. Is the speed sensor for the cruise control amp connected to the cable in some way? or the speedo? or the transmission.
3. if the speedometer died, is it ok to put in a different mph one in? i have an 85mph speedometer.

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1980 300D

The cable is about to fail.

The cruise gets it's speed signal at the speedo, there is a two wire connector on the back where it picks up.

A different speed speedo is fine as long as it's from the same model car (77-81 300D).

ok.thanx so much.
well the speedo has failed. it has completely stopped...not working at all.
so that would explain why the cruise quit as well. is it more likely that it is the cable or the speedometer itself?

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1980 300D

The way you describe the initial failure, my bet would be the cable. Easy way to check, remove cable from speedo so you can see the end, chock the car so it won't roll, jack up one rear tire, put trans in neutral and rotate the tire, you should see the cable move (it will turn slowly unless you're really winging the tire along by hand) if no rotation, there you go, it's either the cable or drive gear in the trans. Pull the cable from trans, look inside the socket on the trans while someone rotates the tire, if the socket turns, its the cable. When the cable is unplugged from the trans, you should be able to freely spin it with no feel of resistance, and see the other end turn. I do not know if MB uses a metal or plastic drive gear in the trans, from past experience on other brands, what I have seen is the cable goes dry and gummy, making the bouncy needle & makes it stiffer to turn which eventually causes the cable to wallow out and strip the square drive socket on the plastic trans gear.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale

In the past when my speedo cables broke, I was able to pull cable out of the plastic tube it's encased in. If you pull the instrument cluster, most disconnect the speedo cable, and grab a hold of the square nub sticking out and pull. If it's broken the cable will come out. If it doesn't pull out than is't still attached, and the cable isn't broken.

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