84SD wife was driving around town at speeds 0-40. She said sometimes the speedo read correctly. Other times it would jerk. Where is the speedo pickup? What else besides the sender or wire? Where to look 1st?

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

That will be an old fashioned cable I think. The cable is self destructing. Sometimes they can be saved for a while with some lube by taking the cable off and pouring oil down the sleeve.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I was under the impression that the speedo is electric because the cluster only has electric connections and not the old mechanical cable connection like for instance my 78 Z car. I was assuming that there is a sensor somewhere in the trans or differential.

The speedo has never been a problem until today.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I can't speak with absolute certainly for the 84SD but for my 83 300D the cable is definitely mechanical.

All W126 speedometers are electrical. A pickup is located in the transmission. Signal goes up to a connector by the accelerator then up into the cluster.

Could be a ground issue, could be a circuit issue on the speedometer circuit board.

I would check the cluster grounds first, also check battery voltage/alternator.

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Agreed, either its a cluster grounding issue, or the connector to the pickup on the transmission is damaged/loose/filthy.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)