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ICQ>
John ---
The pickup for your speedometer is an electronic sensor mounted on the differential rear cover, to the right of and above the filler plug. It senses the magnetic pulses of a "vane"-like disc mounted against the ring gear. It has an electrical connector and wiring harness attached.
From your description, it's difficult to troubleshoot precisely -- the problem could be in the dash instrument cluster, speedometer gauge, wiring, connector, sensor, or even the internal "vane" (although that's very unlikely -- but I've seen it once on a 740).
I would start by examining the connectors behind the dash cluster -- be sure they are firmly (and correctly) plugged in -- as far as I know, they only go one way. Then check the connector at the sensor and confirm that the connections are clean, that the wiring harness appears to be intact, and that it's plugged in firmly.
BTW, while you're sniffing around down there, examine the differential cover for signs of pinholes. This will look like a thick, oily, grease build up of black, gooey grime on the cover (don't confuse it with undercoat). The older 240s sometimes experience rusting of that cover, and over time, your differential oil can leak out. I've found two of my cars with that problem -- and removed, cleaned, soldered, painted, and reinstalled the covers. Replacing a differential is very expensive -- and buying a new cover isn't cheap (like around ($50-75). But you can easily repair them.
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