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The E30 BMW uses a magnetic pick-up on the diff. to send a speed signal to an electronic speedo head. Problems with circuit boards are common as well as the memory battery leaking and causing a slow board death. There are tests designed to find this problem that I use to trouble shoot circuits. Most probably is in the speedo head unit, but don't throw parts at it. Have a BMW tech check it out for you. I am an ASE Master tech and have 30 years of BMW wrenching.
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