I've had this problem with my ABS before too. First, if key is turned on and before the car even starts moving, if the ABS comes on and doesn't go off, check the OVP. IF the car moves and the light comes on check the wiring from the wheel hubs (usually the front) to the plugs in the engine bay near the wheel well. Check for continuity, I forget the values, but unplug the connectors and check resistance on the actual sensor by placing one lead in the center female opening, and the other lead on the part just outside the inner opening, but still within the connector. I think its something like 1.1, at least this is the case of my 201.

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1990 190E 3.0L