The speedometer in my '94 940 only works when I start driving after it sits overnight then quits after about the first 1/4 mile of driving. Do you think it is the sensor on the rear-end? The ABS light does not come on. I checked the wiring that come out of the sensor and the wires are not broken. The problem started after I had rear brakes and new rotors and calipers put on it, the installer hit the old rotors with a hammer straight on the end of the axles with a 3-5 lb. hammer to loosen the rotors. Would that damage the sensor on the rear-end by any chance? Or it might be a coincidence. Is there a chance of corrosion at the connections of the harness? What is the easiest way to troubleshoot the sensor?