The speedometer in my '86 240 was erratic ever since I bought it four years ago. It would work fine most of the time, but sometimes the needle would just drop to zero. Other times itwould jump somewhere between zero and the current speed. It was worse in cold weather. I removed a shim from below the speed sensor, and that seemed to help for a while. Recently I replaced the wire from the sensor to the dash; then the speedo died completely. I got a used unit on ebay, but it doesn't work either. I connected a multimeter to the wires right where they plug into the speedometer. I set the meter to read ohms, and the needle jerked as I drove. Is this the best way to test this?