My tach and speedo are not working. My odometer and all other gauges are working fine. When driving, the spedometer raises and stays around 40 mph no mater what speed I'm going. It drops to zero once the car is shut off. The tach bounces all around at times, but usually stays at zero. Any ideas???

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1983 300D, 247K - Just bought, my 1st
2002 Ford F350 7.3 Powerstroke Diesel - Love this truck