I use this truck for grain hauling and farm use. When I bought it, the speedo didn't work. I replaced the sensor on the tail shaft and it still didn't work, then one day driving down the road it came on and worked flawlessly for two years. We didn't use the truck much and it sat for about four months...speedo quit again. 1991 379 with cummins n14celect 460e with 15 spd. Any help? Thanks. Tach is also recently doa. It tries to move but won't go above 300 rpm. Thanks for any thoughts.