1998 Chevy K3500 Plow Truck

Chevy K3500 Silverado, 350 Engine, Automatic, 8ft Western Plow. Speedo quit working, I changed both input and outspeed sensors and the throttle position sensor and it still don’t work. The truck has a fresh oil change and new plugs installed. I installed the plow 2 years ago and plowed with it for one year before installing the salter, it did not plow at all last year. The truck does have rust on it but will continue to make a good plow truck. The salter and yellow warning light is not included.