I am working on a Super Lawn Truck. It is a 1989 Isuzu NPR. The truck runs great and for the deal i got on it, i really couldn't go wrong. The truck drives, shifts and runs great but i cannot get the truck to shift into the high gear and it will not down shift to passing gear when put under heavy load. Anyone have any suggestions. The previous owner told me about an "overdrive switch" but they are not sure and i've never worked on these older trucks before. thanks in advance.