94 K2500 4l80e Overdrive Problems

[notyepcars]

I have a 94 K2500 4x4 6.5 diesel with the 4l80e tranny. Truck has 300,000 miles on it and this is the original transmission. Recently tranny won't shift into overdrive, especially if I have a load behind the truck. I have changed the fluid and filter about 30,000 miles ago. Has not thrown any codes on the dash. Will a good computer scanner give any tranny info on these earlier trucks? Any ideas? I an looking a buying a duramax, and don'y won't to drop the tranny if it might be something that could be checked or fixed with it in the truck. Fluid is full and not burnt. This truck has pulled 100's of traliers, first three speeds and reverse are perfect, just the overdrive has a mind of its own, especially with a load on the truck. Thanks guys.