Hi I have a 93 3500 with a 6.5L I just bought the truck and the trans looks kinda new and has a sticker that says that its a rebuilt trans. The trans works in all gears but overdrive, I get the truck going over 50mph and I can move the shifter from regular drive to overdrive with nothing happening. I checked the fuses and all look good I also looked under by the trans and all plugs are on. If anyone has any idea what it could be please let me know Thanks

The throttle position sensor is a very important input for the trans. Make sure it is adjusted propery. How does the fluid look/smell? Does it shift hard in other gears? See if a code has set.

I changed the oil and filter as soon as I got it The old oil was still kinda red and did not smell burned or anything