Are you getting a flashing O/D light? If so then you need to have the truck hooked to a scanner. I would be suspect of a bad solenoid pack. I just put a new stage 3 trans in my truck and the company I bought it from had gotten a bad solenoid pack that they put in then when I put the trans in I had to have them send me two more until I finally got a good one. They ended up ordering a new one from ford for me. The reman company they were using named Recon has very poor quality control. I wouldn't get one from them. Moral of the story is that ypu either have a bad solenoid pack or an internal seal failure. The pack is easy to change. The seal would require a tear down though unless your valve body is sticking in which case you could swap out valve bodies.
A forum community dedicated to Ford Power Stroke owners and enthusiasts. Come join the discussion about diesel performance, modifications, EGR deletes, troubleshooting, lift kits, tires, wheels, maintenance, and more!