I have a 4R100 Transmission that I bought for an engine swap project that was supposedly rebuilt 30,000 miles ago. When I test drove my truck, I noticed that the transmission wasn't wanting to shift up from second (even at 55-60MPH) and then when it finally did, I got the feeling that the transmission was slipping and wasn't putting all the engine's power to the ground. At 71 MPH the truck gave me Diagnostic Code P0783 (3-4 Shift Malfunction).

My questions are:

What could be causing this problem and would it be easier to repair my current transmission or look for another?

Thanks!

 

I would start with the solenoid pack. How is the fluid?