Please Help - 4l80e transmission not shifting to 3rd or 4th

[Rookiebuilder]

Please help, I’m lost and not sure what to try next. I put a 5.7 350TBI and 4l80e transmission (from a 1993 GMC Vandura – G3500) into a ’67 C10 truck. Both the engine and tranny were rebuilt with ALL new parts. There’s not an old part on either. I have an Affordable Fuel Injection harness built and I’m using the original VSS buffer as a stand-alone harness. I am not getting 3 or OD – just Reverse, 1st and 2nd gear. I can stop and go and it shift really smooth into 2nd gear, but no 3rd or OD. I have gotten 3 & 4th one time in the last three weeks. Any suggestions would be greatly appreciated – I took 15 months to build this ride and still can’t drive it out of my neighborhood.