Have a 99 F150 2wd 4R70W. Was slipping in first then finally quit. I rebuilt it, utilizing master overhaul kit. The clutches were worn all the way down, especially the forward drum so I figured fault was just high mileage normal wear. Reinstalled the trans, reverse is great but still no forward. If I raise rear wheels, therefore no load on it, then wheels do move forward and will actually get to a point where it shifts to 3rd.
I did do something stupid, as I didn't disassemble the fwd and direct drums and therefore didn't replace those piston seals. (Please flog me after answering my questions though!) Otherwise I replaced everything else. There was no evidence of physical damage ie snap ring, ring groove, etc. Obviously the piston seal is a mystery. I am confident I put it back together correctly.
I have learned through these forums that the problem is not electrical.
I am guessing that the fault is most likely the forward drum piston seal leaking by causing minimal fluid pressure to the clutch pack (just enough to engage with no load). I don't think the direct clutch would be a culprit since it works in revers.
Asking the experts if this sounds like the problem and can it be verified via a pressure tester connected to the port on the case?
Sorry this is long but didn't want to say "it's broke, how come?".
Thanks!
Chris

 

Well it looks like it was the forward drum piston seal. Got it out and apart yesterday and the seal had two spots where it was missing a small piece. Which fits the symptoms. Lesson learned the hard way but I'm glad I found definitive proof of a faulty part. Will prob get it back in the truck next week and will update with the results.