4L80e 4th gear problem???
 

May not have gotten lucky this time with a junkyard 80e /w an HD2... trans drives perfect and pulls strong in all gears but 4th is giving me problems. Before I have a definite answer Im going to check over the shift linkage to make sure its fully engaging.
In "D4" it shifts 1-3 fine,when it shifts 4th it holds for a second then neutrals. Shift it back into "3" drives damn near perfect. Converter locks up and everything. Weird thing is that last night while driving in "D4" it was driving PERFECT converter was locking spot on and everything. Trans temp doesnt go over 120*. Its been staying very cool around 104*-112* with spirited driving actually. I basically have a TH400 at this point lol

if i shift up from "3" to "D4" I can feel it lock up and the rpms drop by around 400-500 then it neutrals until it downshifts to third

I am having the exact same problem with a fresh built 4l80 with hd2 in a buddies truck. Got the trans in last night and went for a test drive and shifts 1-3 perfect but hits 4 and then neutral. I scanned it while driving and commanded 4 and got the same results. His trans is running 180 which is odd because it has a pretty big cooler and we were driving very easy on it. Hopefully someone a lot smarter than me can help both of us out on this. Thanks

I talked to jake ( the transmission builder) last night through email and he believes its the AFL valve. He told me maybe I should replace the stock AFL valve with a sonnax unit

What is an afl valve and can you do that without dropping the trans?