Not going into OD once warmed up

[Tannerm]

Noticed this the last two times driving. Going to work in the dead heat of the afternoon. It'll do its job until she's good n hot. Then when I take off and cruise up to where 4th should engage, it doesnt. I wanna say it's a 4L80e trans. No slipping or hard shifting. Doesn't lead me to believe a rebuild is in order. Just won't engage OD. I assume it's electronic. Fluid is at correct level. Engine temp never goes past the half way hash mark.

Driving her home yesterday morning I don't recall it doing it. So it's obviously temp related. Anyone experience this? If so what's the common fix?

96 K2500 7.4L auto

[JC316]

The radiator used in the K2500 in absolute shit, probably not cooling the fluid enough. Might try an aftermarket transmission cooler.