'91 f150 supercab 5.0, E4OD transmission, 3.55LS.

When the truck is cold, the transmission shifts 1 - 3 normally, with the torque converter unlocked. A mile or so later, it shifts down into overdrive, with it unlocked. It will lock, and then for 2 or 3 miles later, it will unlock with the slightest pressing of the gas up a hill or when a large truck goes by (wind). Once it reaches full operating temperature, it runs normally, where it wont unlock unless you almost floor it, when it shifts down. Is this normal?

I never really noticed it (I likely wasnt paying attention), but its been doing this ever since I put my tailgate back on and keep it up. The truck did have 31x10.5x15 tires on it, now has stock 235/75/15's.

 

I don't know the specifics on your particular truck computer file, but there is a cold shift mode thats different from when its at normal operating temp.

Darrin

 

I think I figured it out. The genious's at the tire shop put a 235/70/15 on the back right instead of a 235/75/15. Took it back to the shop after work, got the right size tire put on, and it didnt do it at all on the way home.