1988 GMC pickup 305 v8, a700R4 transmission wont shift out of 2nd gear

i have a 1988 GMC pickup 1/2 ton rear wheel drive,305 v8,with an automatic/overdrive transmission believed to be a700R4 trans. i have had it overhauled,and i have attempted an overhaul myself,the trans. seems very strong in low, and 2nd, it even seems to work fine when it has set over night, but when warm it will not shift out of 2nd, any ideas?”

Visit a good transmission mechanic when it won’t shift out of second. They can hook up their transmission computer to the vehicle and do a couple simple tests. This could tell you if it’s a mechanical problem, such as a bad automatic clutch, or an electrical problem, like a bad solenoid inside the transmission. You might get lucky and find an electronic problem exists. Your only other option is to pull the tranny off again and pull it apart to see if a mechanical part has failed. Good Luck,