1988 Chevy pickup 400 tranny after about a half mile truck wont shift gears changed tran mywith used one same thig happens could ther be something else other than tranny?

Answer 1

The turbo 400 uses an electric downshift solenoid to drop it into a lower gear for passing when you floor the gas pedal. Since you changed the transmission and have the exact same problem perhaps the switch that actuates passing gear is stuck and feeding voltage to the downshift solenoid even when you aren't punching the gas pedal. The switch is either mounted on the throttle linkage or under the dashboard by the gas pedal.

If you want to see if this is the problem just unhook the wire to the downshift solenoid and drive it to see if the problem is gone. The wire is right behind where the shift linkage bolts to the side of the transmission. You won't have automatic downshift for passing when you punch the gas pedal as long as the wire is unhooked, but if it shifts OK with it unhooked then you'll have isolated the problem and will know what needs to be done to fix it.