I have 1998 chevy 3500 HD one ton truck 6.5 L turb-desiel 4L80E transmission, Drive the truck shifts fine for about fifteen miles, stop at a stop sign truck will not shift from second to third, I can reset this by taking ground from batteries, there are no computer codes and check engine light never come's onneed to know what the problem is and or what to trouble shoot, Could this be something to do with power from the key switch? Thank you

 

when that happened to me it was the computer BUT this can also be caused by sticking trans valves, do you leave it running when you reset or disconect the bat???

 

No I have always turned the truck off

 

ok check all the plugs at trans lot time build up with **** clean them see what happens and check all grounds and clean