What is wrong when it only go forward but no back up at all?

Common for the reverse band to break in these transmissions. Need to have it scanned for possible codes. There is a very simple test that can be done just by driving the vehicle but every time i tell folks what to do they ALWAYS get it wrong! So just let a MECHANIC test drive it, can tell rather quickly if it is a mechanical or electrical problem, just by driving it and knowing how to test for manual low! Very simple and bulletproff! Good luck with it.