Base on my ATSG Manual on 4L80Es, it states condition: First and Second Gear only, inspect component: Solenoid B - Stuck in "off" position, loose connector, no voltage to solenoid, solenoid o-ring failure, no PCM signal to solenoid. Inspect component: 2-3 Shift Valve - Stuck. Inspect component: Quad Driver Module - Failed.
Possible codes: 68 Overdrive Ratio, 81 QDM & Solenoid B Fault, 85 Undefined Ratio, 87 Solenoid B Stuck Off.
What does all that mean, it means to check solenoid B since it's that most likely problem. Get a multimeter and put it on ohms so you can check resistance of solenoid B (2-3 shift solenoid). Unplugged the big plug on the side of the transmission and you will probe the to prongs of B and E to check the resistance. If the resistance says OL on the multimeter then the solenoid B is either not plugged in, solenoid B is bad, or the wiring harness in the transmission could be wrong.
Here's 2 pics of the wiring diagrams to help you check.
https://i.pinimg.com/564x/db/68/41/d...b452b51e19.jpg
Even though this says diesel the transmission still is wired and works the same.
https://www.dieselplace.com/attachme...am-png.622578/