I came by this site while looking up some diagnostic information regarding 4L80E in my pals '93 Chevy Winnebago 6.2 liter diesel (LS3 engine I believe)
The thing is stuck in limp mode, most likely for some electrical issue. I do have an ALDL diagnostic cable and bought license for the GMTD software which I use with it. However my previous experience with ALDL are from TBI engines.
The GMTD drops random numbers on the Input Speed reading (-2000s as well as +400s and +3500s) while the engine is idling, which would most likely be the issue of the limp mode (the transmission is stuck at 3rd or 4th gear). Before I got hands on 4L80E code book pdf which contains loads of useful information, we tried replacing both the sensors (input & output) as they were both old and not that expensive. When the situation didn't improve, I used my multimeter to take AC readings off of the wires leading to the PCM, which was pretty much okay - about 6 VAC idling, 13.5 VAC at higher idle. The van has no tachometer, but I guess the higher idle would have been somewhere around 1000 rpms. Most importantly, the readings were constant rather than random as they were in the GMTD software.
Now I am wondering, is there anything else I could check before deciding the PCM is faulty and getting a spare one? Is there maybe a different software that could hook up into the diesel PCM? I have TunerProRT, but I have no .bin/.ads/.adx files for the diesel.
Thanks and have a great day!