1999 cummins with auto tourqe converter issue

[Kael]

so normally I hop in my truck and let the fuel system prime (I have a fass 160 and stock injectors) fire it up and let it idle for about 10 min. Then I go to drive it and it shifts into every gear great. Then when I get into the last gear and anywhere above 50 mph I will let off the throttle and the tourqe converter won’t unlock? It will just sit at around 1900 rpm. But on my way home from work it will unlock just fine and drop down to about 1300 rpm?

 

[Joe Mc]

Assuming truck has 100k+ miles...
Usually OD lock/unlock issues are related to electronic noise. The wires for the OD run in the same harness as the alternator. Some folks isolate the OD wire in the harness by wrapping with aluminum foil, others acquire and install a noise filter that can be sourced from most of the big diesel vendors IE: ATS, DTT, Gorend (spelling) search on TDR, Geno's Garage may even have one.
Interesting that it works in the morning but not on the drive home? Could be solenoid/transducer? And, most will tell you, check and clean ground connections before you start throwing parts. Hopefully someone with more knowledge than I will chime in. You could call Banks or ATT tech support for help.