Hey sorry if this is covered I have no internet right now, stealing short-term from a grocery store. I have a 1999 ram with built auto, new wiring all around and batteries, fresh injection pump/lift pump/turbo as of 2 months ago. Has been running fantastic, changed trans fluid 6 weeks ago to drive up here for work, today after 10 minutes driving it suddenly stopped shifting below 3rd. Starts from a light in 3rd, shifts fine to 4th and locks converter as per normal in both 3rd and 4th with OD engaged or disengaged, reverse works fine and shifts are normal and smooth. Fluid is good, no slip or shudder, no bubbles, doesn't heat up except on 3rd gear starts of course, and disengages lockup smoothly on decel. Only change is a new TH420 model TPS installed 2 weeks ago, put 50k on it since and only diff after that swap was needing more rpm or a throttle cut to grab second, then suddenly this. It is at the dealer now, but the labor rate here is huge and if I can figure it out somehow I'd rather do that. Trans was built by A&C in Edmonton, AB 3-4 years and 100k ago, serviced perfectly till my last fluid job where they neglected to adjust the bands for unknown reasons.
I may be able to update posts on here from phone if more details are needed but it's hit and miss.
Thanks for time everyone!

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Had something come up, took to dealer, shift solenoid and transducer... both on back order, so couldn't get my job done anyway... but I'll leave this up for a bit in case someone has similar issue in near future. Apparently there's a DTT kit for one of these? Looking into that now so I don't have to wait 3 weeks for fix. If anyone has a better recommend feel free to post, sorry if this was an easy topic!