ThirtyFive 21

So I actually have two trucks, one is a 1043A2 and the other is an 1123 with the same problem. Right now we're only working on the 1043 so ill tell you whats up with that one. We test drive the 1043 and its hard to tell if it shifts from 1st to 2nd or if its locked in 2nd. Either way it wont go past 30mph, ive tried shifting it from manual 1, 2, then D but its the same thing... no shift. The fluid level is good and the fluid looks clean as well. After dropping the pan the fluid down there looked dirtier and there was a good amount of metal on the magnet. so we swapped the filter and we're raising the pan and re-filling the fluid today and hopefully I get some shift now. Ill update as soon as we find out.

Did you try the tranny reset behind the passenger seat?
Try putting in 4L to easier see if you are shifting.
Do you have a Trannex 2000? (because you can shift with this hooked up, through all the gears to see if there is a problem)

Have you guys pulled the codes. If you do not know how let me know and i will guide you through that. I am also going to post a TM for the tranny.

Honestly, most of our trannys here that won't shift, we adjust the TPS sensor and it usually fixes it.