I have 2017 cascadia with dt12. The issue I’m having is when downshifting the truck gets stuck on 8th and 7th gear and then won’t move until I reset the truck. But if I do it manually it downshift fine. Also there is revving when shifting. Any idea what’s the issue ? I’ll post the picture of the fault code down below
For the issue of being stuck on 8th and 7th gear, I wonder if that could be related to the range cylinder housing leaking or corrosion around the port for air coming from transmission in the range cylinder housing. In the past I was tasked with removing range cylinder housing due to shifting issue and found corrosion had pushed the o-ring of the port for air partially closed. Due to skip-shifting a range up or down shifting issue might happening with a 5-7 shift or 6-8 shift. One of the first steps is to inspect the air lines and fittings from the shift controller to the range cylinder housing. If are leaks present. Repair. Another step is to connect Diagnostic Link and road test and try to duplicate the shifting issue. Next, review log file and chart the range group actuator position, air supply pressure, actual gear and desired gear. Look for hesitations, incomplete shifts or drop in air pressure. View the transmissions shift abort counter. If shift aborts are present, than inspect the Range piston housing for corrosion. If found, reseal the range cylinder o-rings and piston. Reset abort counter. Then run the countershaft brake test. For the engine revving shifts, could that also be called "Engine RPM Flare during upshifts" check out this old thread were I attached diagnostic steps Dt12 transmission air problems For your spn 520970 FMI 7 with description "Clutch-Unable to open fully" the diagnostics is pretty straight forward. Connect Diagnostic link., Start engine and idle for two minutes. If transmission air supply less than 100 psi check for leaks and repair as necessary.. Otherwise run the clutch apply leak test (test my have to be performed 3 times in row in order to get a "pass".) If it passes, then check if the clutch limit stop maximum value greater than 49mm (1.93 in) If it is greater than remove transmission and replace clutch. Now, if the clutch apply leak test does not pass or it failed and you replaced the clutch, then you are to perform a road test. If you have issues after that you are to replace the AMT shift controller. Or if no shifting issues, release the vehicle. It seems verification of proper air supply and not having transmission air leaks is common to all three issues and logical for an air shifted transmission. I hope this information helps give you some idea of the issue and tests necessary to narrow it down. A reputable repair shop with experience with the DT12 transmission might be your best option. I would really appreciate hearing back how you solved your issues.
