2021 Western Star 4900 - Transmission Hunting/Hard Shifts

Question

I have a 2021 4900 with the DT12 automated manual. Recently it has started shifting very hard, almost slamming into gear, specifically during the 6-7 and 7-8 upshifts. It also seems to 'hunt' for gears when I'm on a slight incline, shifting back and forth more than it used to.

There are no codes on the dash, and the transmission fluid level is correct. I had the clutch calibration done at the dealer about 3 months ago, and it was fine until now. Is there a software update for the TCM or am I looking at a mechanical issue with the shift actuators?

NightShiftNate · Accepted Answer

✅ Confirmed fix: The problem was the range cylinder O-内部 seals. They were leaking slightly, preventing the shift actuator from completing the range change quickly enough, leading to the 'slamming' feel as the TCM tried to compensate. Replaced the range actuator seals and performed a high-speed transmission learn procedure. Shifting back to smooth as silk.

MackMarty · Answer

Check your CAC (Charge Air Cooler) for leaks. Sometimes if the engine isn't making the expected torque due to a boost leak, the TCM gets confused on the shift points because the engine load data is incorrect. It'll cause hunting and weird shift timing.

2021 Western Star 4900 - Transmission Hunting/Hard Shifts

Suggest a repair