We have an 2011 Cummins 6.7, trucks been a nightmare. At 120000 new motor not to many miles later a tranny. The tranny was a pain it has been in and out of the truck 6+ times n a year they kept messing it up. We ended up putting a new oem tranny cooler and an aftermarket external fan to keep it cool. Shop was told we wanted a sun coast well we got shafted with a jasper. It was rebuilt 2x in a month, now issue is back need advise if we have a tranny issue or maybe just a torque converter issue. Under a load only (this load was 6000 lb , trailer is 8000lb, was a tall load total height 13ft, strong head wind) tranny was running at 120 n all of a sudden it don't want to shift to 6th and temp shot to 210, he let it cool then it stayed around 180 (still warmer than usual) it would shift 1/2 time to 6th then decided to only shift to 6th going down hill. I drove up n swapped trucks n tranny runs cool n shifts fine with out a load. Any advise I'm at a loss anymore about what direction to go with this truck.

 

mine is the same thing. but sometimes i can turn off the truck an turn it back on and the tranny will shift fine for awhile. i was thinking of putting a newer low mileage tranny in put idk if that will even solve my issue. let me know what route you went an if it works

 

heres a thought seek new mechanic