I posted this in the Diesel forum, didn't get a response, so I thought maybe this forum might be a little more helpful.

Scenario; Traveling on interstate highway empty, without trailer, at or below posted speed limit with cruise control w/o tow-haul engaged, and crest small hill and begin long descent. Truck immediately begins to downshift and slow truck. Within ¼ mile truck has decelerated to 60 mph from near 80, and engine is taching somewhere near 4500 rpm. I was able to regain control by shutting off cruise and manually controlling speed. This happened once before, on a different grade, a similar scenario played out. Except, I had one heck of a time resuming speed. I was unsure, at that time what I may have done to contribute to the issue since on long descents like this, with curves, I will sometimes use manual mode to downshift into a lower gear to control speed. Yesterday’s incident, truck was in drive, with cruise, and starting descent of 5+% grade. I usually don’t do anything on this hill for the first several miles because road is straight w/80 speed limit.
Any suggestions or thoughts would be appreciated. My thought (guess) may be a program glitch in the tranny control module?

 

Take it somewhere and see if there is any pending codes present in the TCM? Do the 6.7s have an engine brake? Or is this some kind of descent option. Might pull the book out and read up, it could be normal or may be able to be shut off?

 

Thanks for replying.

I was hoping not to have to bring it in to read codes, that always costs $$$. No engine brake on the 2012 6.7, and I believe any hill descent feature should at least keep you at the cruise setting, just not let you exceed it. It has only done this twice, and I have driven these roads in excess of a 300 times.