Trans downshift

[PJ]

On my way home from California, I was on a 6% downhill grade for about 6 miles, cruise control set at 65mph, foot off all peddels and in overdrive. All of a sudden, the truck did a down shift and the rpm's went over 3500. It just about put my head through the windshield. It felt like I slammed on the brakes. Ive never noticed this on any other of my automatic Cummins. Anyone know if this is normal?

 

[CAgnoli]

Yup, the cruise control WILL downshift to slow the truck. I'd say don't use the cruise if you're running heavy down a steep grade.