Automatic transmission with Jake's going down the hill

What engine/trans combo is it using?
Are you using auto or manual mode? I ask because you said you had it in 6th gear.

 

What is x15?

I only know the Detroit DD15 with the DT12 trans, if that is what you are referring to.

In that case, it is better to keep it in auto and let that combo and computer do the work.
That combo is designed for the engine to get to 2,200 rpm going downhill. Nothing to worry about.
I spoke to one of the people that actually developed it.

You set your speed with the cruise control and let the computer do the work for you.
There is a setting in there for plus or minus speed, and the computer will keep it in that range.
Drop it down a few mph and it will keep your desired speed, downshifting as needed.

Sometimes not with a very heavy load or very steep grade, and then you will have to intervene.
Nothing is perfect, but that combo is pretty good. You just need to learn how it really works.

 

X15 is a Cummins engine.

 

Detroit and Volvo have Descent Mode. On Detroit you turn on stage 1 engine brake and slow truck to speed you want. Then set cruise control. The engine and transmission work together. Basically the transmission will drop 1 or more gear to hold truck at speed you set. The transmission will bring engine up to 2200 rpms and turn stage 3 engine brake automatically. Then switch between stage 1-3 to keep the set speed you set cruise control. They claim it can hold back 80k at 8%. You have watch and make sure you don’t go over 2400 rpms if the engine brake and transmission can hold it back you will have to apply brake.

you can do the same thing manually on Detroit, just turn on stage 3 engine brake and it will bring engine up to 2200 rpms if you keep slowing down at 1800 switch to stage 2 and let truck speed up and as you get to 2100 or 2200 rpms switch to stage 3 again and let truck slow down again.