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Automatic transmission with Jake's going down the hill

Discussion in 'Experienced Truckers' Advice' started by mitmaks, Dec 19, 2025 at 2:17 AM.

  1. Dec 19, 2025 at 2:17 AM #1
    mitmaks

    mitmaks Road Train Member

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    I was put in a new truck kw t680 with 12spd auto trans and when I was going through PA hills with some 7-8% grades the jakes weren't able to hold it at certain speed (20mph) I had to constantly get in the brakes to keep it from running away from me. I had it in 6th gear and it would just accelerate over 1500 rpm and reaching over 2000rpm if I didn't step on brakes to slow it down. Is there any adjustment for the jakes to hold it better down the grade?
    .in my T660 10spd I can hold 80k down the steepest grades no problem. What am I doing wrong?
     
    mitmaks, Dec 19, 2025 at 2:17 AM
    mitmaks, Dec 19, 2025 at 2:17 AM
    #1
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  3. Dec 19, 2025 at 2:23 AM #2
    Moosetek13

    Moosetek13 Road Train Member

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    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.
     
    Moosetek13, Dec 19, 2025 at 2:23 AM
    Moosetek13, Dec 19, 2025 at 2:23 AM
    #2
  4. Dec 19, 2025 at 2:25 AM #3
    mitmaks

    mitmaks Road Train Member

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    It's x15 12speed Auto and I did use manual mode to keep it in 6th gear going down the steep mountain
     
    mitmaks, Dec 19, 2025 at 2:25 AM
    mitmaks, Dec 19, 2025 at 2:25 AM
    #3
  5. Dec 19, 2025 at 3:44 AM #4
    Moosetek13

    Moosetek13 Road Train Member

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    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.
     
    Moosetek13, Dec 19, 2025 at 3:44 AM
    Moosetek13, Dec 19, 2025 at 3:44 AM
    #4
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