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Automatic transmission shifting down speeds by itself

Discussion in 'International Forum' started by Eddozer, Aug 8, 2021.

  1. Aug 8, 2021 #1
    Eddozer

    Eddozer Bobtail Member

    8
    6
    Dec 17, 2019
    0
    2012 Prostar with a Max13 and an automatic transmission.
    When on the highway going at 10 speed running ok; suddenly the transmission
    starts to shift down by itself pretty quick from 10 to 9 to 8 gear and so on;
    and if I let it, it will go down to 1st gear in the middle of the freeway and stop since
    the engine is compressing down as the tranny shifts down and the truck could stop in the middle of the road,
    Its a scary situation, when it starts doing the downshifts, I have to be quick to push the
    shifter to neutral to stop it from shifting down then I put it back in gear and it runs ok for a few days
    until it does it again. Not driving it any more for now but the dealer
    doesn't seem to find the problem, the whole clutch job was done about 60,000 miles ago and
    it never gave me this problem before.
     
    Eddozer, Aug 8, 2021
    Eddozer, Aug 8, 2021
    #1
    Team818 Thanks this.
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  3. Aug 8, 2021 #2
    Team818

    Team818 Medium Load Member

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    626
    May 5, 2019
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    New tranny time, or rebuilt swap out. Seems like Big Rigs last only about 10 years anymore, then major breakdown, shopped, bills come soaring out the shop for the company, or oo's. Take care.

    PS- already had trans work done at 60,000. Its shopped now. Well at least you don't have to drive that thing. It will work out, it has too. Don't give up. Its in the right place, right now.
     
    Last edited: Aug 8, 2021
    Reason for edit: adjustments made.
    Team818, Aug 8, 2021
    Team818, Aug 8, 2021
    #2
  4. Aug 8, 2021 #3
    Judge

    Judge Road Train Member

    13,511
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    Arkansas
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    Try putting it in Manual mode while driving.
    At least it shouldn’t start downshifting
     
    Judge, Aug 8, 2021
    Judge, Aug 8, 2021
    #3
    Eddozer, Doealex and Trucker61016 Thank this.
  5. Aug 8, 2021 #4
    circle h

    circle h Light Load Member

    240
    375
    Oct 22, 2020
    0
    Sounds to me like it is putting itself in low. Shifter, harness from shifter to tcu or tcu. I would follow the harness from the shifter and see if it's worn or pinched. Perhaps a bare wire causing it to select low.. Good luck.
     
    circle h, Aug 8, 2021
    circle h, Aug 8, 2021
    #4
    Eddozer, Doealex and Team818 Thank this.
  7. Aug 8, 2021 #5
    ChuckMartel

    ChuckMartel Light Load Member

    160
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    Jan 22, 2021
    0
    Any codes come up? Seems like a code is setting and defaulting you to start gear.
     
    ChuckMartel, Aug 8, 2021
    ChuckMartel, Aug 8, 2021
    #5
    Trucker61016 Thanks this.
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