10 speed not downshifting

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by elliott, Aug 25, 2020.

  1. elliott

    elliott Light Load Member

    102
    30
    Sep 3, 2007
    0
    I've spent too many years living in a truck, so now I'm taking classes for a diesel tech degree. I'm going to fix broken trucks and clock out at five and go home to a meal not made at a truckstop. Sounds glorious.

    Anyways, at the school we have a daycab International for show-and-tell purposes; it has a standard 10 speed. The instructor says it won't downshift. It'll upshift just fine but will not downshift. I have not personally verified this; I'm just taking his word on it.

    In the 17 years that I've had my cdl and been driving trucks, I've never had the problem---I've never even heard of anyone having the problem---of a transmission that can't be downshifted, assuming the driver knows what they're doing. He asked for my thoughts on the matter since I have a lot of years and miles behind me, but I'm stumped. It doesn't make sense. Just match RPM and road speed and the appropriate gear, and it should slide right in.

    So, assuming that the instructor knows how to match RPM and road speed and gear selection to downshift, what could cause a tranny to be upshifted but not downshifted? In other words, if you shift up from 3rd gear to 4th, what would have to be going on right then to be unable to shift back down from 4 to 3? Could 3 still be spinning? Is that even possible? This just doesn't make sense to me, but I don't know anything about trannys (yet), so that's why I'm reaching out.
     
  2. Truckers Report Jobs

    Trucking Jobs in 30 seconds

    Every month 400 people find a job with the help of TruckersReport.

  3. AModelCat

    AModelCat Road Train Member

    27,627
    144,554
    Jul 7, 2015
    Canuckistan
    0
    My 15 speed did that. Shift rails and interlock were seized up and binding. Really hard coming out of gear at times.
     
    elliott and Dave_in_AZ Thank this.
  4. elliott

    elliott Light Load Member

    102
    30
    Sep 3, 2007
    0
    Interesting. And that could happen between any gears?
     
  5. elliott

    elliott Light Load Member

    102
    30
    Sep 3, 2007
    0
    I just did some research on interlocks based on the previous comment, and it seems that if an interlock pin/ball is seized, then it will fail to keep the other gears/shift rails locked in place. So, if I'm wanting to downshift from 5th to 4th but an interlock is seized, then another gear(s) would be free to move, and it could actually be 8th gear (or whichever) that is trying to line up. If I'm misunderstanding the function of the interlocks, please correct me.
     
  6. BoxCarKidd

    BoxCarKidd Road Train Member

    4,167
    5,981
    Aug 26, 2014
    0
    There is an interlock for high and low range to the air shuttle valve on the top left center of the main case.
    Do you have correct regulated pressure? Do you get air to each side of the range piston when requested? Does it even try to shift when you change the range valve in neutral?
     
    Hammer166, elliott and AModelCat Thank this.
  7. elliott

    elliott Light Load Member

    102
    30
    Sep 3, 2007
    0
    I will ask about these things. Thank you.
     
  8. Hammer166

    Hammer166 Crusty Information Officer

    6,920
    23,775
    Aug 18, 2007
    ~8600+' and loving it!
    0
    I don't think he's talking about the range shift, I believe he means it literally won't downshift.

    All the gears are meshed together all the time, so it's not like you have a gear spinning too fast or too slow. When you shift a transmission, all you were doing is locking the various gears together with the main shaft.

    But we need more details, you say it won't downshift, what does that mean? Does it go back to neutral? Does it grind when you try to downshift it? And is it all the gears you are unable to downshift?
     
    BoxCarKidd Thanks this.
  9. Cattleman84

    Cattleman84 Road Train Member

    9,600
    66,989
    Nov 1, 2017
    The Sticks, Idaho
    0
    My first thought if someone told me that thier truck wouldnt down shift would be... "Do you know how to downshift?"

    But assumimg they do know how, then I'm at a loss as well.
     
  • Truckers Report Jobs

    Trucking Jobs in 30 seconds

    Every month 400 people find a job with the help of TruckersReport.