How do you use the jake brake on a truck with an automatic transmission?

Discussion in 'Experienced Truckers' Advice' started by expedite_it, Oct 22, 2020.

  1. expedite_it

    expedite_it Light Load Member

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    I have almost 7 years of trucking experience, but 6 of those 7 years were driving a truck with a standard (manual) transmission. I know how to use a jake brake on a truck with a standard transmission. I have been driving a truck with an automatic transmission for one year. Today I am getting a one year inspection on the truck, and the mechanic could tell I've not been using the jake brake. The mechanic says I am burning up the brakes way too fast by not using the jake brake.

    When I first started driving the truck with an automatic transmission, I tried to use the jake brake two or three different times when I had a heavy load, and when I turned the jake brake on and applied the jake brake, the jake brake would slow the truck down, but the truck would automatically downshift to a lower gear (even though I did not want to downshift to a lower gear). Then the RPMs would be too high for the gear that the truck downshifted to, and I would have to brake even further just because the RPMs were too high. But my having to brake even further just to reduce the RPM's was problematic because I did not need to brake further just to keep the brakes cool enough. Then the jake brake would slow me down even further, and the truck would automatically downshift again, even though I did not want to downshift. Then my RPM's would be too high again, And I would have to brake even further again to get the RPM's down to the proper level. This was just a never-ending cycle. Next thing you know I am going 15 mph down a downgrade that I could safely go down at 40 mph with a jake brake on a standard transmission.

    So how do you use the jake brake on an automatic transmission without having to go UNNECESSARILY slow due to the automatic transmission automatically downshifting (when you don't want to downshift) and then having to slow down even further because the RPM's are dangerously high for the new lower gear?
     
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  3. AModelCat

    AModelCat Road Train Member

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    And what exactly is "too high" of RPM?
     
  4. Redtwin

    Redtwin Road Train Member

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    Use a lower level of engine brake. You can also select a higher gear with automatics and they usually hold that gear if the engine brake isn't on the highest setting.

    Would be helpful to know which year and model truck you have as different automatics have varied modes of operation.

    Can't hurt to have whoever assigned you the truck teach you how to use it assuming the user manual is not available.
     
  5. singlescrewshaker

    singlescrewshaker Road Train Member

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    Most cases jakes gonna be most effective between 1800-2200rpm. Let her spin.!

    Also, these new autos are synced up with the engine. I'd bet it won't downshift to an unsafe rpm..
     
  6. expedite_it

    expedite_it Light Load Member

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    My truck is in the repair shop right now, and I am not allowed to enter my truck right now.

    As I recall, the RPM's were around 2100 or 2200 when they were too high. On my tachometer, there is a red line at a certain point, and if the dial of the tachometer is in the red line, that means that the RPMs are excessive. As I recall, the dial of the tachometer was in the red line when I thought the RPM's were too high, but I don't definitely know that the dial was in the red line. It was a whole year ago.

    When I get my truck out of the shop and start driving again, I will try using the jake brake the next time I have a heavy load and am going down a significant downgrade, and I will note how high the RPM's go.
     
    Last edited: Oct 22, 2020
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  7. expedite_it

    expedite_it Light Load Member

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    You might be on to something here. When I used the engine brake before, I might have always been on the highest setting of engine brake.


    The only way I know of to go to a higher gear when I have the jake brake applied is to press on the accelerator, which is problematic for two reasons: 1# it speeds the truck up, which works against my goal of keeping the brakes cool and 2# pressing on the accelerator turns off the jake brake. Is there any way to select a higher gear with an automatic when I have the jake brake applied without pressing on the accelerator?

    2020 Freightliner Cascadia

    As I recall, my owner's manual just says how to maintain and repair the truck, not how to drive the truck.
     
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  8. expedite_it

    expedite_it Light Load Member

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    I will post on what RPM's on the tachometer is in the red line when i get my truck out of the repair shop.

    If the RPM's are in the red line, isn't it safe to assume that the RPM's are excessively high?

    It might be the case that I thought that the RPM's were too high when they were not too high.
     
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  9. Roguefox

    Roguefox Light Load Member

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    2200 rpm on the jake in an automatic is just about right.

    If you start to slow too much just press the accelerator to disengage the jake. Release the accelerator to engage them again.

    If you're doing it right you shouldnt even need to press brakes.
     
  10. Jenn72

    Jenn72 Medium Load Member

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    On my 2020 cascadia when I first used the Jake brake my RPM's went up high too. If you tap on the foot brake it will reduce the RPM'S but your Jakes are still on when you let off the foot brake.

    Once you learn how they work and set it correctly you won't need to touch your foot brake at all. When the Jake is set, it will let you go 5 over what is set. So remember to set it a little under going down to compensate for it.
     
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