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Thread: 68rfe sprag fix/lr clutch adjustment

  1. #1
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    68rfe sprag fix/lr clutch adjustment

    Hey everybody Im rather new to tuning and looking for a few answers to some questions on the transmission side of things. So I recently acquired a 07.5 6.7 with the 68rfe, but it shifts way too low (lugs) and Im afraid to push it very hard with all of the horror stories about the low reverse sprag. My question is what is the magic number for the upper end of the lr clutch? I have the stock file read out and saved and have been messing with it a bit. But if I change the value for the lr if it shifts for second will it not disengage untill it reaches the set value? Im kind of stumped here and definitely dont want to set the value too high and burn clutches. Sorry for punctuation spellcheck only helps so much.
    Last edited by Johnsonboys97; 04-08-2019 at 12:48 AM.

  2. #2
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    Raising your upshift and downshift points, lockup and unlock points is the key to keep it from lugging

  3. #3
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    256 is what Dodge updated the newer trucks to. You can go higher, but there is the risk that if something fails you could end up with the low/reverse clutch applied when it shifts. It would normally never happen, because that passage does exist in the valve body. You can data log and see what is the maximum output shaft speed is when your truck does the 1-2 shift. If you keep the value below that, you will be safe.

    Paul

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    Paul that is exactly the details I was Looking for thank you.

  5. #5
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    Everything I?ve read says to raise lr to 900-950 so that it stays engaged thru the 1-2 shift..

    I just tuned an 07.5 and I set it like that and it?s working great....

  6. #6
    I have my upper limit set at 1500. Not sure if it is right, but I have never had an issue with the 1-2 shift.
    2019 CCSB Bighorn - Excited HP Tuners finally supported Cummins CM2350&2450 platforms

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    I have mine even higher than that and haven?t had any issues either

  8. #8
    So the question I always have is if the clutches can stay engaged through the shift, why does the transmission have the sprag? Why not just have the clutches?
    2019 CCSB Bighorn - Excited HP Tuners finally supported Cummins CM2350&2450 platforms

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    No idea, I haven?t been able to find much for info on the low/reverse sprag

  10. #10
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    Older revisions the clutches were only applied from 100-150rpm output shaft speed but later revisions have it applied from 100-256rpm output shaft speed, if I remember correctly. Since broken sprags were caused from the clutches releasing and dropping back onto the sprag during hard accelerations, I just raise the Upper Limit to ensure the clutches stay applied till it will for sure be out of 2nd gear.

  11. #11
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    Quote Originally Posted by jfreemanak View Post
    So the question I always have is if the clutches can stay engaged through the shift, why does the transmission have the sprag? Why not just have the clutches?
    Overrun...

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    What do you mean by overrun Paul?

    The 68rfe is a bit of an elusive one on some stuff in the forums like the L/R Sprag.

    Have you noticed in Warp that the 1-2 shift for tow/haul doesn?t follow commanded unless it is the same as normal 1-2? Seems to be an issue I have that I don?t get why.

  13. #13
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    Quote Originally Posted by Jim P View Post
    What do you mean by overrun Paul?
    Overrun occurs anytime the driven part turns faster then the driving part. Mainly in this case, coasting. If the clutch is engaged then the truck will be trying to increase the engine speed, during coasting conditions, in first gear.

    Paul

  14. #14
    That seems like a rare occurrence. Also, since the TCC cannot lock in 1st, at least there isn't a direct couple for that situation.
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    Thanks for the explaination Paul!