18 speed grinds while cruise is set

Discussion in 'Trucks [ Eighteen Wheelers ]' started by greasehorn, Feb 19, 2020.

  1. greasehorn

    greasehorn Bobtail Member

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    Feb 6, 2020
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    a truck i maintain is a 389 peterbuilt c 15 with an 18 speed in it, all is well until you have had the cruise set at 60-70 on flat ground for a few min, then it starts making an aweful grinding noise,(17/18 gear) at first you could just pump the clutch and it would quit, now sometimes it makes it seem like its sticking in gear, oh btw when the grinding starts the RPMs go up, we initally thought it was a clutch (even tho clutches dont make this grinding noise) and put a clutch in it, we have checked rear ends everything we can think of, im reaching out to yall because in 2 weeks the warranty runs out on this transmission, and i would like to solve the issue before that day comes.....thanks in advance
     
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  3. Dave1837

    Dave1837 Road Train Member

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    Try adjusting the suspension. If it's too high (or maybe too low I'm not sure) it will rattle at highway speeds and sound like the trans is going to come apart. Happened to my dad's just this past fall so he wrote it up as the transmission but all they did was adjust the suspension height and it went away, hasn't done it since
     
  4. greasehorn

    greasehorn Bobtail Member

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    Feb 6, 2020
    0
    ill try it, at first we thought it was leaf spring bushings clear out of it and had the rear end sideways, we tried about everything we can think of
     
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  5. Dave1837

    Dave1837 Road Train Member

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    Let me know if it works
     
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  6. Mid-May Trucker

    Mid-May Trucker Road Train Member

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    Maybe go faster seems like you should be going 80 to 90 in 18th gear.
     
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  7. Judge

    Judge Road Train Member

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    Could be different, as I have a 13, but mine started doing the exact same, ended up being a broken spring inside the transmission.
     
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  8. Rideandrepair

    Rideandrepair Road Train Member

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    Noise on acceleration- too high, coasting- too low.
     
  9. booley

    booley Road Train Member

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    I had an 18 spd that started clunking in high gear. Put it into the shop and they replaced the suspension leveling valve and told me it was fixed. Picked the truck up, and it was still clunking in high gear. Went back and took the mechanic for a ride, he says, "Oh". They rebuilt the transmission (still under warranty)
    Said it was something that was installed wrong during initial assembly...no charge
     
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  10. Powder Joints

    Powder Joints Subjective Prognosticator

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    Sounds like the T/O bearing
     
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  11. AModelCat

    AModelCat Road Train Member

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    Only thing I can think of internally that could make the RPM race and cause a grinding sound is if the teeth on the splitter dog clutch are worn/damaged and its being thrown out under torque. Once you let off the throttle, air pressure on the piston puts it back into engagement because its not fighting the torque trying to pop it out.
     
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