99 freightliner c-12 constant high idle

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by rustyshakelford, May 2, 2021.

  1. rustyshakelford

    rustyshakelford Bobtail Member

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    Dec 28, 2017
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    My new to me freightliner century 112 with a c-12 wants to constantly kick into high idle when left alone. Cruise doesn’t work either. Have a CEL and the code was for a cam position sensor and intake air sensor. Swapped both of those with new cat parts and the CEL is still on and high idle still persists.

    Does anyone have some suggestions on what to try next? Buddy thinks I need a new ecm, certainly hope not. Truck has sat unused for several years. I have a thread here called crawler hauler. Doing a ton of work to get it ready to travel with. I can idle it up manually to 1500 and when I let off, it’ll go down to 750ish for a few seconds then idle back up to 1000. When I put it in gear and let out on the clutch it slows back down too. Would like to get this fixed so it’ll be quieter at camp grounds and also reduce the wear and tear on the clutch.

    brett
     
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  3. ClineER

    ClineER Light Load Member

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    Apr 27, 2016
    N.E. Oklahoma
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    What was previous life of truck, could either have a pto mode switch, where it will auto go to higher rpm. Other cause would be if voltage is low, an option on cats is where it will bump idle if the voltage is below a set point
     
  4. jamespmack

    jamespmack Road Train Member

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    Not a cat guy. But how long have you let it run? Up to temp? I recall working on a few that did that for warm up. Then idled down. Or just as simple as reprogram ecm to a lower idle.
     
    rustyshakelford Thanks this.
  5. rustyshakelford

    rustyshakelford Bobtail Member

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    Dec 28, 2017
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    It’s most recent life was hauling a GN loaded with horses to slaughter and before that was a farm that ran cattle pots. Have a fresh reman I’m told and certainly looks like it.

    Ive searched for a high idle switch, don’t believe it’s ever had a pto. Thinking it’s programming at this point. Batteries are brand new and look great. Just replaced alternator due to minor runout in the bearing causing a high pitch squeal

    Brett
     
  6. rustyshakelford

    rustyshakelford Bobtail Member

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    Dec 28, 2017
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    I’ll sit in high idle indefinitely. I think my neighbor has the cat software and we’re going to try to get it plugged in. Hopeful we can get some insight as to what’s going on. We’re using it to pull our camper (52’ trailer) and as much as I love the noise, it might not be appreciated in an RV park

    brett
     
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