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N-14 Freightliner no high idle

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by olpa77, May 21, 2020.

  1. May 21, 2020 #1
    olpa77

    olpa77 Light Load Member

    72
    15
    Apr 9, 2011
    0
    Hey folks have another head scratcher for ya'll. I'm working with a 96 Freightliner FLD Cummins N-14 Celect. When parked the High Idle (pto idle) wont increase the RPM's. This truck used to be able to bump the idle up to 900 RPM's while parked. It used to work regardless of the parking brake set or released. However if you roll / coast the truck slow (walking speed) the High idle will work but as soon as the wheels stop the idle drops back down. Even if you put it in 1st gear and slow roll you can hit the cruse and idle it up to 900 RPM's. Throw it in neutral the rpm stay at 900 until the wheels stop moving then drops back down to a low idle. Cruse control works fine. Clutch switch and brake pedal will knock the cruse off so I assume they are working. ECM checks out okay as well. To say the least I am puzzled and came to you folks for help. Hopefully there is someone out there who has experienced this before that can point me to the right direction.
     
    olpa77, May 21, 2020
    olpa77, May 21, 2020
    #1
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  3. May 21, 2020 #2
    SmallPackage

    SmallPackage Road Train Member

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    Dec 20, 2019
    Marion Texas
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    Speed sensor on transmission. Possibly.
     
    SmallPackage, May 21, 2020
    SmallPackage, May 21, 2020
    #2
    olpa77 and Rideandrepair Thank this.
  4. May 21, 2020 #3
    Final Drive

    Final Drive Road Train Member

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    2,505
    Jan 21, 2015
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    Check trailer brake handle if it's slightly applied high idle won't work..
     
    Final Drive, May 21, 2020
    Final Drive, May 21, 2020
    #3
  5. May 29, 2020 #4
    olpa77

    olpa77 Light Load Member

    72
    15
    Apr 9, 2011
    0
    I think your right on the money. Thanks for your help. I was thinking no way it's the speed sensor because cruise works and the speedo works. However I finally got a code pulled from it and it's showing 241 Vehicle Speed Sensor.
     
    olpa77, May 29, 2020
    olpa77, May 29, 2020
    #4
    SmallPackage Thanks this.
  7. Feb 2, 2025 #5
    Rocketman 39470

    Rocketman 39470 Bobtail Member

    3
    3
    Jan 18, 2018
    0
    I finally solved my non working high idle issue after weeks and weeks of research. I changed all the air switches and the vehicle speed sensor on my 1998 FLD 120 with the 12.7 Detroit with no help. What I finally determined was that the high idle circuit passes through the hood tilt switch and is disabled when the hood is opened. If that switch is bad or the wiring to the switch is bad and that circuit is open, your high idle will not work even though your cruise control will not be affected. I was able to bypass my hood tilt switch by unplugging it from the harness and used a large paper clip as a jumper to close the circuit. My high idle started working immediately. Hopes this will help someone with this same issue.
     
    Rocketman 39470, Feb 2, 2025
    Rocketman 39470, Feb 2, 2025
    #5
    Speedy356 and SmallPackage Thank this.
  8. Feb 2, 2025 #6
    SmallPackage

    SmallPackage Road Train Member

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    Dec 20, 2019
    Marion Texas
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    Never heard of a hood tilt switch. Sounds like a dumb idea. Lol.
     
    SmallPackage, Feb 2, 2025
    SmallPackage, Feb 2, 2025
    #6
  9. Feb 2, 2025 #7
    Rocketman 39470

    Rocketman 39470 Bobtail Member

    3
    3
    Jan 18, 2018
    0
    It’s a small square box mounted inside the hood near the drivers side headlight box and plugs into the harness that tells the ecm the hood is open, therefore disabling the high idle feature of the cruise control system.
     
    Rocketman 39470, Feb 2, 2025
    Rocketman 39470, Feb 2, 2025
    #7
    SmallPackage Thanks this.
  10. Feb 2, 2025 #8
    IH9300SBA

    IH9300SBA Road Train Member

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    Feb 26, 2021
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    It was used so truck wouldn't start via Opt-Idle if hood was open.
     
    IH9300SBA, Feb 2, 2025
    IH9300SBA, Feb 2, 2025
    #8
    SmallPackage Thanks this.
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