Peterbilt 379 Cruise not working

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Akkaid, Dec 27, 2020.

  1. Akkaid

    Akkaid Bobtail Member

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    Oct 26, 2017
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    Hey y'all, read all the other cruise control posts, still can't seem to fix it. Truck is a 99 peterbilt 379 with detroit 12.7. cruise worked ok, had to replace a brake switch behind the dash a while ago. Cruise started working intermittently, would have to shut truck off for a bit to get it back. Now it's just dead.

    Jakes work, clutch shuts off jakes just fine. High idle works, though clutch will not shut it off (don't think it ever did). No blown fuses. Pulling on clutch or brake does nothing. Replaced brake switch again.

    Any help would be appreciated!
     
    blairandgretchen Thanks this.
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  3. Roger McG

    Roger McG Light Load Member

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    Speed sensor on the back of transmission? I assume there are 2 sensors at rear housing. If the speedometer works I would swap the plugs on those sensors to see if you move your trouble from the cruise to the speedometer
     
  4. blairandgretchen

    blairandgretchen Road Train Member

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    Dec 9, 2011
    South west Missouri
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    Mine took a while to find.

    Ultimately it was that someone had used the ground circuit that the cruise/jake/park/brake etc - and tapped into it for a vehicular ground.

    Once we removed the offending wire, all was good.

    If yours is the same, most of the harness wires are white, and coded with a number. Maybe 941 is that circuit?

    Other issue can be NO/NC - normally open or normally closed air over electrical switches - getting the wrong switch.

    Not very good at explaining mechanical issues - hope some of this helps.
     
    Bean Jr. and baha Thank this.
  5. Akkaid

    Akkaid Bobtail Member

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    Oct 26, 2017
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    Good suggestions. I've only got the one outlet, and bought the wrong sensor. If that don't work, I'll chase them wires down. I'll post the fix if I find one, hate when folks don't do that.
     
  6. Akkaid

    Akkaid Bobtail Member

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    Oct 26, 2017
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    Hey y'all,

    Replaced speed sensor, still no joy. Time to chase wires.

    My first suspect is the wiring for the brake pressure switch behind the dash. But since it's a normal open circuit, it seems like it wouldn't kill the cruise if the wires was bad, unless there's a short...am I missing something? Could I unattach these wires, or short them if I'm wrong, at a point nearer the ecm as a quick test?

    Short version, where near the ecm can I find and short the brake pressure switch?
     
  7. TTNJ

    TTNJ Medium Load Member

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    Someone can correct me if I am wrong but if the wiring to the brake pressure switch is messed up wouldn’t that prevent the Jakes from working? I thought you indicated Jakes were working?
     
  8. Inderjit

    Inderjit Light Load Member

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    Sep 17, 2017
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    Why did you replace the first brake switch? Can you have someone scan
    it and verify all the switches that are required by the cruise control are working.
    You may have more than one brake pressure switch.

    I just dealt with a similar problem on a Volvo VN today. Volvo's software said there was a
    brake pressure switch that was intermittent but could not tell me which one.
    I told the dealer to send me all four. There is a park brake pressure switch a brake light pressure
    switch and two three terminal pressure switches fed by both delivery ports on the treadle valve.
    Apparently there is also a micro switch on the Volvo treadle valve as well.
     
  9. 6cuda6

    6cuda6 Light Load Member

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    Oct 9, 2020
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    Any other issues or things not working.?...what i'm looking at here is your comment "that if i shut the truck off for a bit.......". so something like "oil pressure" "oil temp" "water temp" etc something that indicates to the ECM that the engine is running.
     
  10. Akkaid

    Akkaid Bobtail Member

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    Oct 26, 2017
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    Naw...when it first started, stopping the truck, pulling the brakes, waiting 5 minutes, would make it with again. Everything else, save a freshly bad wheel seal, is great.
     
    6cuda6 Thanks this.
  11. TTNJ

    TTNJ Medium Load Member

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    Nov 21, 2017
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    I am going to circle back to a sticking brake foot valve
     
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