Fld120 air switch question

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Dino soar, Oct 24, 2020.

  1. Dino soar

    Dino soar Road Train Member

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    1995 fld120.

    I had an air leak at the air switches if I push the brake pedal very hard.

    So I bought a brake light switch figuring that was the problem.

    I found a leaking switch but it is not the brake light switch. Now where the switch came out from there is air that comes out that Port when you hit the brakes. When the brakes are not applied there's no leak.

    All of the air switches are lined up basically behind the steering column, but this switch was up higher kind of like where your carpet Ends by the steering column. You could see it there with the carpet installed.

    Also this switch that I took off goes to zero when you test it for ohms.

    The brake light switch goes to Infinity.

    I have to call Freightliner on Monday and see what they say, but does anyone know what that switch is or what it is for?

    Everything else seems to be working on the truck.

    It must be for something.
     
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  3. Rideandrepair

    Rideandrepair Road Train Member

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    The only 2 air switches I’ve dealt with are brake light, and low air. Both are mounted on my parking brake. Ones normally open, others normally closed. They are marked nc or no, along with the part #, maybe even the psi that switching takes place. I’m not sure. I guess you could test it, see if it kicks in below 60psi, along with the low air buzzer. Either way, if you can read the # on the switch, maybe you can narrow it down with dealers help.
     
  4. BoxCarKidd

    BoxCarKidd Road Train Member

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    I thank that is also a brake light switch. Look for a part number on the metal right next to the plastic. May need a magnifying glass to read it.
     
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  5. mslashbar

    mslashbar Medium Load Member

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    If your truck has cruise control it could be to shut the cruise control off when you hit the brakes.
     
  6. Dino soar

    Dino soar Road Train Member

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    That's what's kind of crazy about all of this.

    Since it is normally closed it should be some kind of a low air warning I think.

    But my low air buzzer and light is working like it should and my brake lights are working like it should without that switch being in there or plugged in.

    I'm going to put a plug in there and make sure I don't have any other leaks, but I wonder what the heck that thing is all about?
     
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  7. Dino soar

    Dino soar Road Train Member

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    Wow that's interesting I never thought anything like that.

    I put a big cam Cummins in my truck so no need to worry about computerized cruise control.

    That makes me just want to plug it and forget it.
     
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  8. BoxCarKidd

    BoxCarKidd Road Train Member

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    Trucks have 2 brake and two low air switches.
     
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  9. Dino soar

    Dino soar Road Train Member

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    I think the mystery is solved.

    @mslashbar has the right idea.

    I'm going to double check again tomorrow but that wire that came off of that switch was number 543.

    I found a diagram online of all of the switches and it showed wire 543 going to the brake switch to cut off the cruise control.

    Like I said I'll double-check it tomorrow but I'm 99.9% sure that's what that is.

    So I can put a plug in it and be done with it.
     
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  10. SmallPackage

    SmallPackage Road Train Member

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    That switch and 543 wire are there for the cruise. Just plug it and wrap the hot wire end with tape.
     
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  11. Dino soar

    Dino soar Road Train Member

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    Got it plugged no more leak everything is working.

    I will put the covers back on and all that tomorrow.
     
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