1995 Freightliner FLD Low Air Warning

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Trugo, Dec 20, 2018.

  1. Trugo

    Trugo Light Load Member

    71
    50
    Dec 8, 2015
    0
    im working on getting an old 1995 FLD back on road. Currently I’m stumped on the low air warning system (it’s not working). The original buzzer and light were located in the dash info bar. But that device was damaged and has been removed.

    I’ve located the low air sensors, there are two of them behind the steering column. I can verify this because they both have red stripes which indicate the LAW system. But they’re so deep in there I can’t reach them. I’d have to dismantle the steering column to reach these things.

    Even if I do reach them, I don’t know how the circuit works. Do these sensors work together to put out one signal? Would it be a 12v or ground signal? Does the signal come on at 60psi?

    I’d love to find the wire still existing in the dash, hook it up to a light and be done. it probably still in there. But there’s nothing but 1000 yellow wires in there plus I don’t know if I’m looking for a ground or 12v signal or if it comes on or turns off at the 60psi.

    Any help on this would be great.
     
  2. Truckers Report Jobs

    Trucking Jobs in 30 seconds

    Every month 400 people find a job with the help of TruckersReport.

  3. Working2party

    Working2party Medium Load Member

    608
    724
    Feb 10, 2018
    Kansas
    0
    Those should be normally closed switches it takes 60psi to open them and kill the circuit. They are wired in parallel so that either sensor will set the alarm. I’m not for sure if they are controlling the power or ground side of the circuit. But you should be able to check with a test light if you can get the kickpanel / carpet pulled back on the left side of the column.

    That’s all I got, hope it helps.
     
    Trugo and BoxCarKidd Thank this.
  4. Rideandrepair

    Rideandrepair Road Train Member

    16,539
    53,983
    Aug 8, 2015
    0
    I always thought they ran off the Parking brake . Air switch screwed into it
     
    Trugo Thanks this.
  5. BoxCarKidd

    BoxCarKidd Road Train Member

    4,249
    6,127
    Aug 26, 2014
    0
    That has been a while and the Workingtoparty,s post just came in. One switch is for the primary and the other for the secondary air tanks. They are independent and either should sent off the alarm.
    If the dash unit is damaged you can buy an old school buzzer and wire it into one or both of those switches. I have never heard of the system being tested on both tanks independently. You may want to install and wire a light to it also although I do not thank it is required. I prefer to maintain as original but sometimes we have to run and make ends meet.
    This system is out of service criteria if not functioning and has been battled here before. The light is for the deaf driver and the buzzard is for the blind driver.
     
  6. Trugo

    Trugo Light Load Member

    71
    50
    Dec 8, 2015
    0
    Ok great info. So I should be looking for the signal to shut off at 60 psi. I’ll rip out the carpet and see if I can get in there.

    There are some air switches at the parking brake. I tested those but they’re not for the air alarm. Other threads pointed me to the steering column and indeed there are more air switches there, 2 of which have red stripes which supposedly are for the low air.

    If I can find the right wires I think I’ll just hook it up to the existing air suspension dump buzzer and add a light. That shouldn’t be too hard with the right wire and signal. Yep oos criteria. I don’t need or want a low air warning system. However it seems to be a top concern at the local scales.
     
    Rideandrepair Thanks this.
  7. Trugo

    Trugo Light Load Member

    71
    50
    Dec 8, 2015
    0
    I went out and tore out the carpet behind the steering column. By this point I could see 4 air switches. 2 of them are looped together with one going to ground and the the other going up to dash. I was able to trace that one behind the speedometer. This wire does work using a test light. I’m getting a ground signal above 60psi. Which is good news.

    But this presents a challenge for my basic wiring skill. I need a ground below 60 psi to activate a light and buzzer.

    How to wire such a situation?
     
  8. BoxCarKidd

    BoxCarKidd Road Train Member

    4,249
    6,127
    Aug 26, 2014
    0
    Use that to operate a 5 terminal relay or install normally closed switches.
     
    AModelCat Thanks this.
  9. Working2party

    Working2party Medium Load Member

    608
    724
    Feb 10, 2018
    Kansas
    0
    I’ve only seen it found once. The dot officer had the luck of the getting the “Unprotected ” system to drop before the protected system, so the buzzer sounded when on was a t 60 and the other was @ 50. He’d found more obvious things and red flag the truck. This was one of the items that he required we fix before we could move the truck!!!
     
  10. Trugo

    Trugo Light Load Member

    71
    50
    Dec 8, 2015
    0
    Oh good, now I understand the purpose of a 5 pin relay. Ill go with that, the truck is full of them I should be able steal one from somewhere.
     
  11. Trugo

    Trugo Light Load Member

    71
    50
    Dec 8, 2015
    0
    this 5 pin relay works. the light and buzzer work good.
     
  • Truckers Report Jobs

    Trucking Jobs in 30 seconds

    Every month 400 people find a job with the help of TruckersReport.