No power to ABS pin on trailer wiring. No brake lights

Discussion in 'Freightliner Forum' started by irishluck09, Mar 18, 2021.

  1. irishluck09

    irishluck09 Light Load Member

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    Nov 24, 2019
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    2014 freightliner cascadia
    2006 48 flat spreadaxle utility

    I have no break lights on my trailer. All other lights are working. Has new wiring from truck to trailer and new plugs. I have 14.63v at the truck wiring harness for brake lights. On back of trailer i have 6.23v for brake light. No brake lights. Cant see any broken wires anywhere. No shirts that i can see

    Tested the plugs center pin (blue wire) for the abs/auxiliary and getting 0v on truck side. Does that blue wire affect my trailer brake lights at all?
     
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  3. xsetra

    xsetra Road Train Member

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    The blue wire powers the ABS system.
    Your brake lights operate off a different pin.

    Open the trailer socket for the pig tail. See if you have corrosion or a broke wire in there.

    If you can try a different pig tail, before you take the plug apart.
    Good luck
     
    Last edited: Mar 18, 2021
  4. RocketyMan

    RocketyMan Medium Load Member

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    Sounds like a wiring problem. There are different types of 7 way trailer connectors depending on truck configuration and such.

    Last 6 of vin I can look up some factory wiring.
     
  5. irishluck09

    irishluck09 Light Load Member

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    So we finally figured out the issue being the 5-pin relay on the firewall. Not sure how it caused my brake lights to not work because i had 14v at the eiring going to the trailer but only 8v at the actual brake light.

    But this brings out another issue as why i have no voltage on my abs pin?
     
  6. SWM_TECH

    SWM_TECH Light Load Member

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    For your no voltage on the abs pin, I would start by checking your relays and fuses on the SAM Chassis on firewall by steering column. Fuse F11 is a 30 amp and fuse F14 is 30 amp. Also, check for relay R6 it is a 5 pin relay. Also, check relay R8 it should have a 4 pin relay for trailer abs power with key on. If you put a 5 pin relay in R8 you will have trailer abs power continually. Normal way is abs power with key on.

    Post the last six of your VIN for most accurate information.
     
    QUALITYTRUCK Thanks this.
  7. irishluck09

    irishluck09 Light Load Member

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    My last 6 is fn7378
     
  8. Snailexpress

    Snailexpress Road Train Member

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    Looks like you have no ground wire between truck and trailer or bad ground wire on the trailer.
     
  9. RocketyMan

    RocketyMan Medium Load Member

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    @SWM_TECH is correct about your configuration if you want power to be supplied all the time or only with IGN based upon a 5 or 4 pin relay, respectively. That is why on the schematic you will see "Trailer PWR BAT OR IGN." On this unit, you have a P3 Cascadia. I think some refers might need the 5 pin relay for continuous power or some configs like that.

    At any rate, your SAMCHAS is responsible for your BOC 7-way.

    upload_2021-3-25_8-16-5.png


    I would check the wiring from the SAMCHAS to your 7-way. You could have a compromised circuit 45 or something in the middle. Here in green is your 7-way on the INSIDE. Labeled are the connectors to check.

    upload_2021-3-25_8-22-25.png


    Here is that intermediate harness for the 7-way. Maybe something is going on in here:
    upload_2021-3-25_8-24-50.png


    Then on the other side to the SAMCHAS it could be this wiring:
    upload_2021-3-25_8-30-0.png

    upload_2021-3-25_8-29-47.png


    Let us know what you find out.

    P.S. 14.63v is too high. Your truck has a 36SI alternator. You should be utilizing the voltage sense wire for the batteries to be at 14.2volts. If this circuit is compromised in any form, you'll get high voltage spikes. If it is open circuit, it will go to a "nominal" voltage regulation level and could possibly end up under charging the batteries.
    upload_2021-3-25_8-2-45.png
     
    SWM_TECH Thanks this.
  10. irishluck09

    irishluck09 Light Load Member

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    #### how the hell did you get all that info? Lol ill do some testing. I pull flatbed but my trailer does have Abs.

    On a side note which could be related to this as well. Ive heard this is a xommon deal on these trucks but my right rear brake light on my truck would always stay on. Truck off and everything. HVe been told the box (cant member proper term) is known to cause this issue and has to be replaced. Since then ive just cut the wire from that brake light and wired it into the other brake light. Could this possibly be all in the same issue?
     
  11. Batman13

    Batman13 Light Load Member

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    May 20, 2020
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    Need info about a 4 pin abs harness on. 14 cascadia..its in my rear chassis connects to my meritor under 5th wheel..I need the exact name of that harness..
     
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