Trailer abs light doesn’t come up on dash

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Greggg, Oct 19, 2019.

  1. Greggg

    Greggg Light Load Member

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    wondering if someone can guide me please. I’m gonna try to diagnose

    When I turn key on or start my trailer abs doesn’t show up on dash and abs light on trailer doesn’t blink like it’s supposed to.

    Do I just test every connection with a volt meter with key on or do I have to start the truck ?
    Any other steps I can take ?
     
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  3. Intothesunset

    Intothesunset Road Train Member

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  4. Greggg

    Greggg Light Load Member

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    No I still checked but the “trailer abs” doesn’t even come on and go off when o turn the truck on either so my abs ecu isn’t getting power somehow. I’m thinking just running new wire if I have trouble isolating the problem.

    Just wondering if there are any tips or something I should look for. I’m no mechanic or electrical guy unfortunately.
     
  5. Intothesunset

    Intothesunset Road Train Member

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    Is this a truck, or trailer problem?
    Class 8 truck?
     
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  6. Rideandrepair

    Rideandrepair Road Train Member

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    Well,from my understanding, if the Trailer has a light, it doesn’t send the signal to the Trucks light on dash. My Trailer abs wasn’t working.it wouldn’t come on with key. ( normal test mode) . I had to replace tha main power cord. It went from a plug near the Left side rail, to the abs module. Once it was replaced, light stayed on, then did blink code diagnosis, and replaced a wire from module to sensor. So starting at module testing supply wire for power, working your way back towards, and including ( if necessary) the pigtail, and to fuse box. ( if necessary). I’m terrible with electrical, if I could figure it out, so can you.
     
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  7. Rideandrepair

    Rideandrepair Road Train Member

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    Just a note, If I remember correctly, the blue wire in pigtail receptacle supplies the power to Trailer abs. Only the newer trailers without a yellow abs light at left rear, send a signal to the dash.
     
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  8. Greggg

    Greggg Light Load Member

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    Did you test with key on or engine running ? if it gives me too much trouble I might just run a new wire from the pigtail back to where it’s getting voltage, if that’s the case.
     
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  9. Rideandrepair

    Rideandrepair Road Train Member

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    854334B9-E46A-4FE3-A885-AA3F11D1D0C5.png Check with key on. Blue has constant power. abstroubleshooting.com has Wabco codes. Although mine had been spliced together by Penske, before I bought it. It’s against DOT rules to splice abs wires, from what I’ve read.
     
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  10. Working2party

    Working2party Medium Load Member

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    First Question on trailer ABS is where either Truck or Trailer built on or before July 1, 1997? Older units are not required to have ABS.
    Next Question is was either unit built before 2000? Older units did not require the truck to have a working ABS light. Or in other words truck to trailer communications weren’t required.

    If both are newer than the dates above, turn Key on (engine off) and listen for ABS modulator clicks on truck and trailer (lights on trailer should come on at this time) . This checks that the Blue Constant (key power) Wire has power. (Center pin of ABS cord)

    If there are no clicks at the trailer. Press the brake pedal and again listen for the clicks, (this time they may “chuff”) and make sure the trailer brake lights come on. This checks that the Red brake light wire circuit is getting power. (Opposite of the 7way cord Ground pin) If it fails this test then you need to look for a grounding issue.
     
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  11. Working2party

    Working2party Medium Load Member

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    I haven’t heard that splicing is illegal, but crimp/butt connectors in either the red stop light circuit or the Blue constant circuit can interrupt the Power-Line Communication (PLC). PLC is where they lay communication on top of the 12V power and allows for a Trailer ABS light on the dash.
     
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