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Ground or Short: All trailer lights work except running lights...

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  • Ground or Short: All trailer lights work except running lights...

    Have the chronic issue where brake and turn signal lights work but running lights don't. I believe I have some strange grounding issue. Here's what I've done:
    1) With a multimeter, I verified that I'm seeing 12V off of the round to flat adapter plug for my 2004 Silverado.
    2) With the wire harness attached to to the flatplug on the truck, I've cut (and since recrimped) the running lights wire and verified 12 volts of the trailer harness.
    3) I have a 'Y" for the running lights wire that lead back to each light. I only have one light (back right) wired to the 12V brown running light wire (no side markers are hooked up). I verified I have 12 V off each end of the brown Y ends.

    I have the ground wire running all the way back to the back right light mounting bracked and have the ground wire attached to the mounting bolt. I have 12 volts between the brown wire and the mounting bolt. If I discount the grounding wire (with trailer tounge still on ball, I still read 12V.

    As soon as I connect the brown lead !2V wire to the light (which is mounted and now reattached to the grounding wire), the voltage instantly drops to nothing.

    Is this a ground, or some weird short. I thought it might be a short so I bought a whole new light package... Same thing. Please note, the 12 running light brown wire is not hooked to anything else. THe other end of the Y is standing free (not hooked to other rear light; side markers are not hooked up).

    Hearing from others that the running lights draw more amperage (although this only makes sense if both tail lights are connected and the side markers are connected), I even 'added' and ran a 2nd grounding wire (thicker gauge) back to the same rear mounting bracket; same thing.

    Here's what's also strange, If i attach the positive lead of the multimeter to brown 12V lead wire and the negative terminal of the multimeter to the brown lead wire of the mounted light, I read 12 V. Isn't this strange?

    I really appreciate any help. Is there a short somewhere within the light? any other things I can do to troubleshoot this with my multimeter?
    Last edited by sailerbyday; September 17th, 2017, 09:43 PM.

  • #2
    The one thing I can guarantee is that you DO NOT have a short. If you had a short you would blow your trailer light fuse in your fuse panel under the hood of your truck.

    I have a Shorelander trailer with hydraulic brakes. The problem was the steel brake line ran inside the frame rail on starboard side of the trailer, as did the wiring harness for that side of the trailer. Over the years the wire must have gotten between the brake line and the frame and from normal vibration and movement the wire insulation got worn away. The temporary guess/solution was to pull about 1 foot of wire harness out of the trailer essentially relocating the uninsulated part of the wire harness to place where it won't short.

    Yeah, I was blowing trailer light fuses hand over fist.

    This winter the trailer gets an entire new wiring harness, , from Shorelander.


    • #3
      The trailer light fuse under the hood of your truck is a 15 amp fuse. "Running lights draw more amperage", , than what? The bulbs are smaller, the only way they will draw more amperage is if you have like 20 marker lights.

      You said that if you connect the brown wire (with confirmed 12 volts) to the light, the voltage drops to 0. So if you disconnect the wire do you get 12 volts back?

      I don't think this should matter, but did you try wiring up both back marker lights? Just wondering if that would make some weird "complete circuit" or something like that.

      As for getting 12 volts when you connect each lead of your meter to a 12+ wire. I have no idea whatsoever, that shouldn't happen.


      • #4
        Check and make sure you have the proper bulbs in the socket and the connector in the socket hasn't turned or gone out of position.

        Boat Pro International
        Restorations, Appraisals & Detailing


        • #5
          Standard 4-wire wiring.
          White = Ground (Don't rely the hitch for ground or you will have intermittent problems forever.)
          Brown = Tail & Marker Lights. Yellow = Left Turn Signal/Stop Light. Green = Right Turn/Stop Light. (Green is NOT Ground.)

          Connect the White wire to the trailer frame on both sides of any trailer junction/connection and if you can at each light. This connection accounts for most trailer lighting faults. The rest is a piece of cake.
          Non Sequitur


          • #6
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            Sailor hasnt been back on iboats since starting this topic.

            Money spent @ Boat Restoration should be measured in Smiles & Pride SHOP IBOATS-BOAT SPECS-FORUM HELP-REPAIR/RESTO