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  • #16
    Re: 12V DC circuit mystery

    Sometimes the bulbs look good, measure good and do not light when in the lamp socket
    can you use alligator clip leads to get the 12 volts exterior to the lamp socket?
    Then clip them to the bulb to see if it lights.

    Many time in auto type interior light assemblies the spring tension of the contact is
    poor and although the bulb seems to seat into the lamp socket, the bulb
    tip [bottom] is not making contact with the positive contact.

    You can try a new bulb or use a needle nose pliers to stretch the spring slightly

    what bulb number are you using and is it a two pin or single pin style?


    Cactus Man

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    • #17
      Re: 12V DC circuit mystery

      I bet Rick's first suggestion will show the failure. There is only one ground (white wire - two if you count the hitch) so if the brakes and other lights work it must be the supply. You will likely find that the 12V at the truck is corroded and you need to replace your trucks 7 pin connector.

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      • #18
        Re: 12V DC circuit mystery

        BHD, speedbump, cactusman & wbrooks, thank you for taking the time to read through this thread and reply and thanks again everyone else who has offered their input - very much appreciated.

        Long day today with arms overhead and I'm just whipped so I haven't had a chance to try what you've suggested. I have maintenance scheduled for tomorrow so I'll put your great ideas into practice and hopefully my little mystery will be resolved before noon if all goes well.

        BHD, I have one of those test probes with the lamp in it somewhere in my mess of stuff. I think I got it when I was a teenager tinkering with my old jalopies. Great suggestion!

        Cheers!
        "Good enough" isn't really "good"...

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        • #19
          Re: 12V DC circuit mystery

          Bizarre any kind of talk seems..
          Wilson Rackets|Babolat Rackets|Head Rackets|Prince Rackets

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          • #20
            Re: 12V DC circuit mystery
            Many house type trailers have dual interior lights. They work on 120vac and 12vdc. I'm thinking perhaps something going on with that maybe? Also, when reading voltage with a voltmeter there is VERY LITTLE current being drawn so you can easily get a reading with either a bad connection on the plus or ground side. Once you add a good load a bad connection will show its face. Try this, one one interior light check the voltage at the same time another light is turned on and see if you have a massive Vdrop on the line.

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            • #21
              Re: 12V DC circuit mystery

              Originally posted by QROKING View Post
              Many house type trailers have dual interior lights. They work on 120vac and 12vdc. I'm thinking perhaps something going on with that maybe? Also, when reading voltage with a voltmeter there is VERY LITTLE current being drawn so you can easily get a reading with either a bad connection on the plus or ground side. Once you add a good load a bad connection will show its face. Try this, one one interior light check the voltage at the same time another light is turned on and see if you have a massive Vdrop on the line.
              Thanks for the suggestion QROKING. My tailer is a 2008 Continental Cargo Tailwind cargo trailer that I use for work. 12V DC power only through 7-pin connection to tow rig. Two cabin light fixtures c/w their own switch and one bulb each, wired in parallel with a common switch.

              I was hoping to get to the trailer today but other matters required my undivided. I'll include your suggestion in my troubleshooting efforts tomorrow.
              "Good enough" isn't really "good"...

              Comment


              • #22
                Ground or connection not sufficient for amperage?

                Enough to show 12 volts on a volmeter but not able to conduct the amperage needed to light filaments?
                Suspecthat some connections or pins need to be cleaned and tightened.
                Last edited by Robert Gift; 02-18-2012, 11:16 AM.
                I'd take an educated guess - but I'm unqualified.
                It ain't just soot, it's paydirt.
                "I swear, wherever Gift goes, argument follows." -Youtube comment

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                • #23
                  Re: 12V DC circuit mystery

                  Mystery solved!!!

                  Well after much head scratching, brow furrowing and blank stares at the driveway I finally figured out what the problem was with my auxiliary power circuit. The "+" pin (roughly 1:00), on the 7-pin connector on my truck was indicating 12.4 volts but the current was insufficient to illuminate my trusty test lamp.

                  I had my truck in for some warranty work recently and asked them to fix this while they were at it. "No problem" says the young fellow, "we'll take care of it." Great! Not so great! I go to pick up my truck. Mechanic can't fault the connector. "Checked voltage at connector, 12.4 volts - OK. Check trailer wiring" he says. :-(

                  You gotta be kidding me! I make another appointment. This time I take my multi meter and my test lamp along with me for a little show and tell. I perform the diagnostic on the 7-pin for the young service advisor and he sees that all of the pins light up except for the "+" pin even though its showing 12.4 volts. "Huh?", he says. "Yup, I'm pretty sure the problem lies somewhere between here and one of the two batteries up front!", I reply. Big grin!

                  Anyway, long story short, the shop's electrical wizard spends an hour on it tracing and finds that when I bought the truck 7 years ago and had some nice heavy after market mud flaps put on, one of the self tapping screws grazed the jacket on the hot wire and after 100,000 clicks of bumps and vibration finally wore through it far enough to rob the circuit of current but not blow the fuse.

                  The bad news is that it was not covered by warranty and the repair cost me $400 clams. They did throw a new 7-pin connector and a length of wire on for me at no charge though.

                  The dealership that put on the mud flaps way back when changed hands some time ago and would likley not honour the repair so I'm not even going to bother going after them. I'm just glad the mystery is solved and the cabin lights in my trailer are burning brighter than they ever did to begin with. :-)

                  Thanks again to everyone and all your creative trubleshooting suggestions. I learned a lot from you!
                  "Good enough" isn't really "good"...

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                  • #24
                    Re: 12V DC circuit mystery

                    Awesome! I'm dealing with an electrical issue with my truck, and man, its a nightmare! I might have to pull the wire harness for the engine and unwrap it to find my problem...

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                    • #25
                      Re: 12V DC circuit mystery

                      Originally posted by masterbeavis View Post
                      Run a jumper from the tow rig to the trailer, clean bare metal. Have you physically removed and cleaned the grounds to bare metal and liberal doses of no-ox applied to help stop corrosion? Wires can and do rot inside the insulation. Do you have a 110/12v DC inverter? Does the problem occur on shore power?

                      Tightening ground screws is not enough, corrosion can hide between the ring terminal and frame. I would suggest a continuity test from your fixture ground to ground on your battery or inverter, lots of resistance will show you do have a problem. Does your trailer have batteries?
                      You beat Me to it, SPOT ON !
                      I can build anything You want , if you draw a picture of it , on the back of a big enough check .

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                      • #26
                        Re: Electrical fire due to short!

                        Originally posted by Alphacowboy View Post
                        Awesome! I'm dealing with an electrical issue with my truck, and man, its a nightmare! I might have to pull the wire harness for the engine and unwrap it to find my problem...
                        The sooner you figure that out the better my friend! Unresolved electrical issues in engine compartments can lead to extremely serious problems. A short story to share;

                        A few years ago I recall watching a fellow's car burst into flames from my office window because of an electrical short. I was at my desk working on something and I heard a car horn blaring away out front of the office building. I looked up and there's a car stopped in the middle of the road, the driver is out of his car with his head under the dash trying to figure out why his horn his stuck when all of a sudden I see flames start shooting up through the grill between the engine hood and the windshield, you know along where the wipers mount.

                        I ran down the stairs and opened the front door and yelled across the parking lot, "Hey, your car is on fire, get out of there!" He jumped up and opened the hood just as someone from another office ran to him with a fire extinguisher and put out the fire. He was a lucky man. That situtation certainly could have had a tragic outcome.

                        Good luck with your troubleshooting!
                        "Good enough" isn't really "good"...

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                        • #27
                          Re: Electrical fire due to short!

                          My diagnosis was right on

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