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12V DC circuit mystery

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  • 12V DC circuit mystery

    Hoping for a little insight from all you electrical wizards out there. Today I decided to look after a little long overdue maintenace on my trailer, (haven't had a day off for awhile). I replaced a couple clearance lights and installed a new 7 pin plug. Finally, the loose connection is fixed and everything will work properly! Brakes, running lights, signals and the one that was driving me crazy the interior lights!

    Not so fast... The interior lights have been intermittent for awhile which I used to "fix" just by wiggling the 7 pin plug in the socket. I was sure that replacing the plug would have solved this problem but no, it actually made matters worse. Now the lights don't work at all! I've checked everything I can think of. Continuity through all switches, grounds, bulbs, etc. Ground screws are tight.

    Here's the part that has me completely befuddled. When I remove the bulbs from the sockets and test across the socket I'm getting 12V through my meter. Bulbs are good for crying out loud! I'm not an electrician but this suggests to me that the bulb should light. Why in the world will the bulbs not light?
    "Good enough" isn't really "good"...

  • #2
    Re: 12V DC circuit mystery

    check ground on the truck to 7-pin

    Comment


    • #3
      Re: 12V DC circuit mystery

      Originally posted by jeff View Post
      check ground on the truck to 7-pin
      Thanks for the suggestion Jeff. Just double checked this and it's good. Actually, I wouldn't have 12V at the light sockets in the trailer if I had no ground on the 7-pin...would I?
      "Good enough" isn't really "good"...

      Comment


      • #4
        Re: 12V DC circuit mystery

        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?
        Last edited by masterbeavis; 02-05-2012, 11:09 PM.
        We don't have preventative maintenance around here, we have CRISIS MANAGEMENT!

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        • #5
          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?
          Thanks masterbeavis. No inverter or on board batteries. All the trailer power comes from the 7-pin connection to the truck which appears to be good, (pending the results of your suggested continuity test). While I was checking continuity in the trailer I did not encounter resistance anywhere. However I didn't remove any screws to check the condition of the ground ring terminals either. I'll try your suggestions tomorrow.

          The thing that's still got me bugged is that I've got 12V in the light sockets and I'm sure my bulbs are good! I wonder, is it possible that I've got a short somewhere consuming enough current to prevent the bulbs from lighting but letting enough through to give me a meter reading at the socket? How would I test this theory?
          "Good enough" isn't really "good"...

          Comment


          • #6
            Re: 12V DC circuit mystery



            You stated "I'm pretty sure my bulbs are good". To be sure and because it hasn't been mentioned, run a continuity test on the bulbs also so you know for sure if they are indeed good.


            Comment


            • #7
              Re: 12V DC circuit mystery

              Originally posted by lreops View Post


              You stated "I'm pretty sure my bulbs are good". To be sure and because it hasn't been mentioned, run a continuity test on the bulbs also so you know for sure if they are indeed good.


              Thanks Ireops. Actually I stated, "I'm sure my bulbs are good". I know this because I checked them for continuity. I really do wish the solution was that simple.
              "Good enough" isn't really "good"...

              Comment


              • #8
                Re: 12V DC circuit mystery

                Put your volt meter on one of the light terminals, and the other probe to ground(metal chassis of trailer) , see if what you got, then do the other, make sure your trailer isn't "hot".

                Comment


                • #9
                  Re: 12V DC circuit mystery

                  Originally posted by Alphacowboy View Post
                  Put your volt meter on one of the light terminals, and the other probe to ground(metal chassis of trailer) , see if what you got, then do the other, make sure your trailer isn't "hot".
                  Thanks Alphacowboy. I'm assuming you're answering my question about checking for a short? The wiring design in my trailer has the chassis as ground for all the lights. One side of the switch on each light fixture runs back to a ring terminal screwed into the chasis. I don't quite understand part of your suggestion, "see if what you got". Thinking I should have 12V on one side and 0V on the other if all is good. Please clarify.
                  Last edited by gadgetarian; 02-06-2012, 02:11 AM. Reason: Elaboration on my reply
                  "Good enough" isn't really "good"...

                  Comment


                  • #10
                    Re: 12V DC circuit mystery

                    Trace the wire from 7-way connector to the light. Sounds like the wire in pinched somewhere. Probably between the frame & a board.

                    Comment


                    • #11
                      Re: 12V DC circuit mystery

                      identify the 12v+ at the 7 pin on the truck. verify it's hot when the truck is off and truck on. then install a jumper wire to that pin to allow for a larger load test. more than just a meter. but an actual load like a 55 watt head light.

                      test the hot at the truck to ground. then the hot at trailer jack to ground. then make a long enough jumper for the ground at your light fixtures to your 55watt test lamp. this way you can test for a good ground from the trailer.

                      if your grounding was bad at the trailer, then you would have issues with your running lights and signals too. is it only 1 switch that controls the interior lights? check that too.

                      rick.
                      phoebe it is

                      Comment


                      • #12
                        Re: 12V DC circuit mystery

                        Originally posted by MR.FUDD View Post
                        Trace the wire from 7-way connector to the light. Sounds like the wire in pinched somewhere. Probably between the frame & a board.
                        Thanks MR.FUDD. Yah, unfortunately you're probably right. The thought of emptying the trailer out and pulling the wall panels off the inside doesn't turn me on.
                        "Good enough" isn't really "good"...

                        Comment


                        • #13
                          Re: 12V DC circuit mystery

                          Originally posted by PLUMBER RICK View Post
                          identify the 12v+ at the 7 pin on the truck. verify it's hot when the truck is off and truck on. then install a jumper wire to that pin to allow for a larger load test. more than just a meter. but an actual load like a 55 watt head light.

                          test the hot at the truck to ground. then the hot at trailer jack to ground. then make a long enough jumper for the ground at your light fixtures to your 55watt test lamp. this way you can test for a good ground from the trailer.

                          if your grounding was bad at the trailer, then you would have issues with your running lights and signals too. is it only 1 switch that controls the interior lights? check that too.

                          rick.
                          Thanks for the detailed reply and suggestion Rick. The electric brakes and running lights are all good. It's just the interior cabin lights that don't work. Yes, just the one light switch. Will try your suggestion after work today, (if any steam left).
                          "Good enough" isn't really "good"...

                          Comment


                          • #14
                            Re: 12V DC circuit mystery

                            my guess is you have a bad connection, some where, that is allowing your meter to read it but not enough to light the bulb,
                            similar to when you have a corroded batter in the car/truck and the lights will come on but it will not start, clean the batter terminals and ever thing works,
                            (no I am not saying it is your battery terminals, but I think it is corrosion in a splice or a connection),

                            on your light bulbs, take the bulb and rub the little lead point on the base of the bulb on your pant leg, and polish it up, I have had tail light that that was all it was,

                            check any splice that is just taped up, and even if there is a wire nut on it turn it a turn tighter,

                            jumper the switch,

                            could take a know wire that is one piece and use as a jumper and do some Ohm tests on sections of the system, when you find resistance of any amount your probably in the neighbor hood of the fault,

                            most likely I would take the switch out and try a bulb at that point, see if it lights, if it does then check it after the switch, (one may be able to get a test light, one at one time could get what looked like a awl with a wire out the handle with a clip on it, bulb was in the handle ), or one could take a test wire with clips on it, and solder it to a know good bulb and use that for testing, or go to the auto parts and get a socket for the bulb, so you can get better grounds and connections when testing
                            Last edited by BHD; 02-06-2012, 11:47 AM.
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                            • #15
                              Re: 12V DC circuit mystery

                              For what it's worth, I agree with all the guys who are suggesting bad grounds. Nine times out of ten when I have had issues like yours, it's the ground. Meters can read voltage, but the circuit not have enough of a ground to actually handle the load, so no light. Ricks suggestion is a great one. That should settle it once and for all.
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