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Car Trailer lights problem

  1. Mar 5, 2008 #1
    hey guys, i could as easily post this in an automotive type forum but i'm not a member of any!! no specific car knowlege nescessary!

    Here is the problem
    Where i'm working now, there are two jeeps and one Car type trailer
    jeep 1 operates all the lights on the trailer successfully
    jeep 2 will only operate the indicators, no other trailer lights function at all

    i was given the task of investigating why jeep 2 will not power all lights (side lights brake lights etc)
    i have determined the trailer and jeep1 are wired correctly

    to determine if jeep 2 is wired conventionally i used a multimeter while getting someone to turn on various indicators, lights etc
    all terminals on the socket produced a voltage when they should, so i reasoned it was wired correctly

    now heres the situation:
    i connect the voltmeter across the grnd and tail lights on the lights socket on the jeep with the trailer lights plugged OUT.
    with the jeep lights on, this shows a few volts (cant remeber how many maybe 8-11v) as i'd expect
    now i plug in the trailer lights, the voltage drops to ZERO!!

    so i think, hmmm what could be causing this?
    could it be that the wire is grounded or shrt ccted furter up on the jeep end... but i figured that if it were, i would not read such a high voltage with the trailer plugged out as 99% of the voltage would drop accross the short?

    so secondly i considered that there is some kind of high resistance somewhere along the positive supply, i.e. a high resistance in series
    so i'd have battery+, high reistance, bulb, battery-
    i figure that this high reistance in series wouldnt affect the voltage i'm reading across the terminals. but when the cct is closed (i.e. i plug in my trailer lights) 99% of my voltage drops across the big resistance fault, leaving only a few mV across my bulb, or even ZERO.

    IS MY THINKING CORRECT and if so does anyone know how such a big resistance can suddenly appear (the jeep did function with trailer lights previously i'm told) or is there another type of fault that could cause this to happen.

    in any case thanks for reading and i'd be interested to read any replies!
  2. jcsd
  3. Mar 5, 2008 #2


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    I'm a little too drunk and too rushed for time (pool match in 1/2 hour) to properly read your post. I'll try again later. For now, though, having lived through a horrendous few months working at a U-Haul outlet (as an adjunct to my real job at the same place), I would recommend that you run an auxilliary ground from the trailer to the vehicle. You can't trust the ones that come with the harness. Just run a nice heavy (#10 or better) wire from the vehicle frame to the trailer frame and see if it helps.
  4. Mar 6, 2008 #3


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    Danger is probably on the right track. Some circuits will have power on one side of the bulb and a switch grounds the other side of the bulb. No ground, no light.
  5. Mar 7, 2008 #4
    thanks guys
    no joy i'm afraid
    i'm pretty sure all lights share a common ground and the indicator work fine, so i'm guessing the ground works ok.
    i guess i need to know where the socket connection on the jeep is fed from
    i suppose its a bank of relays somewhere, but i cant see anything under the bonnet. i'll keep digging and if i turn up with anything i'll post back !!!
  6. Mar 7, 2008 #5


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    Yes, your thinking is correct. I'd say this is your problem. How is the trailer connector wired? Most of the time they are spliced into the tail light harness at the back of the vehicle. A poor connection at the splice is most likely the cause and is what you refer to as the 'high resistance'. This is not likely at all a ground problem.
  7. Mar 8, 2008 #6


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    I would validate the ground first.
    You can get loopback currents with the multiple lights.
    So it isn't unusual to have some work even though the ground is bad.
  8. Mar 13, 2008 #7
    well i finally got some time to have a good look at it.
    seems there were 2 probs as far as i could see.
    the trailer lights seem to be fed straight from the jeeps lights, looks like a bad job.
    under the jeep there was a joint in the cable where the wires were joined with "crimps"?
    and the whole thing wrapped in insulation tape. anyhow two of theese were pretty much disconnected!, also at the socket where the trailer plugs in, some of the wires seemed to have become brittle and snapped, i undid the trailer plug and twisted the wires onto the jeep wires and all worked, so tomorow morning i'm going to buy some crimps and a new socket for the jeep and i expect all to be well!!!
  9. Mar 13, 2008 #8


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    Oh well... I'm use to being wrong. :redface:
    I'm glad that you got it sorted out.
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