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  1. #1

    Trailer lights not working properly but I know the trailer is fine, truck harness?

    I have a 2007 Ram 1500 with the towing package. I've hooked up a few trailers to it and the lights have never worked right, but I was never sure if it was the trailer or the truck until I bought my jet skis - the trailer has new lights and a new harness and it worked perfectly on the seller's truck but on my truck the 'lights' work (if I turn the headlights on on the truck the tail lights on the trailer light up as normal) but neither blinkers nor brake lights work. I'm guessing it's just the wiring harness on my truck. Is it likely I can just buy a new $15 wiring harness that plugs into the existing connector under my truck or is it likely something else more expensive is damaged, and is there any way to check for sure where the problem might be? I have a voltmeter. Thanks in advance!


  2. #2
    SurfRider's Avatar
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    In most of the newer vehicles they now have a separate bulb for Brake and Turn signals. The ski trailers use a "combined" brake and turn signal (single wire). If you vehicle has separate bulbs, then keep reading.

    My 99 chevy van is such. So what it has is a little black box that is called a "3 to 2 tail light converter". It takes the separate wires for the Brake and Turn signal bulbs and combines them into one wire. I had my left side go bad in that a few months back, so I had to replace it. I found a "heavy duty" on one on eBay for less than $10 shipped. I did not find any that cheap tonight. It seems odd that both would be burned out, but it is possible. You will need to trace the wires back from your harness that goes to the trailer and they should go to somewhere around your rear lights. The box would be installed between the two.

    The first thing would be to test the connections at the harness, on the vehicle, to see what does and doesn't work. The single male pin on the vehicle connector is the ground. Hook your volt meter or 12v bulb to that and then to the first female connector next to it. Turn on your parking lights and turn on your hazard lights. You should see a steady 12v for the parking lights (I believe is the 3rd hole after the ground) and a blinking for the left or right brake/turn signals. It sounds like you are only getting the parking lights, so that could be an indication that the black box is completely dead. Now what you would need to do is check both the brake or turn signal wires *before* the black box (where it is tapped into the cars harness). If you now get the blinking bulb (or 12v) and then you check the "combined" signal out of the black box and get nothing, you know that the black box is dead.

    Hope all that makes sense?

  3. #3
    I think that makes sense - I actually found a trailer light tester for $5 free shipping on amazon.com yesterday so I ordered that, so I will use that to test the actual harness on the truck and see if anything isn't working. I guess if everything lights up on my harness then I need a converter for the trailer 3 into 2? If some things don't light up then I should check the wiring "before" the box...although I don't know if my truck even has a box - it has the towing package and from what I understand the trailer harness actually is a separate cable piece that plugs right into a pinned plug under the truck, but I haven't looked yet. I will test the wiring at the harness first and see what happens.

  4. #4
    SurfRider's Avatar
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    Yes that sounds correct. I guess it depends on how your truck was setup from the factory/dealer. That black box may be part of their "towing package", although it is possible that the manufacturer had built something in. But usually in order to save 2 cents, it would only be added if you were doing a towing package. They wouldn't put it in every truck if it may not be used. Does your truck have a 4 way or 7 way? Electronic brake controller? Even still, that really shouldn't matter. Do this. Get someone to press your brakes. See what bulb lights. Next turn on the turn signal. Same bulb or different bulb?

  5. #5
    I'll try all that when the light tester comes in. My truck came with a trailer hitch underneath the bumper, straight from the factor, plus a 4 way connector underneath the truck and a 7 way connector (I assume, never had to use it) built right into the bumper. I don't think it has any special electronic related braking or anything though.

    Right now if I hook the trailer up to the 4 way connector, I only get running lights and nothing else (even hitting the brakes doesn't make the lights change, and if I turn off my headlights nothing lights up on the trailer at all no matter whether I use the blinkers, the brakes, or anything else).

  6. #6
    I have a similar problem with my 03 Ram 3500 and from the research I had to do for mine, your Ram has a Totally Integrated Power Module (TIPM), which has circuits for trailer lights separate from the truck's lights. From what I gather, if the TIPM breaks a circuit, it has to be reset electronically by the dealer, for a fee of course

    Mine doesn't have the same TIPM as yours, but it uses separate fuses for the trailer lights. The problem I have is is that the fuses were stronger than the relays and circuit board in the fuse block can handle, so either relays or parts of the board burn up before the fuse blows. In all cases, the TIPMs are expensive to replace if necessary.

    What a lot of guys are doing is using an isolator (I just ordered a Modulite Ultra controller) that only draws milliamps off of the vehicle circuits and isolates them from shorts or overdraws on the trailer lights.

    Hope this helps...

  7. #7
    Alright so I got the tester in today and tested the trailer harness on my truck and everything is fine, but the only lights that work on the trailer are the running lights - the blinkers and brake lights dont work (but they were working on the sellers truck) so I'm guessing I need the 3-2 converter. Heres one I found on amazon, I'm guessing this will be OK?

    http://www.amazon.com/Curt-Manufactu.../dp/B001EP0GI6

    Also, how do I connect it? Do I have to actually splice it into new connectors or is there a way to 'easy-plug' it into my existing harness or what? Thanks!

  8. #8
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    All 4-way connectors all work (are wired) the same. Now what did you test with this tester? Did you test the truck end or did you check the trailer or both? I'm not clear on what you did. Do not order anything yet. JZ said that the RAM trucks may have their own custom electronics. That doesn't mean that you cannot rewire it differently though.

    If your truck tested correctly, then it would have to be your trailer. It is possible that the trailer has a bad ground and that is what needs to be fixed.

  9. #9
    I just tested the truck (basically hooked up a 4 way 'tester' into the truck and the left light came on on the tester, hit the brakes all 3 lights came on, then left and right blinker second and third light blinked), not sure how to test the trailer, though the tester I got has plugs on both sides, not sure what plugging it into both the truck and trailer at the same time would do. Any idea how to test the trailer or the ground on the trailer?

  10. #10
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    I guess you'd have to check instructions with the tester, but unless it can supply 12v, it probably cannot test the trailer.

    Almost all wiring harness have the ground wired up directly to the trailers frame. The only exception I have seen to this is the Triton Aluminum trailers. So on the harness that plugs into the truck, you would have to look at the wire that goes to the single female pin and see where that is attached to the trailer frame. It may be pop-riveted on or it may have a crimp ring that is bolted on. That would be the first thing to check. Then each of the tail lights would also have their own ground wire attached directly to the frame.

    My guess is that it's the ground on the connector end due to the fact that neither light is working properly. The parking lights can sometimes get a false ground thru the brake/turn wires. So that would be the first thing to check. You could probably take a set of jumper cables and attach it somewhere to the truck where you can get a good connection and then put the other end on a metal part of the trailer and see if the lights work then. It may have been possible that the other guy made a ground between the trailer hitch/ball and to the frame.

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