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Trailer Tail Lights Not Working

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  • Trailer Tail Lights Not Working

    The tail lights on my boat trailer isn't working. The brake lights and the turn signals work fine but no tail lights. I checked the white ground wire and it is bolted to the frame of the trailer but I noticed it doesn' run back into the plug-in...its just hanging disconnected. I'm going to wire it back into the plug-in and see if it fixes it. If it doesn't, what else could it be? A problem with a tail light wire (loose, torn, etc?) I'm hoping the ground wire is the problem???


  • #2
    Re: Trailer Tail Lights Not Working

    Ground wire could be the fix, if not, get a test light and check the tow vehicle plug and be sure you have power on the taillight terminal, next I would check the bulbs and while there if bulbs look ok, check for power at the light socket, check the ground for the light, make sure both are well grounded.
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    • #3
      Re: Trailer Tail Lights Not Working

      The ground wire (usually white) needs to be connected to the trailer. Don't rely on the ball and hitch to complete the circuit. It's preferable that it run to each individual light but most kits aren't designed that way.

      From there, the easiest place to start is checking with a volt meter to ensure the harness on the tow vehicle is working properly. Then pull the bulbs and see if they're getting voltage. Check the bulbs to make sure they're good. Polish all the connections (I like to use a rotary tool with a nylon brush) and coat them with dielectric grease. Check any connectors to ensure they're secure. At this point you've covered everything. Problem solved...but it's easier said than done. I have something wrong with one/both tail lights almost every single time I hook up. I actually grab a screwdriver, dielectric grease, and a volt meter every time I'm hooking up to leave but there's ALWAYS one that's decided to stop working properly.

      What I really need to do is invest in a good quality submersible LED lighting kit. Maybe you should do the same.
      People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf.

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      • #4
        Re: Trailer Tail Lights Not Working

        The brown wire in the trailer harness is the feed for both tail lights and any side marker lights. Yes -- the ground wire needs to be connected to the trailer plug and the mating side of the tow vehical harness must also be grounded. Have you checked the bulbs. Did you get a new tow vehicle? The reason for asking is the new vehicle may have separate turn and brake circuits which then requires a converter.

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        • #5
          Re: Trailer Tail Lights Not Working

          I didn't get a new tow vehicle but the boat was bought after the truck and the lights have always worked like this. I wasn't out after dark fishing so I thought the turn signals and brake lights would keep me legal until I got the time to fix the tail light problem. Plus, I'm going to make a lake trip this weekend and won't be back before dark so now is the time to do it. Like you guys said, I will check and test the bulbs as well. Theres a good chance that they could've blown when the man owned the boat before me...backing the boat into the water after the constant on tail lights have gotten warm usually blows them. I'm going to work on it this evening when I get home. I will let you guys know if I have any more questions. Thanks for everyone's input.

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          • #6
            Re: Trailer Tail Lights Not Working

            Weird that some work and some don't. What you might want to try is adding a dedicated ground from the trailer to the tow vehicle. I ran a 12G (or heavier) wire with a large alligator clip on one end and anchored the other end to the trailer. I clip the alligator end to my hitch. This provided a good ground circuit so all my lights work.

            U-haul has done this in the past with previous vehicles I've towed with so it might be something you want to look into.
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            • #7
              Re: Trailer Tail Lights Not Working

              Like silvertip says, the brown is your tail lights the green and yellow are your brake and turn signals. You can't light any of the bulbs unless the circuit is complete so since your brake lights work the ground is fine. It's the brown wire you need to look at for 12v when the vehicle lights are on.

              Just so you know, the white negative (-) wire is more commonly connected than not and it doesn't hurt to have it. There is nothing different between connecting it and dropping the ball on the hitch though, there's only one negative (-) pole on the battery and all negatives end up there eventually. If the vehicle has separate brake and tail lights, you'll need the adapter to combine them at the trailer or you'll need a 5 wire trailer wiring harness instead of the 4 you have now and different trailer tail lights with separate brake and signal lamps.

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              • #8
                Re: Trailer Tail Lights Not Working

                Originally posted by Jlawsen View Post
                Like silvertip says, the brown is your tail lights the green and yellow are your brake and turn signals. You can't light any of the bulbs unless the circuit is complete so since your brake lights work the ground is fine. It's the brown wire you need to look at for 12v when the vehicle lights are on.

                Just so you know, the white negative (-) wire is more commonly connected than not and it doesn't hurt to have it. There is nothing different between connecting it and dropping the ball on the hitch though, there's only one negative (-) pole on the battery and all negatives end up there eventually. If the vehicle has separate brake and tail lights, you'll need the adapter to combine them at the trailer or you'll need a 5 wire trailer wiring harness instead of the 4 you have now and different trailer tail lights with separate brake and signal lamps.
                I have to respectfully disagree with "There is nothing different between connecting it and dropping the ball on the hitch though, there's only one negative (-) pole on the battery and all negatives end up there eventually", there is a huge difference. Have you ever watched a trailer going down the road and the light blinking on and off, that is the result of no ground wire and just dropped on the trailer ball. Most electrical problems are due to a bad ground. Not trying to be hard on you, I just have too many years repairing wiring problems and could trace most of them back to a bad ground.
                To the OP, connect the ground wire on both ends, it is very important.
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                • #9
                  Re: Trailer Tail Lights Not Working

                  I fixed the ground wire but it didn't fix the problem. Tomorrow I'm going to test the trailer on a different truck because I've heard Ford F150s are bad about this and thats what I drive. I will also check the bulbs but can you reseal these type of lights after taking them apart? Its the waterproof sealed lights. I checked the tail light wiring under the trailer and everything appears to be fine. I may try to buy the little pig tail tester too.
                  Originally posted by nlain View Post
                  I have to respectfully disagree with "There is nothing different between connecting it and dropping the ball on the hitch though, there's only one negative (-) pole on the battery and all negatives end up there eventually", there is a huge difference. Have you ever watched a trailer going down the road and the light blinking on and off, that is the result of no ground wire and just dropped on the trailer ball. Most electrical problems are due to a bad ground. Not trying to be hard on you, I just have too many years repairing wiring problems and could trace most of them back to a bad ground.
                  To the OP, connect the ground wire on both ends, it is very important.

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                  • #10
                    Re: Trailer Tail Lights Not Working

                    I think the part that gets forgotten here is that you are assuming you have good wires and good connections, you need to measure each and every wire for continuity to its destination. I cannot tell you how many times I have seen a good looking wire turned out to be corroded within the jacket to the point where it failed.


                    Get a Volt Ohm meter and get after it. buy a 25' section of 14 gauge wire also we can teach you the rest.
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                    • #11
                      Re: Trailer Tail Lights Not Working

                      look under the hood of the f-150. theres a fuse that runs the tails under there. on the drivers side firewall. its a 20 amp yellow. seriously.
                      power distrubution center. fuse #6. if its blown replace and go boating. bet its a 98 or newer ??

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                      same thing had me stumped a while back. this was my problem and ill bet a cold beer its yours too.thats the power to the towing tails. the truck will work fine if its blown . brakes and turns on trailer will too. tails however will NOT.
                      sigpic17' spectrum 1700 fish and ski force 50hp 2012 channelcats 173 caught

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                      • #12
                        Re: Trailer Tail Lights Not Working

                        Yes its a 2002 F150. I'll take a look under the hood. I put my dad's 1990 F250 on the trailer this morning and everthing works but there is one tail light still not working. But with the truck's headlights on, one tail light was working.

                        Originally posted by catfishcarl99 View Post
                        look under the hood of the f-150. theres a fuse that runs the tails under there. on the drivers side firewall. its a 20 amp yellow. seriously.
                        power distrubution center. fuse #6. if its blown replace and go boating. bet its a 98 or newer ??

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                        same thing had me stumped a while back. this was my problem and ill bet a cold beer its yours too.thats the power to the towing tails. the truck will work fine if its blown . brakes and turns on trailer will too. tails however will NOT.

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                        • #13
                          Re: Trailer Tail Lights Not Working

                          I replaced the fuse under the hood and it fixed the problem...or it at least works like my dad's truck now (one tail light working and one not working). I could tell by looking that the fuse was blown. I will check the bulb and wiring for that one tail light. Thanks catfishcarl99...I feel like I'm actually making progress now.

                          Originally posted by wjhuskey View Post
                          Yes its a 2002 F150. I'll take a look under the hood. I put my dad's 1990 F250 on the trailer this morning and everthing works but there is one tail light still not working. But with the truck's headlights on, one tail light was working.

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                          • #14
                            Re: Trailer Tail Lights Not Working

                            awesome. glad to help. well at least you know theres power there now.
                            sigpic17' spectrum 1700 fish and ski force 50hp 2012 channelcats 173 caught

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                            • #15
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                              Re: Trailer Tail Lights Not Working

                              I have a 2002 Ford Supercrew so I will have to remember this tip just in case. Trailer lights can be a real pain in the butt so I hope you got yours fixed catfish.
                              I got tired of chasing around bad lights on my trailer so I bought a whole new setup rear and marker lights and sent a designated ground wire to each one and besides a blown bulb the other day I haven't had any issues in 4 seasons. Galvanised metal is not the best conductor so do yourself a favor and run those extra ground wires.
                              For the future, Make sure your plug pins on trailer and truck are not corroded and are making good contact.
                              Happy boating
                              Todd

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