Diagnosing and Repairing Trailer Lights and Wiring

For most consumers, trailer wiring repair can be a frustrating experience. Diagnosing the root of the problem early in the process is the key. I recommend a 12 volt stick-tester to check your wiring and plug for continuity as the first step in any electrical repair. Plug testers are available for all types of trailer plugs, but a stick tester can be used on any receptacle and can test wiring between connections. And they typically cost less than ten bucks.

First, with the trailer wiring plugged into your tow vehicle, turn on your running lights. I recommend doing this with the hitch not connected to the trailer coupler, for reasons we will explain later. Do a walk around the trailer and check to see if all lights are lit. If your tow vehicle blows a fuse during this phase, first remove your trailer tail-light lenses and check for blown bulbs (usually black or creamy white inside if blown), and also check to make sure the bulb is in correctly. The industry standard for trailer tail lights is an “1157″ bulb, which is installed by pushing into the light socket and twisting it so that both contacts rest on the tail-light prongs. If the bulb is only “half-twisted” in, then both filaments will light, as the bulb’s two contacts will touch both tail-light prongs. This will create a dead short when a turn signal or brake is used with the running lights on, and can also result in all marker lights blinking or illuminating along with the turn signals or brake lights. Next check behind the tail lights, and then behind each marker light to see if the hot leads to any of them might be crushed between the light housing and the trailer frame from improper installation. This is a common problem, prevalent on new trailers. A tail light wire could have been crushed behind the housing during installation, but the inevitable short may not surface for long periods of time, perhaps when moisture gets behind the light and completes the circuit. Most lights are connected to the trailer with ¼” bolts and will require a 7/16” wrench to loosen the nuts. Pull the housing away and make sure the wire(s) are run through the provided channel behind the light to avoid smashing them on re-install. If any wires are exposed, then replace that section with new, insulated wire. On most trailer lights, 14 to 16 gauge wire is sufficient.

With the running lights still on, turn your ignition key enough to operate  turn signals and put it in “left” or “right” turn (NOT hazard flashers). Do another walk around and check each turn signal. Are any running lights flashing? If so, and you have already checked the bulbs and wiring on the previous step, then you probably have a bad ground. The ground problem could be on the tow vehicle or trailer, but sometimes you can get a sporadic ground through the hitch ball and coupler connection, although it isn’t a stable enough ground to rely on for lighting and electric brakes. Unplug your trailer and connect your light tester “ground” to the tow vehicle ground terminal. This is the “male” post on a 4 or 5-way flat plug, or at roughly “7 o’clock” as you look into the 7-way plug typically included in factory hitch packages. Turn on running lights and a turn signal and test by putting your stick tester to the terminals. On a 4 way, tail lights are the brown wire, yellow and green are left and right turn/brake respectively. On a 7-way, left turn/brake is at “9:00” and right turn/brake is at “3:00” and running lights are at “11:00” or above the left turn. If all functions are working, move on to the next step. If not, then ground your tester to the tailpipe (the hitch sometimes has too much paint on it to get a ground) or steel bumper and re-test. If the tester is lighting now and didn’t light using the vehicle plug’s ground, then you need to ground the tow vehicle plug. Check for a broken wire coming out of the plug. The ground should be a white wire. Ground this using a wire “eye” terminal directly to the frame using a self-tapping screw and re-test. If no broken connections are found, then open the plug (if possible) by removing the screws in the side of the housing and checking all wire connections to the terminals. On a 4-way, this will not be possible. Some vehicles have 7-way plugs that will not be repairable in this fashion and must be replaced. 4-way plugs sometimes lose their connections inside the molded housing and are not repairable.  Replace as needed. If I am not getting a test light at the terminal, I like to stick the pointed tester into the wire leading to the rear of the plug to see if there is any signal going to the plug, while grounding to a reliable ground source (such as a tailpipe).

If some of the functions in the plug work, the ground is likely fine and you need to check wiring and fuses going to the plug. On aftermarket installations, some vehicles (those with separate turn and brake lights) incorporate a “converter box” to merge those functions so the trailer lights will operate with either turn or brake using the same wire. It is essentially a box of diodes, and these can and do burn out. Test all functions going “into” and “out” of this converter box. If you have turn and brakes (separate functions on this type of vehicle) coming from the vehicle into the converter box, but not working coming out of the box (and merged with the turn signal function) then you have a bad converter. Snip the wires and splice a new one into the wiring harness.

Many vehicles have factory tow packages, which incorporate a separate fuse block for towing functions. This system is preferable, in that a blown fuse from malfunctioning trailer wiring will not affect the tow vehicle lights, which are on a different circuit. If no signal is reaching the plug, check the owner’s manual (usually towards the rear of the manual) to locate the towing package fuse placement and pull them out one at a time. There is usually a separate fuse for running lights, left and right turn (and 12v hot, electric brakes, etc. as needed). Most tow package fuse blocks are located under the hood. All tow packages are already converted if tow vehicle has separate turn and brake functions.

Once the vehicle wiring is deemed to be operating, it is time to plug it back into your trailer and take another look at the lights. If trailer lights still flash on and off with tail lights and a turn signal on, then the trailer has a bad ground. Check the white wire from the plug and make sure it is properly attached to the frame, usually near the coupler. Note: On tilt-bed trailers, the lights are usually attached to a frame that is NOT permanently attached to the tongue of the trailer. A ground wire needs to be run either directly to the trailer light attachment bolts from the plug, or at a minimum the wire needs to be run to the “tilt-bed” part of the trailer. This is the only way the lights will be able to use the tow vehicle’s ground on this type of trailer. Although a sporadic ground is sometimes transferred to the tilt-bed, since the bed moves independant of the main frame, the ground will not be consistent.

For more information, please visit our website: http://www.rwtrailerparts.com

or e-mail me direct: rob@rwtrailerparts.com

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119 Responses to Diagnosing and Repairing Trailer Lights and Wiring

  1. Jason says:

    A lot of great content.. you should consider breaking in down into many blog articles.. Nice work!

  2. Sean Chandler Sr. says:

    Very informative I wouldn’t change a thing.
    I am master trailer tech myself.

  3. Steve says:

    What do you recommend when I have tested the vehicle wiring and it’s good? But on the trailer, only the tail lights come on. No turn signals or brake lights, I have put a new 7 way plug on the trailer and tested each wire in it to make sure they match up with the truck’s outlet, and then made sure to connect each of the wires to the corresponding wires on the trailer. I’m no expert, but I’m at a loss for what to do next.

    • So, you’ve checked the truck plug to make sure the signals work on it? Most factory wiring packages have separate fuses for the trailer turn functions, and although the vehicle’s lights may work, the trailer’s turn signal fuse(s) could be blown. I would first check the truck plug, and if it works, check the turn signals before and after whatever splice you used to connect the 7-way to the trailer (with it plugged in and the truck signals working). If it’s good there, then test behind the tail light to see if there’s power going into it for the signal lights. If there is, you may have bad bulbs. The turn signal is a different filament from the tail light and could be bad (assuming your lights are not LED). If it’s an older truck, say 20 years approximately, it could be that your flasher needs to be upgraded to power the additional lights. Not relevant if you have a factory towing package that you’re plugging into.

    • sean chandler sr. says:

      Check to see if your vehicle has separate turn signals and brake lights, it might require a converter.

  4. sean chandler sr. says:

    Check your bulbs on the trailer, incandescent bulbs are notorious for filament failure.
    If ur running LED’s then check for good clean ground usually a white wire attached to the light bracket.

    • Mezza says:

      Picked up my trailer today which has been fitted with new LED’s. Tail lights work, left indicator doesn’t work and the right indicator is applied the hazard lights come on!!!
      How do I tackle this one…

      • Trailers don’t really have hazard lights. The turn/brake lights flash using the turn signal wire in your plug. I suspect that someone wired your trailer plug wrong. Most trailer harnesses use yellow for left turn, green for right turn and brown for running lights. If someone installed a 7-way RV-style plug, which comes on most new pickups, they may have tried wiring it color-to-color to the code inside the trailer plug. On the 7-way RV, red is left turn, brown is right turn and green is tail/running lights. So your LT wouldn’t work and all your marker/tail lights would flash if you put on your right turn signal.

  5. Diane says:

    I have a very strange situation i have never seen before. My Trailer lights work when the car is off/in Aux, but when the car engine is on, all the lights, including the car lights don’t work. i have a 7 straight plug on the car, and a round on the trailer.
    Any suggestions?

    • sean chandler says:

      Sounds like your trailer plug is miswired, right turn and tail wires are crossed. Is your plug a 7-way round?

  6. Diane says:

    I have just recently purchased a 1987 Jayco Dove Caravan. I am having some issues with the brake lights. It seems whenever we plug the carravan into the car, and press the brakes, they short out the caravan and car brakes. i have not experienced anything like this before so did some nvestigating. As we purchased a new flat to round plug, i checked to make sure all the wiring on the car side was correct, then opened up the caravan plug and checked to make sure it all matched. Well, it did. Then i opened up the caravan tail lights and cleaned all the connections and checked the wiring. all seems ok. The only thing i did noice was that there is no direct earth wire on the tail lights, so i am assuming they somehow connect back to earth thru the van itself???? Anyways cleaning the wiring, etc did nothing to stop the shorts. then i had the RHS brake light globe out on the van when i accidetally asked my husband to depress the brake again..! sure enough the LHS brake light started working normally??? so I changed the globes over, and it shorted again… now the wierd thing is, there are two red wires going from the terminal under the van to the brakes lights. The RHS light has 2 red wires, the LHS light only has one. So I removed one of the wires to the RHS light and both lights went out. then I replaced that wire and removed the other wire on the RHS light and now the RHS brake light works correctly???
    I am very confused as i cannot see where the wires are going and cannot determine what wire is doing what and why it is shorting out the entire brake system.
    Any suggestions would be welcome.

    • Trailers in the US are wired so that the turn/brake lights are the same function. If that were the case, I would try running a ground directly from the trailer plug into the trailer frame (usually a white wire). That usually firms up ground problems on most trailers. If they are separate functions, I would start with the tow vehicle plug and test all of the functions (using the vehicle plug’s ground) with a stick tester, then plug in the trailer and test the wires coming out of the back of the plug and work my way back from there. When you say they shorted out, is it blowing a fuse? Sometimes that’s as simple as a pinched wire behind one of the lights or a bulb not installed properly.

  7. glenn cremeans says:

    My camper lights alk work. But when I try left or right turn signals alm the lights flash like hazard lighfs. And when I apply the brakes all the camper lights go dim. Please help

  8. Classic case of a bad ground. In fact, that’s the method I used to employ to check for a bad ground. Make sure there is a ground going from the trailer plug into the frame. The bad ground could also be on the truck side, but it’s less likely. If your ground is on a breakaway switch, move it. Those switches tend to pivot on their attachment bolt and loosen the ground wire if attached.

  9. I have an xc90 2005 and a coachman 2015 all was working fine on my old tow car but this new to us xc90 came already wired with a 7pin plug. When we attach the caravan electrics the side lights on the van flash on then go off and it knocks out the cars tail lights. have rewired both caravan and car plugs and tested both work fine all in correct order etc. I tried a friends old caravan (2002) with traditional filament bulbs and it did exactly the same flashed then knocked off. we drove a short distance without the tail lights (brake, indicators and fogs all worked) but after a few turns the left indicator failed. Turned on/off to reset but same thing happened after a few left turns.I am tempted to have the car converted to the new 13pin to match van but am worried that the fault may be in car relays, any ideas?

    • Just a thought, but maybe the xc wasn’t wired with the right kind of converter? Those vehicles require a power converter, which draws directly from the battery to power the trailer lights. It’s possible that it had been wired with a standard converter, and since most modern vehicles use very thin wire to power their lights, those same wires cannot usually handle the load of a lot of extra trailer lights (especially incandescents). If you have a power converter, maybe the fuse to it has blown? Or the converter may have gone bad.

  10. Jerry LaHood says:

    On my bumper pull horse trailer (with several running lights all around the trailer) the running lights fuse keeps blowing. With a fresh fuse the running lights work for awhile (except for two that are burned out) then another blown fuse. Could the two burned out lights cause this fuse to blow? All other lights work fine. Thanks for your help.

    • It’s possible, maybe a bulb blew in a way to create a dead short. If you replace the bulbs and still have problems then you should start looking for pinched wires behind any tail or marker lights. Sometimes a pinched wire can go for years without grounding out to the frame, then one day the conditions are right (more wear or lots of moisture maybe?) it starts blowing fuses. Also looked for frayed or pinched wires on the trailer wiring harness. I start at the plug and work my way back.

  11. randall c says:

    My interior 1157s are the that get brighter then dimmer sometimes but why?

  12. Cbourque says:

    We have a 1999 Chevrolet Silverado, pull a trailer for work…when the trailer is plugged in and headlights are turned on a 15A fuse blows (under hood)…but when trailer is unplugged fuse does not blow. Have checked all connections and wires everything is good. All lights on trailer workcorrectly when headlights are off . Anysuggestions?

    • So, does it have a trailer wiring package, and is it the trailer tow package fuse that’s blowing? That would be on a different fuse than the headlights. If it was wired from scratch, then the headlights and running lights (truck and trailer’s) would all be on the same fuse and the trailer lights might be just too much additional amp draw, if the trailer has incandescent lights. I would also check behind each tail and marker light for a pinched wire, and the tail light bulbs to make sure they’re twisted in correctly.

      • Cbourque says:

        It was working fine, then a few days ago the lights on the trailer weren’t working checked fuse (Parking Lamps Trailer Wiring) and it was blown.

  13. Then I would remove each light (tail and marker) one at a time and check for a crushed wire. Sometimes wires get pinched during install but the insulation keeps the metal from contacting the frame for a while.

    • Cbourque says:

      The brown wire for the running lights that runs in the trailer has a short in it. We left it unconnected and capped the ends. Next day off we will rewire

  14. Lance says:

    Excellent article. Bookmarked for future reference. Is there a top shelf wiring connection you would recommend? I currently have the large pin round style, but it is quite old and ready to be replaced. Are the blade styles better? Any brands that have the best reputation? I’m guessing my problem stems from weak ground as well, as the drivers side light on trailer keeps going dim or out. I sand-papered to the raw metal on the area on the fender it grounds to, and that worked for a while, but it continues to eventually go out again. Could be another ground issue as well, or the fender itself is a weak ground vs. the frame. Thank you! Great stuff here!

    • I would go with the blade style simply because it has become the industry standard. You rarely see the round pin 7-way except on tractor trailers now. I like the Pollak 7-pin connector because I think it is easier to wire than other brands I have used. I used to use Bargman but I think it is harder to wire them. I do use molded pre-wired 7-way Bargman plugs though if you’re replacing the cord too.

  15. Roger Watson says:

    I’m having trouble with my turn signals on my travel trailer , they seem to work fine when the running lights are not on. However when they are on the second filament blinks at a low illumination on either side . Got any suggestions?

    • Bad ground, probably on the trailer end. Trace the ground wire and make sure there is a solid connection directly to the trailer frame.

    • Sean chandler Sr. says:

      I concur, travel trailers are notorious for bad grounds especially at the tails and turns.
      Check for loose ground wires inside the light base.

  16. My left turn signal doesnt work on my trailer. All other lights function as they should including my left tail lights on the trailer with the exception of turn signal function. 7 way plug is new(I didnt have to rewire it, just unplug existing one and plug in new one), all lights and harness is new. Truck has what looks to be factory installed 7 way plug.

    One thing I noticed last night I noticed that I have a bulb on the left side that is blown. Im supposed to have a bulb in the lower section of my tail light housing and its not working. If the bulb is blown would this cause my trailer turn signal function on the same side to not work?

    • The bulb is blown on the truck, or the trailer? In any case, if it has a factory package then the truck signals and trailer wiring are independently fused. All I can say is, the left turn filament on the trailer could be blown and everything else on the trailer would work just fine. Start at the truck and test the 7-way to see if the left signal is working there, and then follow the wire back to the trailer tail light.

  17. lew says:

    I love this site! Thanks for your help. I have a landscape trailer with a 4 flat (5 wires coming out). Vehicles all have 4 flat with four wires in and do not have separate brake/turn lights.Grounds seem good on vehicles and trailer. Everything works fine until I reconnect the brake light bar at the end of the trailer and turn the running lights on. . At that point, all lights turn bright as if brake is activated. (The brake light bar came wired to the brown on the driver’s side of the trailer). All other functions are normal. With the running lights off and the brake depressed, the brake bar does not light. Am I wrong in thinking that the brake light bar should only be illuminated when the brake is depressed (as vehicles operate)? If so, what do you suggest I do?

    • That 5-wire plug is a manufacturer’s harness made to split the running light to run down both sides of the trailer. You have a 4-way plug. The brake light bar you’re referring to is probably your ID light bar, which works on the tail/running light circuit. It shouldn’t light up when you hit your brakes. It is a federal requirement for trailers over 80″ in overall width.

  18. Jodi says:

    I have a tow trailer I put all new wiring on it once I hook it to my truck everything works I step on brake nothing works.. I tried everything

    • Bad ground! Check the ground (usually white) going from the trailer plug to the frame for a firm connection. Also make sure the lights have good metal to metal contact if they are self-grounding. If all the lights go out, it’s probably a bad ground at the front.

  19. Robin Barger says:

    I have a little trailer and a bigger trailer and am having the same problem on both I can not get the bottom part of the 1157 bulbs to light. I have running lights on both but turn signals only blink on the running light part of the bulb I have the ground drilled through the trailer and bolted tight. I even soldered the wires together. So when I step on the brakes I lose all lights on trailer also I have tried 4 trucks my chevy and a friends chevy both did same thing we both have factory harness then we tried a brand new ford and got no lights at all on the last two trucks. Help Me Please

    • I still think you have a ground issue. Stepping on the brakes and losing all your lights is a classic sign of a bad ground. It could be inside your plug, possibly a disconnected wire. Also, the ground coming from the plug should connected directly to the frame. If the lights are on a tilting frame, they try connecting a ground from the main body to the tilting framework. Also try prying the trailer’s ground terminal so that it gets strong contact with your tow vehicle plug, and also try some electrolytic grease on the ground terminal.

      • Robin Barger says:

        thanks so just befour I read this I ran another ground from the trailer ground to the back part of the trailer frame and wow all lights are working now. You were right it was ground and yes it has a tilt part that in never use its all solid but I guess the ground didn’t travel through frame good enouf. thanks sooooo much.

  20. mike says:

    i have 6 newer dump trailers- all work fine except my doolittle master dump blows the tail lite fuse in my truck- all other lites are fine on the trailer- i tried the tail lites without the hitch connected and they worked until i connected the hitch to truck– then it blew the fuse again. any suggestions?


    • Does the truck have a factory tow/wiring package? And are the trailer lights all incandescent or LED? I’m thinking maybe if they are incandescent then the draw could be too great on the truck non-factory-package tail light circuit. Maybe the ground is weak and when you connect the hitch to the ball, it gets a ground and then blows the fuse because of the amp draw.

  21. Randy says:

    I have a 2014 6×12 enclosed utility trailer and live in North Dakota. This winter has been heavy snow and wind. Recently the trailer blows the fuse in the truck. I have trouble shot it down to a short in the marker/tail lights. The problem is that when I isolated each brown wire running down the frame right aft of the 4 pin connector, both sides show grounded. Looking at a diagram each side should be independent from the connector split back. What would cause them both to short at the same time when they should be independent of each other? Thank you.

    • Did you separate the wires from the 4-way plug? Most manufacturers harnesses have 2 brown wires, one for each side of the trailer for running lights, but both wires meet at the 4-way plug itself, so no matter where the short is, the running light wire is basically shorted everywhere. I like to test the wires independently to determine if the short is on the driver or passenger side, eliminating half of my work, then if one side shorts I start looking for pinched, cut, or chafed wires. Don’t forget to check behind marker lights because often the wires get pinched between the base and trailer skin and don’t blow fuses right away, but after a period of time and most often when conditions are moist.

      If you cut the wires from the 4-way and both wires continue to blow a fuse, then unfortunately you may just have shorts on each side. Are you sure your ground is good?

      • Randy says:

        I’ll checked the ground and it was good. And both side trailer wires (brown & brown/green) have been cut from the plug and tested seperately and both are grounded somewhere aft of the hitch frame as far as I can determine. Both go up and inside the front wall beams right aft of the hitch frame. Just really confusing that both sides are bad at the same time when they are independent of each other. Going to check one additional spot then looks like I’ll be running new wires.

  22. Tyler says:

    I just put new lights on my trailer and all the lights, turn signals and side markers work except the left taillight doesn’t. What could the problem be

    • The problem has to be in the taillight. Either a bad bulb, a bulb that has been inserted incorrectly, a bad light socket, maybe even a bad ground at the light. They usually get their ground thru the bolts that attach it to the trailer. Check to make sure there’s metal to metal contact at the attachment bolts. If the tail light has a side marker that’s working, then the bulb or light socket must be bad.

  23. Ron Brown says:

    Left rear tail light wire broke on my trailer I fixed it back just like it had been then I fixed the ground wire on the plug in wire at front of trailer grounding onto the trailer. When I turned on the lights the left turn signal on my chevy 2500 wouldn’t blink but all the lights were blinking on the trailer when I turn the right blinker on it works fine? What did I do?

    • It sounds like you may have wired the turn signal wire in the trailer harness to the running light wire in the tail light. That would cause all your lights to blink when left turn was on. Maybe it wasn’t blinking on the truck itself because of the huge amperage draw going to all the trailer lights.

  24. V Nicks says:

    Small 5X8 trailer about a year old- My right signal does not work (brake or signal) and my left signal works fine. I have checked the tow vehicle’s plug with both a plug tester and a voltage tester, and I get a good signal. I have checked the right side wire all the way down to the light, and get voltage from both signal wire and brake wire. As soon as I connect the wires to the light fixture, the signal stops. I have tried three different light fixtures to test, and the same thing happens. During the course of this I replaced the tow vehicle plug, the trailer plug, and all the trailer wire as well as brand new lights – but nothing I do will make the right signal work even though the power is getting to it. I *know* it’s got to be something with the ground – but it’s grounded directly to the trailer, and the other side works!
    Any help would be greatly appreciated!!!

    • How does it get its ground? Thru the light housing’s attachment bolts? If so, you can try cleaning out the holes on the trailer’s light bracket to give strong metal to metal contact when you attach the light. I like to jam the bolts hard against the side of the hole they’re going into to try to maximize the ground connection. You can also try running a new ground from the trailer frame to the attachment bolts so it won’t rely on the bracket to get its ground.

  25. V Nicks says:

    Yes, it grounds through the attachment bolts. I actually took the light out of the bracket and ran the whole thing across the trailer and hooked it up to the other light’s bracket to see if that would work, lol- no dice.
    Oh, a wire directly from the frame to the bolt! I’ll try that today! Thank you!

  26. V Nicks says:

    OK – so I connected the bolt of the right signal to the trailer with a ground wire – and nothing happened. I went back to test back at the car again (starting back from scratch) and noticed something that I didn’t notice before. When I connect the voltage tester’s ground to the plug’s ground connector, the right signal connector on the plug lights up the voltage tester. When I connect the voltage tester’s ground to the car itself, clipping it to the frame, the right signal connector on the plug *doesn’t* light up the voltage tester. Something wrong with the vehicle-side connector’s ground? If so, though, wouldn’t that affect the left signal as well?
    I’m beginning to think it’s gremlins! Or that I’ve lost my mind. LOL

  27. Maybe you should run a ground straight from the plug to the tail light, since that ground is working. Sometimes I can’t find a decent ground clipping to a vehicle frame. Either rust, or undercarriage coating, or sometimes the body is non-conductive on some newer cars.

  28. BrokeDad says:

    I just bought a used 2000 KieferBuilt Alpha Xl4 – 2 horse, 2 axle, slant load trailer. When I turn on the running lights, or the 4-way hazard lights, the electric brakes engage. I’ve done a lot of troubleshooting including pulling bulbs and using my multimeter to check resistance on all the plug leads. It boggles my mind, but every wire in the plug seems to be grounded? I suspect a massive short in the main cable, but haven’t torn into it yet. Dreading it, honestly. What troubleshooting methods can I use to isolate this problem??

    • BrokeDad says:

      btw the 2 sidemarkers on the fenders do not light up. I checked them and they are not defective.

    • You’re probably right about a short in the cable. I would look for an area where the cable is crushed or worn through, maybe it has been dragged or cut/crushed by the trailer jack when unhooking. Also, if the plug is able to be opened, look to see if the ground wire (or other wires) might have come loose inside the plug and touched another terminal. Sometimes those plugs can fill with water, which could cause a short, or severe corrosion. Follow all the wire that you can see. If there is a short it is usually obvious. Look for wads of taped wire connections, frayed or damaged wiring, etc. If the visible wiring looks good, try disconnecting one side of the trailer’s wiring if that’s possible to figure out which side the short is on to narrow your search. After that, start pulling lights to see if there is pinched wiring maybe behind one of the lights that would short into the trailer frame, causing the brakes to also activate, although not at full strength.

      • BrokeDad says:

        This is good advice. Thanks I will take a look at the main cable from the trailer hitch to where it’s a merges into the frame. There is evidence of heavy taping there. Also note that I replaced the original 6 way connector with a 7 wire connector to fit my truck. Note that the problem existed before I replaced the connector and was still there after I replaced it. The previous owner had used an adapter and I thought maybe that was the problem so I carefully replaced it with a native 7 wire connector. I was very careful to not have any loose wires touching each other. When I called the previous owner to see if she had any problems with it, she told me to just adjust the gain down on the brake controller. That was an indication to me that she did have a problem but unwittingly was just adjusting the brakes to not engage. I don’t have a brake controller yet so that is not an option for me. Thanks.

  29. I’m wondering if maybe no ground at all might cause feedback to the brakes if any lights are turned on. I used to check for a bad ground by turning on running lights and one turn signal. If all lights pulse with the signal flash then you have a bad ground between the tow vehicle and trailer. Check to make sure the trailer plug has a strong direct ground to the trailer frame. I like to run the ground for my brakes to the same bolt I use for the trailer cord. Do not ground it to the breakaway switch bolt, as those switches pivot and lose their ground. If all looks good on the trailer side, maybe check the tow vehicle ground. Factory tow package grounds rarely fail.

    • BrokeDad says:

      I may have a breakthrough…I called the Kiefer Built factory, I spoke to a woman in customer service and parts named Rhonda who emailed me a note, “This is a wiring diagram for a 6 prong plug for trailers before 11/02 . Hope this works for you.”
      The wiring diagram she sent clearly shows the BROWN WIRE CONTROLLING THE TRAILER BRAKES AND BLUE WIRES BEING GROUND. I can tell you that I wired opposite of that just because that’s the modern common way to do it. And the existing converter plug the previous owner had installed would have done the same thing. Tomorrow I will switch those wires and report back…fingers crossed!

      For the record here’s the wiring colors per that diagram assigned to the original 6 PIN PLUG using their exact language:
      RED WIRE – TERMINAL (TM) (I wonder if they meant Trailer Markers?)

      I wish I could post a picture here? If you are looking straight at the 6 pin TRAILER plug the relative positions look as follows:
      (top) Red
      O(top left) Brown (top right) Blue wire
      (center) Black
      O(lower left)Green (lower right) Yellow wire

      For those who come after me seeking Kiefer information: The factory has changed owners a few times. Check this link out http://kiefermfg.com/who-we-are/ and/or use the number below:
      Customer Service and Parts
      Kiefer Mfg and Sales, LLC
      2276 Back Valley Rd
      Sneedville, TN. 37869
      PH.(423)733-0000 Ext.315

    • BrokeDad says:

      rwtrailerparts: I think your insight about no ground at all is consistent with the ground and trailer brake wires being crossed. We will find out tomorrow…Thanks.

  30. So, if your 7-way is wired to the normal code, then you should connect it this way:
    Trailer YELLOW to plug RED
    Trailer BLACK to plug YELLOW (if it is indeed backup, my guess is that it’s a 12v hot wire, in which case you would connect to plug BLACK
    Trailer BLUE to plug WHITE
    Trailer RED to plug GREEN (TM is tail/marker)
    Trailer GREEN to plug BROWN
    Trailer BROWN to plug BLUE
    That’s a funky wiring code. No wonder you had trouble following it.

  31. Stuart says:

    I am having a heck of a time adding a new 7-pin trailer receptacle along with a toad recepticle from REMCO. From the RV that has amber turn signals I have:
    BRN – TAIL 12V
    YLW – L/TURN
    GRN – R/TURN
    WHT – GRND
    I have the red/yellow/green/brown and white connected to a BRITE-LIGHT Roadmaster converter so only brown, white, yellow and green come out. When I connect this to my trailer 7-pin (The trailer works fine when plugged into my Excursion and last RV) the tail & turn work great, when the brakes are pushed it only lights the left on the trailer. I swaped out the converter two times with good units I took off other trucks and the same thing happens. I ran a new ground(RV side) and hooked both recepticle grounds and converter ground to just in case to no avail. The trailer has led lights with no resistors so I thought I would put the 1157 standard bulbs back in or add resistors. What is your thought?

    • It sounds like you have it hooked up correctly. If Y,G,R all work going “into” the converter and only Y works when the the brake light (R) is activated, then I would assume I had a bad converter. If turns and tail work otherwise on the trailer, then it isn’t a wiring issue. You also hooked up the 12V wire on the power converter?

  32. Jon says:

    I have a weird problem with my trailer maybe someone can help with. I was pulling my trailer awhile ago (haven’t had time to look at it for awhile) and I either applied the brakes or put a turn signal on, I can’t remember which, and all the lights in my car went crazy. Blew a couple fuses and blew the wire harness on my vehicle. So I took it to a shop and they replaced the vehicle harness and some fuse box in the back of my truck. Took the trailer out again, and it did it again (luckily the shop put a second in for free because I thought it was the work they did) now I think it’s my trailer and I don’t want to hook it up again. How can I test the various wires to determine where the problem is? Can I use a multimeter somehow? I visually inspected the wires and didn’t see any frays. Thanks.

    • It sounds like they may have installed and replaced a converter box. Those things have been known to fail and catch fire even if installed correctly, but they can fail quickly if the trailer wiring has a short in it, which heats up the diodes in the converter and will melt the box itself. To check the trailer wiring, really all you need is a low amperage battery. Ground it and test each wire independently. If there’s a short then you will get a larger spark and the wires will start to heat up rapidly. I have a Hopkins Tow Doctor test kit that tells me right away if there’s a short or ground fault in my wiring.

  33. Peter Maryschuk says:

    I just ripped out all the old wiring on my utility trailer and installed a new Y harness system. Brown and green wires to the right tail light. Brown and yellow wires to the left tail light. Attached the ground wire (white) to the frame of the trailer. Plugged in the four way connector to the SUV and presto–nothing. I even installed new tail lights on both sides and both were working in the store before I brought them home. Any suggestions? Thanks

    • Is this a tilt bed trailer? I’m guessing the ground isn’t being carried to the lights, either because they are on a tilting frame or because the lights do not have good metal to metal contact through the attachment bolts.

  34. C Lester says:

    I just bought a dump bed/utility trailer that was just built. I can’t get the running lights to work on the trailer,but the turn signals, brakes, and I ran the hazard flashers at night coming home with the trailer. I replaced the high current fuse on my truck-ford f350- As soon as I turn the lights back on,it blows the fuse for the running lights. Also have a warning light come on saying trailer brake control module fault. The trailer brakes were working at first, but not now. I know the problem is with the trailer wiring because I replaced the fuse and hooked up the light plug from my camp trailer, and everything works as far as the lights,turn signals, etc. Any ideas what could be causing this? Thank you

    • New trailers are notorious for light failures. Sometimes it’s a wire pinched behind one of the tail lights during manufacturing that has finally worn through the insulation. That wouldn’t explain the brake problem, which could be either a disconnected ground or brake wire somewhere from the inside of the trailer plug to the first braking axle. Turn and brake signals will work pretty well with a poor ground, but not tail/running lights. I’ve seen running light fuses fail when there is no ground from the trailer plug to the frame.

  35. Bertha says:

    i’m in the process of purchasing a used utility trailer however the left signal light is now working and wondering if it is a big expense to get it fixed the owners are not saying what the problem is claiming it was working.

  36. Jeff says:

    I just rewired my trailer and the lights were working fine initially. I just noticed that my left blinker on my trailer and on the vehicle are not working – the vehicle light also does not work now when the trailer is not attached. Could this be a ground wire problem?

  37. herbjanes says:

    My trailer lights worked fine for years, then stopped. I’m trying test/verify the power coming to the 4-way connector. if I disconnect the connector and then circuit test the 4 bare wires i get these results:
    White – always power detected (I’m not sure this is valid … is this a ground wire?)
    Brown – not tested
    Yellow – power detected as expected with four way flashers or brake lights
    Green – same as yellow

    Now here’s the perplexing part. if I attach the connector one wire at a time, I detect the same result through the 4-way connector. However, when I wire ALL of the four wires to the connector, I detect power only from the white wire. Neither green or yellow shows the results I’d expect from the above test.


  38. Bud says:

    Just replaced left side tail light assembly and now the right side doesn’t work. Original installation did have a crushed wire behind left side light which I wrapped in electrical tape and put in the channel when re-installing. Left side works now works just fine, but no functions on right.

    • Bud says:

      Problem solved for now …. somehow managed to blow both bulbs in the RS assembly. Used bulbs from broken LS assembly (parking mishap) and everything seems to work fine.

  39. Jeff says:

    When I connect my trlr wires to the truck my brake control shows no trlr. You can then activate the breakaway switch and it will show connected

    • Sorry these messages have been piling up. I’m working solo at the moment, and the business comes first. Jeff, was that a ground problem? I could see the breakaway feeding back thru the plug is the ground was bad.

  40. mark bennett says:

    Hi folks. I have a question regarding dual filament bulb that operates the rear lights and the rear brake lights. The issue is that both work, that is the bulb lights up when the car lights are on and the bulb lights up further again when the brake is pressed – all of this is of course correct. The issue I have is my car seems to think that the trailer lights are out or the brake lights are out and neither is the case.
    Do dual filament bulbs cause issues for the car in that this message is reported?
    Just to say I confirmed everything had a good ground and that the 2 wires coming in had 12V.

  41. Peter Jones says:

    I have fitted a trailer board ,but when I press brake the brake lights go off after 5 seconds

    • What do you mean when you say you fitted a trailer board?

      • Peter Jones says:

        A light board to fit to a boat trailer ,I fitted a light socket to the van and when I plug the light board in the brake lights only stay on for 5seconds ,I take my foot off brake press again and same happens ,disconnect plug and it’s ok again ,thank you

  42. Robert woods says:

    Have an RV trailer, all lights work. But, when I turn on the turn signal, the light comes on but doesn’t blink. I have been blinking them manually. What???

    • It could be that your flasher is overloaded with having to power the extra lights on the RV. Is it a factory wiring package, or is it running off the truck turn signal circuitry?

  43. Grant Reynolds says:

    We have a vintage 1960 Shasta travel trailer and reused the original Stop, Tail, Turn Lights one bulb assembly – we rewired as original, however when lights are turned on, you can barely see the brake or turn signal light functions. When the lights are turned off , you can clearly see the brake and turn signal light functions. Any suggestions? Sincerely appreciate help.

    • I’d be looking for a bad ground. Some of those old travel trailers used to have a separate ground going from the frame of the trailer to the body, especially if it has a wood frame body structure.

  44. Stan says:

    I was walking out to my trailer which I used last week and the rear light was on in the taillight on the side. Just the one bulb. It is a Wells Cargo 2005 enclosed trailer. The taillight is a wrap around style led that Wells Cargo uses. The trailer is not hooked up to the truck just the on board battery. I found the replacement taillight which is a sealed unit online. Could it be it it something else?

    • The taillight won’t come on unless it has power, so I’d be looking in the junction box to see if the power lead from the trailer’s battery is somehow touching another wire, or even grounding out to the frame. The 12v terminal in a 7-way plug is right next to the right turn terminal. Perhaps a loose wire in the plug or some corrosion between the terminals could cause this problem.

  45. I have an older trailer that I upgraded to LED lights. Grounded to the trailer frame from the harness and then grounded the LED’s to the trailer frame. When connected to the tow vehicle but NOT on the hitch…..everything works fine. As soon I put the trailer on the ball…..nothing works. I’ve checked my grounds, wire brushed the frame but to no avail. I’ve tried this on my other vehicle and the same thing happens. What am I missing?

    • That’s a head scratcher. It sounds like it’s grounding out, like a short, when the car and trailer are coupled. So, I think I would remove the lights one at a time, beginning with the tail lights, and look for a wire pinched behind one of the lights during installation. Let’s say your ground isn’t as good as you think it is, and a tail light wire is pinched against the frame, once you connect to the hitch you would get a decent ground and also inherit a short. Every time you remove a light, re-hitch to figure out if one of them is causing the problem. I’ve also seen brand new 4-way plugs with factory defects like wires touching inside the plug where you wouldn’t see it. I’d like to know how this turns out if you don’t mind.

  46. Tim says:

    I’m having a weird problem. With aluminum open car trailer all hooked up with 7 prong connector I am getting tail lights but no flashers or brakes. With a multi-tester I have voltage at the truck plug, and voltage at the trailer light ends for the turn signals, but when I plug the bulbs in – nothing! If I put the multi-tester on the brass contact when the bulb is plugged in – also nothing. With the bulb out – current. Something about plugging the bulb in is stopping the signals from working. New bulbs, contacts clean and not touching, everything looks normal. BTW, the taillights are getting about 9.5 volts and the flasher contacts (when the bulb is NOT in are flashing at about 6 – 7 volts.)
    Any ideas greatly appreciated….

    • Maybe the tail light assemblies aren’t grounded properly? Are you testing the little contact inside the socket where you screw the bulb in? If so, use the socket itself as the ground and test the contact. If you show current, then the ground and power are good and we have to look at other possibilities, such as not screwing the 1157 bulb in correctly (which is very easy to install incorrectly), making the contacts miss the bulb entirely or sometimes the bulb contacts get caught “in between” the socket connections, essentially shorting them all together. Is the trailer a tilt bed?

      • Tim says:

        Thank you for the reply… the trailer is not a tilt bed, just a normal flat bed. The lights have worked for the whole 6 years I have had the trailer – sometimes I need to jiggle the bulbs in the socket to get the connection but I could always get them working. I used it locally a week ago and noticed that I had taillights and only one turn signal so I decided to replace the bulbs and just make sure everything was OK for the season.
        The current to the working tail lights was about 10 volts at the contacts at the trailer housings, and it was actually only flashing at 4V at the signals. Ah ha – I thought, not enough voltage to get the bulbs to light up, and the copper wiring inside the insulator was black, so I cut it back to where it was fresh looking and spliced in a new section – now I had 9 or so Volts at the flasher contact. But still – with the bulb in and my probe on the brass flasher contact (black on the metal housing) – nothing. I have now managed to blow a fuse (I think) on the truck (2017 Frontier) so will replace that and carry on! I do have a new housing so I suppose that’s the next step, although I don’t think it will help.

  47. Jeff Hopkins says:

    I recently picked up a Reese Towpower 8508211 Brake Control (Proportional BRAKE-EVN). I am connecting it to the Reese Towpower 8506911 Brake Control Adapter Harness (for GM Models), which is plugging into the matching plug socket in my 2017 Chevy Traverse. I have verified that the wires are connected properly, and used a circuit tester to confirm power is good, that the brake line is correct (only comes on when I apply the brake using pedal or manual control on the Reese Controller). The signal lights on the back of the trailer also function properly.

    But I am seeing unexpected codes on the front of the controller. When the trailer is connected, I see .c code, which I expect. When I slide the manual brake control lever, I see the GAIN number in the display, which I also expect. However, when I press the brake pedal, I see n.c. flashing in the display, which leads me to believe that the controller thinks that there is something wrong (trailer not connected?). The n.c stops flashing and it goes back to displaying .c when I release the brake pedal. Additionally, when the trailer 7-pin connector is disconnected from the back of my vehicle, the Reese controller still shows .c on the display (which I think means that the controller still thinks the trailer is connected)
    Not sure where the problem is, since the lights work properly, and I know I have the power and the brake system wires connected properly. I drove the trailer home from the dealer today, not knowing at that time that the n.c. code was a problem. I did not notice any issues in stopping the tow vehicle and trailer using the brake pedal, everything “felt” normal/safe. And I did test the manual controller when leaving the dealer, and it did bring the vehicle to a stop from a slow speed.

    Please advise why my controller is showing n.c. when I press brake pedal, and still shows .c when I unplug the 7-pin from my vehicle. I leave for a camping trip soon, so I would love a fairly quick reply! Thanks so much! Jeff Hopkins jhopkins@cogeco.ca

    • It should only show “nc” when it isn’t connected. With the trailer disconnected, I test the wires going into the brake control. Test the brake switch feed with the brake pedal depressed, then test the blue wire (trailer brakes) with the brake pedal depressed. It should show “c” and then amperage gain. If it’s all good under the dash, I test the 7 way vehicle socket, and possibly the wires going into the back of the plug if I’m not getting continuity at the socket. If it’s all good I look inside the trailer plug for a loose wire. Ground and Brake functions are side by side in a 7-way, and sometimes a loose wire can touch both terminals, which are only 1/4″ apart. If you’re still showing “c” when disconnected, and can’t find anything wrong with your truck wiring, try plugging in another brake control and see if it does the same thing. I get bad brake controls in all the time. With your factory package, swapping brake controls for a quick test is easy.

  48. Ed says:

    I have a trailstar boat trailer that the right marker light by the wheel well won’t work. I took it off and put it on the other side and it works fine. All other lights are working properly. I’m at a loss. I’ve inspected all wiring that I have access to and do not see any pinched wires, etc. I’m assuming there is a loose connection inside the trailer then. Is there any way around this issue without having to put all new wiring?

    • Sorry for the late reply. Having some computer issues. If it works on one side and not the other, did you test the wire going into the light with a 12V tester? If the wire is hot, then you have a bad ground. Boat trailer fenders are sometimes attached with rusty bolts and might not have a good ground to the trailer frame.

  49. Kevin says:

    My trailer lights just stopped working all together yesterday. I checked power at my receiver and have power in every regular driving scenario (Turn signal, braking ect). The fuse is good under the hood also. Could a broken bulb make all the lights stop working all together? Just seems weird that all the lights went out and not just the one light itself. I obviously plan on replacing the bulb, i was just wondering if this is a condition you have seen before. Thanks for your time.

    • The only thing that all the lights share is a ground. If all of the lights stopped functioning, I would be looking for a bad ground. It’s possible that a bad bulb could have tripped a fuse in your wiring, let’s say if you have an after market power converter installed. The fuse could have blown if there was a short and that would shut down all lighting. If you do have aftermarket wiring, look for a fuse link near the battery

  50. Ray Winsett says:

    I just put a new light kit on my trailer The Brake lights Turn signals and flashers work BUT regular lights do not come on

    • Don’t know why it took me three months to see this. I would be looking for a bad ground in the trailer harness, or a blown trailer package fuse on the tow vehicle.

      • Ray Winsett says:

        I fixed the problem I had double checked all the wiring on the trailer before I posted this. What I found was The wiring on MY TRUCK had taped up wiring So I removed the tape and BOOM there is was A broken wire So I replaced that pigtail NOW everything works fine

  51. shelli says:

    Hello, I have a stock trailer, no lights are on when plugged in and truck is on, there are several marker lights and lights inside the trailer that also will not come on when switched. When you hit the brake the brake lights all work in the back and the turn signals work, but the trailer is completely dark otherwise.

    • If your truck has a factory wiring package, look for the trailer tail/marker fuse and make sure it isn’t blown first. Truck lights will work fine if the trailer fuse is blown. If the truck plug is good, look for a bad ground in the trailer wiring leading from the truck to the front of the trailer. Running lights will not work with a bad ground, but often you will still get turn and brake signals.

  52. lisaallen101 says:

    Fantastic and informative article! LED bulbs are definitely the way forward. Higher initial cost but fair more economical in the long run. They work far better for the interior of horse trailers we find. They’re brighter and make loading and unloading horses in the dark a lot easier, as the horse feels much more confident in the lighted area and you’ll be able to better tend to his needs.

  53. John Bryan says:

    I hope you can help me. I have 2018 Equinox Premier and have installed CURT wiring according to their instructions. I also have a 2001 Silverado 1500. When I hook up my trailer to the truck, all lights are good and everything works. When I hook up to the Equinox, the stop light and blinker on the driver’s side doesn’t work. I have checked the wiring and the lights on the check device show everything is correct. Since my Equinox has LED tail lights, do I need to change the trailer lights to LED?

    • That Curt kit plugs into a factory package, so either the package has a blown or missing fuse for the driver side turn/brake, or maybe a bad relay, since the trailer works with another vehicle. Look in the owner’s manual (rear) to find the towing package fuse locations. The kit will output 7.5 amps, which is more than enough to handle brake and turn lights.

  54. John Goff says:

    I have a 2017 travel trailer that the left turn signal/brake light stopped working. Replaced the entire light assembly and have power to the inside of the socket, when you install the bulb it does not flash. the bulb is new.

  55. Darren says:

    Boat trailer led lights. Driver side works perfect. Passenger side much dimmer than driver side. With running lights off, turn signal and brake lights work. Running lights work, but when turned on brake light and turn signal do not work. Help??

    • That light isn’t grounded. Ground the white wire (if applicable) or, if the light gets its ground from the attachment bolts, then clean the hole the bolt goes through or run a fresh ground wire to the light/bolt from nearby the light. The other lights are grounded, just need to ground that light.

      • Darren says:

        Thank you. I did clean that ground wire and nut that held it to the frame, but not the hole the bolt went thru. I will give that a try.

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