Troubleshooting No Running Lights on Trailer but Turn Signals and Brake Lights Work

Updated 06/14/2024 | Published 01/01/2010

Products Featured in This Question

35 Ft 4-Way Trailer Wiring Harness - Wishbone Style - 42" Ground

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$25.10

Hook Probe Circuit Tester

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$12.08

Question:

I have a 4 way on my trailer w/ 5 wires and im plugging into a 4 way w/ 4 wires. I have brake lights and turn signals but cannot get the running lights to work.

asked by: John

Expert Reply:

The 4-way on your trailer is what's known as a wishbone connector like part # A35W42B. Two of the wires are the running light circuit which is a little easier to install since you can run one down each side of the trailer. This way you don't need a jumper wire to go from one side to the other. A wishbone 4-way is compatible with a standard 4-way, so your problem is somewhere in the wiring itself.

I would start by checking the 4-way on the truck with a circuit tester, like part # PTW2993. Testing the truck's connector tells if the problem is on it or the trailer. If everything checks out here, then the issue is on the trailer side. If you have a problem on the truck, then you need to trace the wires coming from the connector looking for any damage. You should also check the ground connection making sure it's clean and solid. It should be grounded to bare metal as well.

If the trailer is the problem, then start at its connector. Make sure the pins are clean and corrosion free. Then you want to inspect the running light wires throughout the trailer. They're traditionally brown wires but may not be. Since you have two, they'll be the second and third wires starting on the side with the ground pin. Take a look at the picture I attached. Check for any damage or pinch points. You should also check the ground connection on the trailer side as well.

expert reply by: Robert G

click to enlarge

Cameron

3/1/2020

Im not sure how old this thread is but its near the tip of the search for this question still so here goes nothing. I just got done rewiring my whole trailer because I had this very same issue. No running lights, but brakes and turn signals worked. You know what the problem was?... Hanging my head in shame... I wasnt turning the lights on in my car...

Chris R.

3/2/2020

I'm smiling to myself only out of recognition, because we've all been there before. Sometimes it just takes complex, time consuming, frustrating troubleshooting adventures before the simplest of solutions is discovered. No need to hang your head. At least it's fixed!

Terry B.

9/14/2020

@ChrisR Ok, I have an led light strip on my tow vehicle, the 4 pin connector has the brown wire with a fuse on it going to the ground where it's normally a white wire, the white and brown wires have traded places. If I leave it as is and plug it into the 4 pin connector that has the wires like they are normally I don't get any brake lights, I do my signals and running lights working fine. If I do a test of swapping the wires coming from the light strip to match to connector installed on my two vehicle I don't get any lights. This is really confusing, it seems like if I just connected the connectors the way they designed I have two negatives. I do know the wires are all correct from my to the tow vehicle. Which is a jeep liberty. So what am missing or not understanding, I just need the brake lights to work with the rest. Thanks

Jon G.

9/15/2020

@TerryB I can say from experience that the color of the wires doesn't always matter and that you need to make sure everything is connected by function. With that being said, it's really weird that you are getting turn signals and no brake because with a standard 4-Way connector those are combined. This makes me wonder if maybe your turn and brake signals have been separated out from each other for some reason. Try looking for a converter along the wiring for your Jeep to see if this would be the cause. To combine your brake and turn signals (if they are separated out) I'd use the Roadmaster # RM-732.

See All (9) Replies to Cameron ∨

4/20/2020

I have retired the entire trailer and have turned the lights on in the truck and still can’t get the running lights to work. If your tail light has 3 wires - red, white and black. What should they be hooked to on a 4 pin? I have the black to the brown, the red to green/yellow and white as ground? What am I doing wrong?

Chris R.

4/22/2020

Are you sure that the issue is on the trailer? If you haven't already, I would definitely test the truck side 4-Way to make sure you're getting power on the running lights pin. It could be something as simple as a blown fuse under the hood (check your owner's manual for the right location). I'm pretty sure you have the colors matched up correct with the trailer wiring. Let me know what you find and we'll go from there.

Kevin S.

4/23/2020

I have turn and break lights, until the point that I turn the head lights on. Then there is no lights at all that work on the trailer. So the result is, no running lights at all on the trailer side. Only have tail lights for the trailer, as long as the headlights are off on the truck. I checked the running light pin on the truck plug, and I do show it is getting power to it.

Chris R.

4/24/2020

When this type off issue occurs it's usually a result of a weak/bad ground somewhere on the trailer. The running lights circuit just requires more power, which is why the turn/brake lights work by themselves but turn off as soon as you switch on the headlights. What I recommend doing is checking the main harness ground along with the individual grounds on each of the trailer light assemblies - making sure they're securely attached to clean, bare pieces of metal. Let me know if the issue persists and we'll go from there.

Mitchell S.

6/14/2024

@ChrisR I have this same issue, but everything appears to be working all the way to the lights. The grounds are all good and I’ve tested all wiring up to the point of entry into the lights. Both lights behave the same way, so I hesitate to think it’s a problem with the lights themselves. Could they have both had some sort of issue together?

Kevin C.

6/14/2024

@MitchellS If you're reading the each light signal all the way through the wire up to the light then it would have to be an issue with the light itself. It would also be worth checking your ground as a weak ground could also cause the issues you're describing.

Pedro J.

5/2/2020

I just bought a trailer light kit, everything works except for the left break light all wires are new and fine all connections fine I’ve put 2 different brand new bulbs in one led one regular and changed the plug on the trailer end. I have several other trailers and this is the only one that does this any ideas

Chris R.

5/5/2020

Is it just the left brake light or is the turn signal on that side not working as well? The brake and turn signals are combined on a trailer, so it would be odd if it was just the brake light giving you issues. Either way, I would check to make sure the ground on the light assembly itself is securely attached to a clean, bare metal surface - whether it ground through a dedicated wire or through its mounting hardware. If this doesn't help let me know.

Todd B.

5/18/2020

I added side marker lights to my new trailer. when it is plugged into my truck still do not work. I can hook it staight to my battery and they will work. I tried even to run hot wire to them and grounded it on truck still doesn't work.

Gerald K.

7/8/2020

I have a 99 Silverado pickup that has the socket installed factory. I am using a adapter to 4 pin connector and plugging a led light strip into it. Brake lights work, turn signal works, but no running light. I purchased a Hopkins connector that has 3 led lights near the pins to check for power. When I turn on the parking lights, the led lights up that's labeled TM. But no running light. Any help would be greatly appreciated.

Chris R.

7/13/2020

So it sounds like you've verified that the trailer running lights circuit IS getting power on the Silverado. Are you able to test the running lights wire going to the new light bar when you have it plugged in? I wonder if the new light itself is bad.

Gerald K.

7/14/2020

@ChrisR I bought another brand and its working great. I found a typeS brand with a 5 flat pin taillight strip. Plugged in easily and looks better than the Walmart brand too. Thanks for helping me..

Lee H.

5/16/2021

@GeraldK I have a 2011 f250. Trl brake and turn signals worked fine but no running lights . I researched till I turned blue , ckd wiring and all and all it turned out to be is the #26 fuse under the hood . A square 30 amp fuse low profile . I didn’t know how to ck the square fuses , but all it is , is a jumper wire across the top of the fuse you can actually see. Hope this helps

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