Diagnosing a trailer light problem

I’ve had my share of trailer light problems in the years that I’ve been towing around boats. I’ve got a new one that I had never previously encountered.

The situation is that the turn/brake lights work just fine but the running lights on the trailer do not work. This includes the side markers and the light bar below the stern of the boat. So, during daylight, no problem b/c all of the appropriate lights work just fine. After dark, however, the normal trailer lighting is all off unless I am signaling or braking.

Most of the trailer lighting problems I’ve encountered have been caused by blown bulbs or bad ground. Because the signal and brake lights work, I’m assuming that my ground is fine. The bulbs in the rear lights are fine. Do the side marker lights run in series, so a blown bulb “upstream” would kill all the lights downstream in that circuit? Do I have a short somewhere in the running light circuit? Whad’ya guys think??

The answer is no. The Brown wire is either broken before or after the trailer connector. If you have add on wiring on your truck you might have a bad connection. If your truck is equipped with factory towing a fuse can be blown and your truck lights will still work. Get a 12V tester and check your brown wire at the plug first.

I still say you have a bad or weak ground some where.When you turn the lights on it pulls enough amps to drain the running lights, dont assume the ground is good.

I second that advice!

All trailer lighting lamps are 12V and therefore all lights that operate at the same time (all running lights) are wired in parallel.

Good trouble shooting technique would say check the brown wire first. If you do not find 12V there then you know you have found the problem. If there is 12V there then you will have to try to track down a bad ground which can be a nightmare…

I’m going to put my .25 cents on the brown wire too.

Either there’s no power at the plug…on the truck side…or there’s a broken wire from the trailer plug back to the lights.

IF the fuse is blown in the truck…THEN you have a wire that’s grounding out somewhere (likely) in the trailer.

Trouble shooting tip #2:

Always check connections first…then look at wires.

You might try wire brushing the rust off your hitch ball and smear some dielectric grease on it.

Same thing happened to me, it was a fuse under my hood, check all your fuses, though it may not solve the problem of it blowing again, been there done that…. I ended up rewiring my whole trailer this fall, I used a outdoor extension cord for the wires running inside the trailer, there’s no way in the life of the trailer that the trailer will rub through and ground that wire.

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If there is 12V there then you will have to try to track down a bad ground which can be a nightmare…

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Isn’t that the truth! I don’t even bother searching for a good ground on the tounge anymore…straight back to the lights. A few to many holes drilled in my last trailer!

BIRDDOG

I had the same problem, and mine was a bad fuse, caused by a short in the brown wire downstream from the connector. Seems I had put on trailer guide ons, and the wire had rubbed on the bolt threads. Simple fix once it is found. Good luck, troubleshooting trailer lights can be a bi@$h.

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I had the same problem, and mine was a bad fuse, caused by a short in the brown wire downstream from the connector. Seems I had put on trailer guide ons, and the wire had rubbed on the bolt threads. Simple fix once it is found. Good luck, troubleshooting trailer lights can be a bi@$h.

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I blew fuses in both of my last trucks due to wires being bared by various means. Fords actually have fuses under the hood especially for trailer lights. Found that out the hard way! I spliced wires, did all the stupid human tricks, only to find out it was the fuse under the hood! I had THE EXACT SYMPTOMS YOU HAVE!!!!!
PM me if you need further directions to the fuses if you have a Ford.
Tuck

the typical trailer, the running lights are not in a sequence. they are pulled off the brown wire, in various spots. i would say you definitely have a problem with either the brown wire out of your truck hook-up, or where it goes through the tongue, before it splits off to go left and right.

glen

My problems have been bad grounds and bulbs. If you have a round 4-5 pin connector at the back of the truck also take that apart on the back side of the bumper. Any green crude will cause a problem. Last year I was ready to rewire the trailer until my son found the problem on the back side of the plug on the bumper. Rig up a test plug and connect to a 12 volt battery and test with out the truck battery. This has always helped.

And the winner is…….Chris Tuckner!!

Chris correctly diagnosed my problem as an under the hood fuse. I went to hood after checking my trailer connector at the truck…0 V with the headlights on. After the fuse change: 11.8 V and the trailer lights work great. Now I just need to find out why the fuse blew.

Chris, I’ll even up with you at the Mille Lacs GTG!

Thanks to all who chimed in. I suspect that someone else will benefit from our collective trailer light wisdom!

Where was the fuse?
Was the fuse in a fuse block with a bunch of other fuses?
Or was it a single in-line fuse somewhere in the engine compartment?

My Dakota has two big blocks of fuses, one in the passenger compt and one under the hood. The block where the offending fuse was located is under the hood, adjacent to the battery.

Just an fyi for Chev users…they two have two fuse blocks…SOME of the trailer light fuses are under the hood..and others are under the back seat (I don’t built’em, I just drive them).

In my case the trailer manufacturer used the little plastic loops with a screw hole on one side to fasten the wires to the bottom of the trailer. One of the loops broke and the wire was exposed to the head of the screw…when my trailer bounce just right…pop went the fuse. Very hard to find because if you didn’t wiggle the wires just right…it wouldn’t short out.

Good luck on the last…but most fun part!

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And the winner is…….Chris Tuckner!!

Chris correctly diagnosed my problem as an under the hood fuse. I went to hood after checking my trailer connector at the truck…0 V with the headlights on. After the fuse change: 11.8 V and the trailer lights work great. Now I just need to find out why the fuse blew.

Chris, I’ll even up with you at the Mille Lacs GTG!

Thanks to all who chimed in. I suspect that someone else will benefit from our collective trailer light wisdom!

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Oh come on that’s total BS, you didn’t even read my post where I told you to check under the hood

Just playing with ya, though I did beat CT to it.

Nick…Eye Guide pays cash to win….

Oh so I have to seek my revenge on Chris on Mille lacs not, Jason….

Was gone on vacation last week and have a lot of reading to catch up on…boy am I glad I scrolled down far enough to see this post. Same exact thing happened to me last week. I’ll cross my fingers and hope it’s the fuse under the hood, as well. If not, plenty of other good tips. Thanks for sharing everyone!

Limp Fish

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I have the same problem. I borrowed my Dad’s truck to pull a friends house and now the trailer plug in is not working. I haven’t checked it with a 12v tester, but thats next. I checked the fuses under the dash and they were fine. Are there more fuses under the hood of a 2002 F150? Here’s a stupid question: which of the wires do I hook the 12v tester up to?

Yes there are more fuses under the hood, (I have an 01 f150)as far as testing with a volt meter, find a good ground, and the brown wire ( I think ) is the wire that should show you a constant 12 volts.

You also need to check your ground wires, and such. I ended up having so many ground issues, I finally attached the white wire to the trailer so the wiring always had a good ground.

Then there’s the other lesson in stupidity that I learned, (which is why I finally ran the white wire) that the trailer needed to be hooked up to the truck to ground the wiring, I just finished rewiring my trailer and I wanted to see if the lights would work, I knew I had power to my connector, but the lights would not work, because I didn’t have it on the hitch, so it wasn’t grounded Took me atleast 30 minutes of looking around before I figured out my stupidity.

I’d also suggest getting yourself a little trailer light tester, I have a little connector that fits on to the connector of my truck , and show me that there is power available to the connector, so I can faster trouble shoot my problems. Cost me something like $4, I bought mine qat checkers, but I just seen my local Gander mountain had them too for the same price. Much easier than a volatge tester and really quick.