Boat trailer light help

I’ll start by saying I have very little electrical knowledge. This last weekend the running lights and side marker lights went out on my trailer. A few details:
I have a 2012 Ford f 150
The running lights on the truck are working
I’ve tried to connect the trailer lights to both styles of plugin on the truck
Break and both turn singles work fine
The trailer harness was replaced by the prior owner. I have unwrapped all the electrical tape and all connections look good.
I have a multimeter but have no idea of how to use it or where to start. I don’t even know what setting to set the multi meter at. I assume that I should start by testing the trailer plug in on the truck itself to see if there is current getting to the running light pole ( I’m not sure what pole that would be )
If one bulb was burnt out it should not stop the other running light or side marker lights from working would it?
I hate electrical problems. I really don’t want to take it someplace to get it worked on
and pay a lot of $’s if it is just something simple

There would not be a truck fuse or relay that would control just the running lights on the trailer would there? I took a quick look at the truck manual but didn’t see anything listed that said anything like trailer running lights

Double check the ground. 12V systems are finicky for a good ground. Also double check the truck fuses, my 14 chev has separate fuses for trailer lights.

Most trucks do have a separate fuse for trailer lights, do you have another trailer to plug in to check?

Wouldn’t his described problem suggest that one wire isnt connecting? Usually brown iirc?

My best guess is a fuse on the truck. One bulb out on the trailer should not affect any others.

If your electrical knowledge is limited, best help would be to search and watch some you tube videos on this issue. Taking the time to watch a few videos will be better than trying to understand written trouble shooting instructions here.

The only thing I see listed for trailer lights in the manual is a relay in the power distribution box. I tried moving another of the same relays to the trailer light spot and still no running lights. I think there was possibly a fuse for trailer lights I will have to look again. I kind of thought if it was the fuse I’d not have any trailer lights
Someone said they sell a rather cheap plug in tester to plug into the truck plug in that shows if all poles are receiving power. I may check the auto parts store on my way to work for one of them

On a side note – someone told me I should unplug my trailer lights prior to backing the trailer into the water. Does anyone here do that. I’ve never done that on any other boat trailer I’ve had

Anonymous wrote:

Double check the ground. 12V systems are finicky for a good ground. Also double check the truck fuses, my 14 chev has separate fuses for trailer lights.

I’ll take off the ground connection and clean it up good and reconnect it when I get home. I kind of thought about that but thought if it was that all the trailer lights would not work. It’s a rather quick check and a good idea to make sure it clean anyway. Thanks

I have a 2012 F150 and I ran into the same issue as you. There is a fuse for the trailer in the engine compartment fuses, I forget which one but it is a square blue one, they are under two bucks at the Ford dealer. I ended up rewiring the trailer this year because I was blowing the fuse every couple months. Since rewiring I haven’t had any issues.

To test your vehicle it is easiest to buy a trailer light tester.

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fishthumper wrote:

On a side note – someone told me I should unplug my trailer lights prior to backing the trailer into the water. Does anyone here do that?

I used to with incandescent bulbs (hot bulb + cool water = POP!), but I never have with LEDs.

mark-bruzek wrote:

To test your vehicle it is easiest to buy a trailer light tester.

Agreed. A couple bucks will easily narrow the source of your issue down.

On most trailers the tail lights are seal all the way around but the bottom. When the trailer is backed into the water air is trapped in the light housing…and keeps the hot light from popping do to cooler water.
A few years ago I started popping one light and found the housing to have a crack in it. That allowed the air to escape and let water in.

Switched over to LED’s since.

I’ve never disconnected my trailer light plug.

I always unplug my trailer. Cold water and hot bulbs don’t get along very well.

Your problem is NOT with the ground. A bad ground will affect all the lights, not just the running lights.

As others have said, the Brown wire is the one to look at.

Those trailer light testers are very handy. In one quick test you can tell if the problem is in the trailer or the truck.

SR

My truck will occasionally not have running trailer lights… I have the indicator lights on my trailer plug. They show break and turn signals work but not running lights.
Then they’ll work for a few weeks and they’ll not work for a few weeks. Really weird how the problem turns on/off.

My trailer is wired with all grounds through the plug and absolutely zero trailer wiring issues… Rewired recently all great connections for the Long haul.

Few weeks back my running lights weren’t working. Went fishing. Came home and let truck idle for 10 minutes while talking… Bam lights start working while standing there. Work for two weeks and several trips to/from landing and then they’ll not work.

I can completely dump all wiring truck harnesses under water or not at the launch and that doesn’t seem to be causing it.

It’s a weird deal. But seems similar to the op problem. I’m blaming the computer in my truck and not a fuse as a fuse is blown or not.

Seems like when I have this problem and I have recognized this several times repeated over a few months now, that leaving the trailer connected with lights on for a decent amount of time can result with the lights working.

Sure can be frustrating when everything is working and then it isn’t.

Update – I stopped and got a trailer light tester on my way to work.? It showed that the plug in is not receiving power to the tail lights – so my problem is most likely on the truck side. Either a fuse or a relay. Since I already tried to replace the relay that says trailer tow with another of the same # relays and that did not fix the problem. My guess is a fuse. Just can figure out which one from the manual. May need to just start pulling possible ones to see if one is blown. I guess it could be the truck plug in itself. May need to crawl under to see if a fitting came loose or wire is broke. I may find someone else’s truck to hook the trailer to to make sure it’s the truck side for sure

Trailer light problems are frustrating enough already. They add a car computer into the mix and I might start drinking again

fuse #67 under the hood (20amp)

(67) could be wrong but its the yellow one and 20amp.

If it immediately blows you have a power short on the trailer.
If you drive it down the road and it blows you have a “very cool” power short that will drive you nuts!!!!

See my post from earlier this year…..

fishthumper wrote:

There would not be a truck fuse or relay that would control just the running lights on the trailer would there? I took a quick look at the truck manual but didn’t see anything listed that said anything like trailer running lights

fishthumper wrote:

Update – I stopped and got a trailer light tester on my way to work.? It showed that the plug in is not receiving power to the tail lights – so my problem is most likely on the truck side. Either a fuse or a relay. Since I already tried to replace the relay that says trailer tow with another of the same # relays and that did not fix the problem. My guess is a fuse.

I’m almost certain it’s a fuse.

In my last vehicle, I had the same issue. I tore the fues box apart and by simply checking every fuse I FINALLY found the culprit.

In the manual it was labeled cryptically as TRNL with no explanation as to what it did, so bottom line is it may not be obvious from looking at the fuse diagram unless your manual really spells it out.

Grouse

sand-burr wrote:

fuse #67 under the hood (20amp)

(67) could be wrong but its the yellow one and 20amp.

If it immediately blows you have a power short on the trailer.
If you drive it down the road and it blows you have a “very cool” power short that will drive you nuts!!!!

See my post from earlier this year…..

Sand-burr – you are the winner and my Hero. #67 in the power distribution box it was. The manual list it as TT Park lamp relay power. $4 for a fuse ( make that $8 because I bought a spare ) and it back to working. Hope the rest about it continuing to blow is not correct. I think I may have found the problem in the rest of my trouble shooting. The previous owner must have wired a new plug in into the harness. He used the screw in type of wire connectors. When I unwrapped all the electrical tape I noticed lots of moisture on the connectors. I’m going to let it really dry out and rewrap it with fresh electrical tape and pray the problem does not return. Do you think it would be better to redo the connections with butt connectors with the built in soder that you heat and it soders and shrinks the connector at the same time. I’ve used them in the past and had good luck.others have told me the twist on type work the best

Thanks everyone for you help and idea’s. I’ve been gone from the site for a rather long time due to a career and home relocation. I guess until now I forgot how much I missed all the info. And knowledge there was on the site. I’m glad to be back. If anyone needs help or Fishing Info. In the St.Cloud area feel free to send me a private message.

Always unplug trailer lights before backing in . I take it a step further and back up just far enough to where I don’t get my bearings wet at all

Waterproof connectors!!!! Not black tape.

Morel King wrote:

Always unplug trailer lights before backing in . I take it a step further and back up just far enough to where I don’t get my bearings wet at all

I have never unplugged mine, LED or incandescent and have never had an issue if the housing was not cracked. Nor have I had a pre-mature bearing failure. My boat trailer sees about 6,000 miles a year. Service is key.

I understand wanting to take care of your equipment, but that’s a little much in my mind. Kinda like a plastic cover on a couch…

“I have never unplugged mine, LED or incandescent and have never had an issue if the housing was not cracked.”

That’s the thing…every time I replace a light assembly I usually find a way to bash into it right away and crack it. Then it leaks, and the first time you back into the water the bulb blows. So I just unplug it; takes two seconds.

I’m waiting for the LED assemblies to get a little cheaper. Looking forward to that.

SR

Morel King wrote:

Always unplug trailer lights before backing in . I take it a step further and back up just far enough to where I don’t get my bearings wet at all

If you have a boat with surge brakes on the trailer, unplugging isn’t a great option. Just get LED lights and leave it plugged in.