Utility trailer running lights lights stay on

[MadDog304]

So I have a small utility trailer with a four pin light connector. When I use my wife’s vehicle, the trailer running lights only come on with the headlights.

On my dad’s 2025 Maverick, the trailer running lights come on when the plug is inserted. The brake and signals work as expected. Typically that would indicate a grounding problem I think, but my utility trailer is fairly new and I don’t have an issue with other vehicles I’ve used. Was wondering if others have experience.

I was going to test it a bit more and even try a 7 pin adapter.

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[scott1070]

This is normal and working as it should

 

[MetalsGeek]

Your problem description leaves out a lot of detail that might guide understanding. Is the trailer hitch a factory option or aftermarket? A lazy installer could have hooked the tail lights to the accessory power plug in the bed, or some other convenient place. The factory trailer lights are separate from the truck's so the BCM can detect the presence of a trailer. Since the trailer's tail light behavior should follow the truck's, you can compare them. When in "auto" mode they may come on unexpectedly, so you may have to override that by manually turning them OFF. All these are just wild guesses without more detail.

 

[MadDog304]

Factory installed

 

[Kenv24]

So I have a small utility trailer with a four pin light connector. When I use my wife’s vehicle, the trailer running lights only come on with the headlights.

On my dad’s 2025 Maverick, the trailer running lights come on when the plug is inserted. The brake and signals work as expected. Typically that would indicate a grounding problem I think, but my utility trailer is fairly new and I don’t have an issue with other vehicles I’ve used. Was wondering if others have experience.

I was going to test it a bit more and even try a 7 pin adapter.

That's because your Mav headlights are set to automatic. Mine defaults to auto when I start the truck. Turn them off and the trailer lights will go off. Turn them back on....the trailer lights will go on.

 

[The Real Maverick]

And by "running lights" (which trailers don't have) I assume you mean "marker lamps" also known as "parking lamps".

Typically they should follow whatever your parking lamps on the truck do.

 

[MadDog304]

So, my dad’s truck is brand new. Was just delivered last week. The first time we hooked the utility trailer to it using the four pin connector, the marker lights came on even with the headlights off. The brakes and turn signals function properly.

Today, when I tried it, the marker lights are not coming on at all. Even when the headlights are turned on. I used an adapter to the seven pen and the same thing. No marker lights, regardless of whether the headlights are on or off.

so then I tried it using my wife’s Armada and it works like it should. The marker lights come on and off with the headlight switch. My utility trailer is fairly new with LED lights.

I am beginning to think that he needs to have the dealership service department check it out. I am beginning to think that it is in the wiring harness on the truck.

 

[The Real Maverick]

How MANY LED marker lamps are on the trailer? If it is too few (like 2 or 4) the current draw (load) could be so low the Maverick is having trouble knowing it is there.

In the 2022-24's there was a dash menu option to "check trailer lamps" from the driver's seat. Do you have that? Does it work? Or does it say something like "no trailer detected"?

 

[MetalsGeek]

Sounds like you do indeed have an issue. I've seen some bizarre electrical problems that arose from a bad ground, so it may be as simple as that, although you would think that the trailer hitch would have enough connectivity when parked that the WHITE (ground) wire would be unnecessary.
The ground pin on the end of the 4-pin trailer connector goes thru a couple of connectors on its way to the truck's chassis ground, so this would be easy to check with an ohm meter. It's the only exposed pin in the truck's 4-pin connector. While you're at it, check the trailer side too.
I picked up this cheap tester at Harbor Fright and it's been useful. My truck is sensitive enough to think a trailer is connected when it's installed.

Looking deeper, the tail lights on the trailer connector are powered from a relay in the BJB (Battery Junction Box) that's controlled by a signal from the BCM (Body Control Module). Tail lamp power also goes thru 20A fuse #55 so if you short it out during testing that would blow and keep it off thereafter. Standard trailer wire colors are: GREEN = Right Turn/Stop, YELLOW = Left Turn/Stop, BROWN = Tail/Marker and WHITE is chassis ground.

 

[MadDog304]

How MANY LED marker lamps are on the trailer? If it is too few (like 2 or 4) the current draw (load) could be so low the Maverick is having trouble knowing it is there.

In the 2022-24's there was a dash menu option to "check trailer lamps" from the driver's seat. Do you have that? Does it work? Or does it say something like "no trailer detected"?

I’ll have to check on that. We’ve only had the Mav for a few days. I’ve not seen that but will look for it,

 

[Kenv24]

Sounds like you do indeed have an issue. I've seen some bizarre electrical problems that arose from a bad ground, so it may be as simple as that, although you would think that the trailer hitch would have enough connectivity when parked that the WHITE (ground) wire would be unnecessary.
The ground pin on the end of the 4-pin trailer connector goes thru a couple of connectors on its way to the truck's chassis ground, so this would be easy to check with an ohm meter. It's the only exposed pin in the truck's 4-pin connector. While you're at it, check the trailer side too.
I picked up this cheap tester at Harbor Fright and it's been useful. My truck is sensitive enough to think a trailer is connected when it's installed.

Looking deeper, the tail lights on the trailer connector are powered from a relay in the BJB (Battery Junction Box) that's controlled by a signal from the BCM (Body Control Module). Tail lamp power also goes thru 20A fuse #55 so if you short it out during testing that would blow and keep it off thereafter. Standard trailer wire colors are: GREEN = Right Turn/Stop, YELLOW = Left Turn/Stop, BROWN = Tail/Marker and WHITE is chassis ground.

He did say the trailer was good connected to his wife’s truck….. so it’s probably something with the Mavs connection.

 

[MetalsGeek]

He did say the trailer was good connected to his wife’s truck….. so it’s probably something with the Mavs connection.

I totally agree with you that's most likely, but if for example the ground wire on the trailer had a bad connection, this circuit would be completed thru the hitch ball. If the Maverick's receiver was new and paint covered the metal enough to interfere with an electrical connection, then you would have no (reliable) ground and odd behavior could result.
Another problem area in electrical troubleshooting is always the connectors. If the socket is worn or damaged it may not connect well with a mating pin.
I even ran into one bizarre situation where troubleshooting resistance with a multimeter led me astray because it put so little voltage & current thru the system that a connection read good even though it would fail under full design load.
Not all trailers are wired in compliance with the 4-pin standard, so it's even possible that the trailer and other car were wired to be compatible with each other, but would not work and/or blow fuses when plugged into the Maverick. That was one reason I suggest the HFT tester, although I didn't mention it at the time.

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