Trailer Hitch Lights not working right

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buzzard86

Pre-takeoff checklist
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Jim R
Totally unrelated to aviation but I'm stumped and frustrated so I thought I'd ask for help from the collective wisdom that is POA.

Trying to get ready for our holiday travels. We have a Thule Terrapin cargo box that fits in the receiver hitch. It partially blocks the vehicle's tail lights, but has it own integrated lights that work off the trailer light plug.

Wiring looks fine (no breaks, kinks, cuts etc) but the cargo box lights only work with the flashers. If I turn on the headlights, they stop working (even if the flashers remain on). No turn signals, brake lights or running lights at all. Thule has been no help.

Vehicle is used regularly to tow and I've had not issues with the trailer lights so I don't suspect it's on the car side. Also, same thing happens if I use the flat 4 or round 7 with adapter. I'm suspecting something with the plug on the Thule side.

Before I cut and splice a new plug, any thoughts on what might be going on?

-Jim
 
on a flat 4, the same wire and same bulb is used for the flashers as the brake. i'm guessing the brake works if the headlights are off? how is this box grounded? i'm guessing "poorly". Run a ground wire to each light assembly instead of counting on a ground through the trailer hitch. Dedicated ground wire is the only way to go with trailer lights.
 
Yeah, the only ground is through the harness. The brake lights on the box don't work, regardless of whether the headlights are on/off. The only thing I can get to work at all are the hazards, which will work it the car is off but stop functioning as soon as the car is turn on. Still thinking funky ground?
 
We have a travel trailer so I am a little familiar with trailer wiring. I would take an electrical tester and check the pins on your tow vehicle in the various scenarios you're talking about. That will tell you if the vehicle wiring is correct, or if the carrier's wiring is off.
 
Yeah, I'll probably do that next just to cover all the bases. I've tried to two different vehicles, one of which is our truck that regularly tows our travel trailer and a utility trailer. Haven't had any issues with the other trailer and both vehicles are doing the same thing with the cargo box, so I'm guessing it's something with the box wiring.
 
I'm assuming you already tried cleaning the contacts at the connector. The next step is to check whether the left and right flashers work independently (as turn signals), or only together (as hazard lights).

If they work both independently and together, then the most likely problem is a short between the white and brown wires or between the brown wire and the chassis.

If they work together but not independently, then the most likely cause is a break in the ground (white wire).

I'd start by removing the bulbs and checking for continuity between the brown and white wires, and then between the brown wire and the chassis, from both ends of the trailer, using an ohmmeter. If there is any continuity at all with the bulbs removed, then there is a short.

If there is absolutely no continuity between the brown and white wires or between the brown wire and the chassis, I would check for resistance from one end of the white wire to the other, using a jumper to either the green or yellow wire. (We know those are good because the flashers work.) If there is no continuity, then there is a break in the white wire.

If there is continuity from one end of the white wire to the other, I would check for resistance between the white wire at the connector, and the trailer chassis; and from the trailer chassis to the ground of the bulb sockets. If there is more than a few ohms of resistance, then the light housings are not grounding through the trailer chassis, possibly due to corrosion. You need to add chassis grounds from both the white wire and the lights. (That would be a good idea in any case, by the way.)

Rich

EDIT: I should have said "cargo box" instead of "trailer." It doesn't change the procedure.
 
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Dealt with something similar on my boat trailer. Found some chafed wires that were shorting to each other.
 
midwestpa24 said:
Dealt with something similar on my boat trailer. Found some chafed wires that were shorting to each other.
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Trailer wiring is notoriously bad. You're lucky to get two or three years out of the original wires. Replacing them with THHN/THWN when the original wires inevitably start to fail is usually worth the cost and effort. Once one goes, they all start to go.

Rich
 
Sounds like a bad ground.
 
People suggest a good check of the ground path, because it is possible for power to go to the trailer on one circuit, say left turn, hit the disconnected but shared ground, then flow backwards through the marker and other turn lights on the trailer, and in turn the ones on the tow vehicle, back to the tow vehicle ground. And a lucky good ground through the draw bar can make up for bad ground wire, but either be intermittent, or fail as things rust, or get replaced with a painted drawbar. Depending on how the tow vehicle turn switch is wired, this can result in lights that work, but dimmer than they are supposed to with the marker lights off, but quit when you turn the marker lights on. Or may appear to work as long as both aren't on at once.

I have once or twice resorted to dragging out a car side plug, and a battery to plug into the trailer, and test what happens when power is applied just to one circuit, and the others are floating.

There is also something to be said for a test plug, or better, a test set of lights, like a magnetic mount light set, to prove the tow vehicle. Occasionally, you have a fuse that only affects the trailer plug, and can cause odd behaviors when out.
 
I’d use a 12v power supply and energize the different pins on the cargo box plug. It sounds like the pin-out may be non typical. If so, once you know what goes where? Easy to fix.
 
The peculiar thing is that the hazard flashers no longer work when the car is started. I really can think of only one explanation for that.

Because trailer wiring is notoriously bad, most trailer lighting controllers have integrated short protection. If the car's BCM polls the lights or briefly illuminates them when the car is started, and if there is a short between the brown wire and either the white wire or the chassis, that would cause the controller to shut down the whole circuit. Typically it wouldn't reset until all power had been removed from the circuit by the engine being shut down and all the lights turned off. During that protected interval, none of the trailer lights would work.

Once the ignition was shut off and the controller reset, the hazard lights would start working again because the short wouldn't be energized (there would be no current to the brown wire, which powers the taillight filaments). But as soon as power were applied to that wire, the short would re-occur, and the trailer light controller would shut everything off again.

My money's on a short in the brown wire. A bad ground is also possible in addition to a short; but I think a short is the main culprit.

Rich
 
Tom-D said:
Sounds like a bad ground.
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That there. If the trailer lights plug's ground is bad, the flashers will work separately because the ground current is finding its way to ground through the taillight filaments of all the bulbs; those filaments have a common ground with the brake/turn filaments within the bulb. The flashing carrier light will route its ground current through the taillight filament of its bulb, and from there it will run up the taillight wiring to all the taillights and the front park lights. Since they're all in parallel, there's little resistance.

Hit the brakes or hazards, and the current doubles. The taillight filaments start to heat up and their resistance goes up. Same with the brake lights. The current can't find such an easy ground and the lights don't work. Turning the taillights on stops everything, since the current is now blocked from running backwards though the taillight system.

You can confirm it by watching closely the taillight filament in the flashing light. It will be seen to glow along with the flasher.

Bad grounds are probably about 80% of trailer light issues.
 
Funny..................I was working on my trailer lights last week. One of the side markers was out and I wanted to replace all of them with LED.

Then, none of the side markers worked. I went back and forth.....

Finally, it turned out that I'd blown a fuse in the Suburban.
 
Dan Thomas said:
That there. If the trailer lights plug's ground is bad, the flashers will work separately because the ground current is finding its way to ground through the taillight filaments of all the bulbs; those filaments have a common ground with the brake/turn filaments within the bulb. The flashing carrier light will route its ground current through the taillight filament of its bulb, and from there it will run up the taillight wiring to all the taillights and the front park lights. Since they're all in parallel, there's little resistance.

Hit the brakes or hazards, and the current doubles. The taillight filaments start to heat up and their resistance goes up. Same with the brake lights. The current can't find such an easy ground and the lights don't work. Turning the taillights on stops everything, since the current is now blocked from running backwards though the taillight system.

You can confirm it by watching closely the taillight filament in the flashing light. It will be seen to glow along with the flasher.
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But that doesn't explain why the hazards go out and stay out when the ignition is turned on.

Even if the taillights come on briefly (or if OP has his foot on the brake) during starting and cause the situation you describe, that would only cause the hazards to go out while current was going to the taillights. Once the current was removed, the hazards would work again.

If it caused a short and tripped the breaker in the trailer light controller, on the other hand, all the trailer lights would stay out until the controller reset itself.

Dan Thomas said:
Bad grounds are probably about 80% of trailer light issues.
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Agreed.That's why I always ground the lights themselves to the trailer with a wire from the grounding post to the frame, and add a ground from the white wire to the frame if it doesn't already exist.

I still suspect a short in this instance, possibly in addition to a bad ground.

Rich
 
My universal laws of electricicals (sic):
- bad lights/accessories.....its always a bad ground.
- poor running (especially after warm up) of a motorcycle....it’s always a bad coil.
- hinky electronics....always needs a reboot (that means turn off and then back on to us non-millennials)

Sparky
 
So Thule randomly got back to me today and offered to send me a new harness and lights under the terms of their lifetime warranty. May be overkill for what sounds likely to be a ground problem, but I'm not going to turn it down.

I'm hoping to take my Instrument written this week, so I'm going to focus on that and let the light project sit until the new harness and lights arrive. If they work then I'll just swap them out. If things are still wonky then I'll obviously have to dig further and will probably start by running separate grounds.

Thanks for all the tips.
 
Replacing the harness might uncover the problem. Then you'll have a spare.
 
I oversee a fleet of almost 200 semi trailers. We see quite a few light issues. Most are pretty straight forward, burned out bulb or bad wire/connector going to the bulb etc. But sometimes you get the mystery problems like brake lights will light up the side markers too, turn signals make the roof markers flash, that kind of thing. In those cases, its almost always a bad ground somewhere. The trick is finding it.
 
Agree sounds like ground issue or burnt bulb shorting...but first test your source at the flat four plug on your vehicle and make sure that is all wired properly with one of these cheapie probe testers below. The four ports on the plug should be ground, L blink, R blink and running lights.

images
 
Dan Thomas said:
Hilarious. McManus was always good for laugh. He's even funnier to those who have actually done this stuff, whether it's fishing or camping or whatever. The secret of good humor is the exaggeration of truth.
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Oh, it's even worse. McManus went to WSU (as did I) and grew up in northern Idaho. I can picture places in my mind when I read his books. My wife could always tell when I was reading one of his books in bed, because the bed was shaking from my laughter. The guy was a genius. RIP, Patrick.
 
Ghery said:
Oh, it's even worse. McManus went to WSU (as did I) and grew up in northern Idaho. I can picture places in my mind when I read his books. My wife could always tell when I was reading one of his books in bed, because the bed was shaking from my laughter. The guy was a genius. RIP, Patrick.
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Yup. Laughing in bed, reading funny stuff. I can imagine guys like McManus when they were kids in school; they were like the guys I knew that had the class in an uproar most of the time, and the teacher pulling her hair out in frustration. Without those guys, school would have been much too boring and soul-deadening.

There was a fellow Canuck, Max Braithwaite, who wrote similar stuff about growing up in rural Saskatchewan. More bedtime laughter.

And some fiction: W. P. Kinsella: The Moccasin Telegraph and following books, and W. O. Mitchell; find a copy of Jake and the Kid.
 
Quick update for anybody that’s wondering. Thule turned out to be great and sent me a new harness and two new lights for free under their lifetime warranty. The existing lights now work perfectly with the new harness so, as the POA crowd correctly speculated, there was something awry in the old harness.

Oh, and I passed my Instrument written.

Happy holidays everyone.

-Jim
 
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