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Curt Trailer Wiring Harness Issues

jolson1228

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Hello all!

I just spend about 4 hours in the garage installing and troubleshooting the Curt Bronco Custom 4-Way Flat Output Trailer Wiring Harness for 2021+ models with LED tail lights (Curt Part #56468). It probably should have been a 1-1.5 hour job. I learned a few things along the way that I think might help others if you end up installing this wiring harness. I did a couple of quick searches from my phone and didn't find answers, so I'm making a quick post that hopefully shows up in search results.

After completing the install, I had two issues. First, my rear passenger blinker was not working. I was getting a hyper blink when I turned on the right side blinkers, and when looking at the lights, nothing was flashing. Second, when I hit the brakes, the brake lights (red) AND turn signal lights (orange) would come on for both rear lights. If I turned on the left blinker while brakes were applied, I would also get a hyper flash for the rear driver blinker not working.

After chasing gremlins for about an hour, I found two things that I did wrong. One was my fault, as I didn't want to drill into my Bronco for a ground point (like the instructions said to do), so I picked a bolt that was not an adequate ground. Once I found a good ground, the driver side tail light started working as intended. For those curious, I split the ground wire off back below the bumper and used one of the bolts holding the bumper to the frame for a ground. I did have to open the connecter up a bit to for it on the bolt. The other thing I did wrong was following the wiring diagram...

I found that connecting the power wire on the harness to the battery, as the diagram and instructions said to do, caused half of the issues I was experiencing. For some reason, when the harness is connected to power, the rear lights had an aneurism and couldn't figure out what lights need to go on. I disconnected the power wire from the battery after fixing the ground point, and everything worked as intended.

To keep it short, if you're installing this wire harness on your Bronco, make sure you have a good ground point, and follow the instructions. If that doesn't work, try disconnecting the power wire and see if that fixes your issues.

Hope this post helps someone out!
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tater_70

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It's been a long time since I wired any taillights, etc. I may be missing something but why would you need a power wire if your just tapping into taillight/brake/running lights? If you added a brake controller that would be different.
 

MrBell

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I recently installed the CURT wiring harness. Had no issues. This YouTube video was very helpful. My Bronco has the LED taillights, so I used a different harness and it did not require modification.

 

swmetal

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If the Curt harness works without having the power wire connected to the battery or some ither positive lead such as an Aux wire, then you still have it setup wrong.
 
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jolson1228

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It's been a long time since I wired any taillights, etc. I may be missing something but why would you need a power wire if your just tapping into taillight/brake/running lights? If you added a brake controller that would be different.
That was my thought and what led me to disconnecting it to test.


I recently installed the CURT wiring harness. Had no issues. This YouTube video was very helpful. My Bronco has the LED taillights, so I used a different harness and it did not require modification.

Very in depth video, thanks!


If the Curt harness works without having the power wire connected to the battery or some ither positive lead such as an Aux wire, then you still have it setup wrong.
I may still have it wrong, but it's right enough to work as intended. As Tater mentioned, above, I think it's getting its power from the tail lights themselves, and doesn't need the positive lead since I'm not using a brake controller, but I could be wrong. That's why I said follow the instructions first though! I could just have a faulty unit that's giving me gremlins, or I could have messed up somewhere along the way, but as far as I'm concerned it works so I'm not going to mess with it more to try to make it look exactly like the diagram.
 

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If the Curt harness works without having the power wire connected to the battery or some ither positive lead such as an Aux wire, then you still have it setup wrong.
I agree, this is super strange. Now that you have the lights on the truck working, have you tested the output to the trailer to see if trailer lights are all working properly too?
 

BigFootie

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The little box needs power to operate. That turns your separate turn signals into the same lights that also are your brakes on your trailer. I’m assuming you are using a 4 pin connector. There is a right turn signal, left turn signal, which together are also the brakes, the lights and ground connections. The Bronco has separate brake and turn signals. This box allows the turn signals and brakes to share the same circuit. The good ground is very important if you want this box to last longer than a year or two.
 

Haustkraft

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Why go thru all that trouble and not do a 6/7 pin? You would have the option to pull anything with brakes, this way you won’t have that circuit.. just my two cents.
 

Rick Astley

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Why go thru all that trouble and not do a 6/7 pin? You would have the option to pull anything with brakes, this way you won’t have that circuit.. just my two cents.
Because the Bronco maxes out at 3,500 lbs towing with adequate brakes for a class 2 hitch and, in general, trailer brakes aren't equipped or an option on small class 2 hitches?

Just spitballing here.
 

Haustkraft

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Because the Bronco maxes out at 3,500 lbs towing with adequate brakes for a class 2 hitch and, in general, trailer brakes aren't equipped or an option on small class 2 hitches?

Just spitballing here.
36 states require brakes on anytime over 3k gvw, BUT it’s not about the weight in my mind, it’s controlling that trailer behind you when you have to lock up the tires. 4 pin is a limitation but you can throw a 6/7 pin to 4 pin adapter if you wanted. Why a class two and not a traditional 2” / class 3?
 

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swmetal

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Why go thru all that trouble and not do a 6/7 pin? You would have the option to pull anything with brakes, this way you won’t have that circuit.. just my two cents.
Not quite true. Most boat trailers have surge brakes and use a 5 pin connector. Of course there are converter plugs for 4 to 5 pin and 7 to 5 pin, with the 7 to 5 pin better since it has a reverse circuit.
 

BigRed

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36 states require brakes on anytime over 3k gvw, BUT it’s not about the weight in my mind, it’s controlling that trailer behind you when you have to lock up the tires. 4 pin is a limitation but you can throw a 6/7 pin to 4 pin adapter if you wanted. Why a class two and not a traditional 2” / class 3?
Disc Surge brakes are the key. I use them on ALL my trailers including the 3,500 and under Aluma's.
 

Haustkraft

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Not quite true. Most boat trailers have surge brakes and use a 5 pin connector. Of course there are converter plugs for 4 to 5 pin and 7 to 5 pin, with the 7 to 5 pin better since it has a reverse circuit.

just Recommending what Ford installs and the most common. I can’t for the life of me find a class II for these trucks ?

how big of a boat ?
 

Haustkraft

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Disc Surge brakes are the key. I use them on ALL my trailers including the 3,500 and under Aluma's.
I agree with those as well.
 

swmetal

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just Recommending what Ford installs and the most common. I can’t for the life of me find a class II for these trucks ?

how big of a boat ?
I suppose it depends on where you live. Here in Florida, surge brakes are common as so many of us have boats. Boat trailers don't typically have electric brakes due to being dunked in water all the time.
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