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Discussion Starter #1
I notice all the wiring kits require a hot wire ran to the battery. What is the reason for this, does anyone know? My jeep has traditional halogen tail lights and my trailer has LED's. Not sure why the wiring would need the extra power from the battery.
 

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Brakes on the trailer.
 

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No. The hot wire is to power the trailer light relay. There isn't enough amperage at the vehicle lights to power the trailer lights. Additional amperage draw on the taillight circuit will (probably) blow a fuse, throw a code, or both.
 

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No. The hot wire is to power the trailer light relay. There isn't enough amperage at the vehicle lights to power the trailer lights. Additional amperage draw on the taillight circuit will (probably) blow a fuse, throw a code, or both.
I’ve never had that problem, and I’ve probably had five different trailers I’ve towed over the years and two different boats. The only one I’ve had a direct battery connection to was a seven pin for a trailer with brakes. So I’m confused why he would have a four pin with a direct battery connect.
I’ve always had utility trailers, no box trailers with high lights. Maybe that’s why you need a relay?
 

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So if I have zero use for trailer brakes I should be ok using the harness without the wire to the battery?
I’ve never used a battery wire on a four pin connector. Never blown a fuse or set any codes. In fact I’ve never heard of setting a code for a blown fuse on a turn signal. If you connect to the battery where does the other wire go? Four wire pin has four wires. Ground. Left turn signal. Right turn signal. Brake lights.
Maybe “CallMeJoe” can explain how to wire the light relay in?
 

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The Mopar and many other harnesses come with a small box that gets buried next to the right taillight. the hot wire goes to the battery and provides output for the circuits that go to the trailer. The tap wires that hook to the Jeep are just to trigger ( typically 200 milliamps or less) the relays that run the trailer lights. If you have LED's on your trailer, you "might" not have a problem, but if any of the taillight circuits draw more than half an amp or so on the JL, you run the risk of problems. The taillight wires are tiny and won't tolerate much amperage. I suspect if you look at the Mopar 4 wire harness kit, you'll see that it has a small box near the end. That's the relay. The Mopar 7 way/4 way trailer harnesses both use this same setup. Of course the 7 way has 3 additional circuits for brakes, battery charge line and backup lights but they are wired in addition to the trailer light relay. I suspect an aftermarket 4 way kit such as Curt, etc uses the same relay or a similar one. Not to worry, it's easy to hook up. The most difficult part is running the relay hot wire through the factory harness protectors up through the firewall to the battery connection . If you try and draw too much amperage from the taillight circuits, you'll be in for trouble.
 

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Discussion Starter #9
Thank you! That does make sense. I was confused as to why all of a sudden there was a battery wire needed on the JL when my JKU harness was plug and play with the tail light only.
 

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I have not installed my kit yet but I was wondering if there is a reason I could not just pull power from the 12 volt power outlet right next to the drivers rear quarter panel? :whistling:
 

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As long as total amperage draw is below the maximum for the outlet that would be a fine place to grab some volts. I pulled power from there to run my overhead console as the outlet can be set for battery constant or ignition on as needed. Keep in mind, I believe there are requirements that the brake lights work whether your jeep is running or not, meaning you should set the fuse for battery constant. That's probably not important for most, but in states where inspection is required, it could be a thing...
 

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It just hit me why there is an extra wire. The harness your looking at is for vehicles that have amber turn signals in the back. That’s two extra bulbs, two extra wires, and a reason for the box that “call me joe” is talking about. Are you sure you’ve got the right harness?
 

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Discussion Starter #13
From what I have been able to find myself and what eTrailer recommends, there is only one style harness available for the JL/JLU. They all have the the wire to the battery. I’m going to buy the harness and try it without the extra run to the battery. If all is well, great. If not, I’ll take the time to run the wire.
 

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The extra wire is to power the relay box, not for euro-style (5 way) taillights.
 

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One possible reason/improvement is to isolate trailer wiring. Prevent excess draw on otherwise tapped circuit. Prevent interruption of vehicle lighting if there is a problem with trailer lights, etc.
Whatever the technical reason, seems like a good idea, especially for those who like to launch a boat with the trailer wires connected...
 

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From what I have been able to find myself and what eTrailer recommends, there is only one style harness available for the JL/JLU. They all have the the wire to the battery. I’m going to buy the harness and try it without the extra run to the battery. If all is well, great. If not, I’ll take the time to run the wire.
I'd run the wire either inside or outside. I don't plan on burying my Wrangler past it's axles so I ran it high underneath in a matter of minutes. I've added a similar harness to Ford and Mazda sedans I've owned without ever having issues. You can be safe and run it inside but I'd recommend running a separate wire.
 
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