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Routing the Power Wire for a Trailer Wiring Harness


Some trailer wiring harness 4-Way and 7-Way connectors require a power wire to be routed from the harness to the vehicle battery. The power wire is required on trailer wiring harnesses for some vehicles because the electrical system on those vehicles cannot handle the amperage draw associated with trailer lights. The power converters and ModuLites used in these applications connect to the vehicle battery to provide the power for the trailer lights, protecting the vehicle's sensitive electrical system. On vehicles that require a 7-Way connector, a power wire will typically be routed to the battery via a 40-amp circuit breaker to allow the 12-volt circuit in the connector to function. Note that the power wire should not be connected to any power source other than the battery. For instance, you should never connect the power wire to the 12-volt socket found in some vehicles' cargo areas. Connecting the power wire to an alternative power source could overload the circuit and damage your vehicle's wiring.


Modulite wiring kit Labeled wiring connector




How Do I Route the Wire?


First, you need to know where the battery is located. Typical locations include:


Once you know where your battery is located, you can decide which of 3 methods will work best for you. There are 3 ways or paths you can take when routing the power wire to the vehicle battery.


Each method has its pros and cons. You can use any method, or a combination of methods, for the results that suit your specific needs and application.





Method 1: Under the Vehicle


The most basic method is also the fastest and easiest way to route the power wire from your wiring harness or 7-Way to the battery. First you should follow the installation instructions for the wiring harness or 7-Way leading up to routing the power wire.


In most vehicles, the power converter will be located in the trunk or cargo area or behind a tail light of the vehicle. On some vehicles, the power converter is mounted underneath the vehicle on the outside. A 7-Way is mounted outside of the vehicle at the back.


Read in more detail about how to install a 7-Way


Grommet and opening in car body - for wire routing Underside of auto body - for routing wiring




Wiring ties fuse holder Battery post of circuit breaker



The benefits of this method are that it is fast and easy. A drawback is that the wire is not as protected from the elements as it could be.





Method 2: Through the Frame


The second method will help protect the wire better than leaving it exposed under the vehicle. This method requires that you route the power wire through the frame of the vehicle toward the front where the battery is located.


Routing the wire through the frame can be tricky. Every frame is different, so this method may or may not be an option.



Underside of car body


The benefit of this method is that the wire will be more protected from the elements. The drawbacks are that installation is not as easy, takes longer, and makes it more difficult to troubleshoot the wire should something malfunction. Also, some frames are only partially open, so you would need to weave the wire in and out of them.





Method 3: Through the Vehicle


The third method of routing the power wire involves running the wire through the vehicle interior. This is usually a must if the battery is located in the trunk or cargo area or under the backseat. Every vehicle interior is different, so the path you choose to take is up to you.


Usually, this method requires that you route the power wire first through the trunk or cargo area. For a 7-Way installation, unless the battery is located in the interior of the vehicle, Method 1 or 2 is recommended.


You can then loosen or remove some interior trim panels or carpeting to hide the wire before entering through the trunk to the backseat area of the vehicle.



Wire routing path - trunk through back seat

From this point there are several options:


Once you have reached the front of the vehicle, you can route the wire through the firewall and into the engine compartment. When you go through the firewall, you may find an existing wire bundle going through a grommet that you can use.


You may find that you will need to drill through the firewall. If you do have to drill, use caution and do not drill through any components on the other side. Once the wire is in the engine bay, connect the power wire to the battery or circuit breaker as described in Method 1.


The benefit of this method is that the wire is the most protected from the elements. The drawbacks are that it is the most time-consuming method of installation, it can be difficult to troubleshoot the wire, and you may need extra wire to reach the battery.





Tools Needed for Routing the Power Wire of a Trailer Wiring Harness


Additional tools for routing wires

Most of the wiring harnesses that require a direct connection to the battery, such as #119176KIT, come in a kit that includes wire ties for the Method 1 installation. If you choose Method 2 or 3, you may need some additional tools:



Updated by: Raymond P.

Last updated: 9/17/18





Roy H.

7/31/2022

i have a 2011 tocoma prerunner the lights on truck work fine but the trailor hitch has no power at all for the lights the hitch part is all factory but when you plug in the trailor you have no power to anything etc no brake, taillights or siginal

David B.

8/1/2022

If you have no power at all, I suggest checking the fuse(s) as well as your grounds. Check and make sure there isn't any damage either that is causing a short. I would also get a circuit tester and see if you have power at the plug while it is connected to a trailer, try a different trailer to see if it works as well. Check the grounds and, if applicable, fuses on the trailer.

Danny B.

5/26/2022

Like many here, I am a bit confused as to how a 7 pin ties into a 4 pin flat connector. Tow vehicle is 2018 Rogue. Here are my 2 or 3 dilemmas: 1. I found a 12v constant power source on drivers side, behind rear panel in trunk (actually, a black and blue bundle).. It does produce a constant 12 volts, it seems (some called in factory Tow ready package?). Anyway, we used that wire to connect the 4 pin flat connector and the black module; connected black wire from module to the wire we found by tire jack on drivers side). Trailer lights / turn signals / brake lights work fine. Seems to work, so why would running the power wire to the battery be necessary? 2 . how do you now connect the 7 pin conversion kit? Specifically, WHERE would the black wire from the 7 pin connect to? (remember, I already used the power wire to connect to the the black wire on the 4 pin module; do i simply splice INTO it? Or do I remove it?

David B.

5/27/2022

The power wire is required on trailer wiring harnesses for some vehicles because the electrical system on those vehicles cannot handle the amperage draw associated with trailer lights. I would read the first paragraph on the "routing the power wire" faq page it will give reason why the power wire needs to be ran separate. I recommend watching the install video of the T-one wire harness on this 2018 Rogue.

Niels D.

9/1/2021

Thank you for the instructions. I currently have the 12V line protected with a 10A fuse at the battery, and a 15A BUSS self-resetting circuit breaker after the wiring harness. Would you recommend I replace this with the 40A breaker? If so, where would you recommend I put said breaker?

Victoria B.

9/7/2021

@NielsD I've never heard of a wiring setup like that. Yes, I would recommend replacing your current fuse and breaker with a 40amp breaker in-line on your 12V hot lead wire. This 40 amp breaker is typically installed close to the battery, somewhere inside the engine compartment. I've attached a help article about 7-Way connectors that will offer you some help. Under the Vehicle Side, Step 2: Make Vehicle Connections is where you'll get the most beneficial information.

Matt

8/29/2021

Am I able to connect a 4 plug trailer connector to the led taillights of my vehicle if I have led trailer lights for power? (As opposed to running a wire from the battery to the back of the vehicle?

Victoria B.

9/7/2021

@Matt Depending on your vehicle you may be able to hardwire a 4-way trailer connector into the vehicle's lighting system. If you are using a wiring harness that has a powered modulite converter, it definitely needs to be powered from the vehicle's battery. A benefit to having a harness that uses a power wire is that the trailer's lights will pull power from the battery and only use your vehicle's lights for signal. Even LED trailer lights pulling power from your vehicle's LED lights and wiring could put a strain on your vehicle's lighting system.

Joe S.

3/18/2021

I'll be replacing my AGM battery with a lithium battery in my travel trailer very soon. I don't necessarily want the power wire to charge my battery. I just want to make sure my lithium won't draw a huge amperage across the small 12V wire trailer plug. Is this something I need to be concerned about?

Etrailer Expert

Kef G.

3/23/2021

The change to a lithium battery shouldn't be an issue. It won't draw more than the 12V that your vehicle's wire is capable of sending. The 12V wire is mainly there to ensure that your battery doesn't completely die. Really, you'll end up with very similar results to your old battery in terms of charging from the 12V wire.


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