Trailer Wiring Diagrams
Trailer Wiring Connectors
5-Way Connectors
6-Way Connectors
7-Way Connectors
Mounting Your Trailer Wiring Harness
Trailer Wiring Adapters
Troubleshoot Your Trailer Wiring
Problem: There is no signal going into the box, meaning something is incorrect on the vehicle. Solution: Solve the problem by checking for blown fuses and then check the installation points. Use a circuit tester to check that the correct wires are tapped into, the wiring harness connectors are plugged into the correct vehicle connectors, and check that connectors are plugged in all the way. Problem: If a signal is going in and coming out on the appropriate wires, then there is a problem with the trailer wiring. Solution: Solve the problem by inspecting the wiring on the trailer to make sure all of the connections are correct and ground wires are connected properly. Most likely, the ground wire on the trailer is not secured properly. A trailer wiring system is grounded to the frame near the coupler and each light also needs to be grounded. If there is not a white ground wire coming out of a trailer light, then the light is grounded through its mounting studs. Too much paint, dirt, or rust can cause bad grounds, so make sure the ground is secured to the bare metal frame. Also look for pinched or cut wires; these are often found when wires are routed above suspension components or behind taillights. Problem: If a signal is going into the box and not coming out or a signal is going into the box and coming out on the wrong wires the ground on the vehicle may not be connected properly. If the white ground wire is not installed properly on the vehicle, then the green and yellow wires will not carry signal coming out of the box. Solution: Solve the problem by checking to make sure the ground wire is installed properly on the vehicle. To make sure you have a suitable ground, first connect the circuit tester's ground wire to the same location as the ground wire on the wiring harness. Then test the signal going into the converter or modulite box. If there is no signal, connect the circuit tester to another grounding location such as the exhaust pipe. If a signal is present, the grounding location for the wiring harness is bad, and a new location needs to be found. On a few vehicles, the frame and body components are not grounded. In these rare instances, look for a ground wire coming out of the license plate light.
Issue: Converter Shorting Out
Too many lights on the trailer. Each incandescent taillight draws about 2 amps and side marker lights typically draw about 0.5 amps each. Most converters allow up to 4 amps to pass through them. A standard converter cannot have any more than 1 taillight on each side. Water. Trailer connectors get wet, causing too many amps to be pulled through the converter. Water can unite all 4 wires on the trailer connector causing it to draw too many amps. This often happens when a boat trailer is backed into water with the trailer wiring still connected to the vehicle. To solve the problem of connections getting wet, always disconnect the trailer connections before backing into the water. Also, covers can help prevent water from getting into trailer connectors when it rains or when a vehicle is taken through the car wash. Bare wires touch each other or the trailer frame. This causes too many amps to be pulled through the converter box. The coating on trailer wires can wear thin, often at the connection to the trailer's taillight or where the wires pass over the trailer suspension. If wires are hung too loosely, they can get pinched and smashed between the trailer frame and suspension U-bolts. To solve the problem, inspect the trailer wires and replace any that are damaged or frayed.
Issue: Weak Ground
The connector on the vehicle should have a ground wire secured tightly to a clean surface on the frame. A wire attached to the body or a surface with undercoating or rust can cause a ground problem. The connector on the trailer should have a wire from the connector secured tightly to a clean surface on the trailer. Each tail lamp assembly at the back of the trailer also must be grounded. This is done in one of two ways. The first is by a separate wire coming from the back of each tail lamp assembly being secured to the frame. The second, and most popular, ground is achieved through the bolts of the tail lamp assembly. In this case, the lamp housing must be attached to the frame of the trailer. If the lights are mounted on wood or PVC material, the light will not receive ground. The last possibility to explore is in the trailer design. If the trailer has a tilting bed, it is possible the ground is not passing through the pivot point, resulting in a poor ground. The solution to this is to route a ground wire from the connector at the front of the trailer to each lamp assembly at the back of the trailer, bypassing this pivot point.
How to Wire your Car or Truck to Pull a Trailer
T-One Connector
Custom Hardwire Kit
Standard 4-Pole Wiring Harness - For use with vehicles that have adequate power and standard wiring system, these simply connect into existing wires on the vehicle and have a 4-pole flat connector to attach a trailer. Converter - For use with vehicles that have separate turn and brake light wires. Some vehicles send only one signal per wire, creating what is called a 3-wire system: one wire for the left turn, one wire for the right turn, and one wire for the brake signal (common on vehicles with amber turn signals). A converter will reduce it to a standard 2-wire system needed for wiring a trailer. There are still additional wires for the running lights and for the ground. Any vehicle with amber turn signals will need a converter. However, there are some vehicles with all-red tail lights that can also require a converter. A wiring harness with a converter has a black box built in it. Five wires go into the box, and only 4 come out. The converter transfers the brake signal on the vehicle into the left and right turn signals for the trailer wiring system. Modulite or Powered Converter - Used with vehicles that do not provide enough electrical power to handle the additional strain of powering trailer lights, the Modulite or powered system draws power directly from the battery but still connects to the vehicle's wiring system to determine when to power the lights on the trailer. If there are too many lights on the trailer for the vehicle's electrical system to provide adequate power, use a modulite or powered converter. A modulite installs the same way as a standard converter except an extra wire must be run to the battery. Instead of drawing power from the vehicle wiring system, a modulite draws power directly from the vehicle battery. This is safer because the extra amps to power the trailer are no longer going through the expensive electrical components of the vehicle. More vehicles are using thinner gauge wire and require a modulite, regardless of how many lights are on the trailer, simply to protect their wiring system.
Daulton R.
11/8/2023
Thank you for providing the guidance and instruction that you do. It is extremely helpful for a lot of people. The more I visit your site the more I want to come back for help, and purchase from you.
Jenny N.
11/16/2023
@DaultonR Hey thank you for the kind words! Feel free to post a comment or ask a question at any time. We appreciate ya!
Daulton
11/17/2023
Come to think of it I do have a question related to a trailer modification. I tore down an old nasty Shasta trailer and I am making some modifications to the frame. I haven't done a title search but I do have a bill of sale. Should I have taken the trailer to the DMV before I did any changes to the frame? It still has the original numbers stamped on the tongue. I am adding a foot of length. Do I have to have the trailer bed finished or can I take it in with just the frame. I have finders and will add lights, etc next. Thank you for your comments and your time.
Jenny N.
11/17/2023
@Daulton Sounds like a challenge and I would love to be able to answer that for you however each state has their own rules when it comes to trailer regulations. I know it may not be the most fun of experiences however reaching out to your local DMV is going to be the best bet to answer this question,
Pj
10/26/2023
We bought a brand new 2023 dump trailer at auction. It was made in Mexico and does not have standard color-coded wiring. We have all lights and brakes, but when we turn on the headlights of the tow vehicle, the brakes on the trailer come on and stay on.
Mike L.
11/12/2023
@Pj The connector on your trailer is wired incorrectly. You'll want to wire it by function, not by wire color. You'll want to apply 12V to each wire on the trailer's connector to see which function each circuit carries then wire it correctly according to the chart in the linked article.
Helpful Links
Char
10/21/2023
I have a 2008 Dodge Ram 1500, with an unplugged blue plug labelled "Electric Brake" under the dash, but only a 4-flat connector at the rear. What do I need in order to utilize the electric brakes on my trailer?
Nik C.
10/24/2023
@Char you need a trailer brake controller. I use and reccomend a Tekonsha Prodigy P3 controller, available here at etrailer.
Mike L.
11/9/2023
@Char Nic C. is correct. Install your brake controller of choice, use the correct brake controller harness like # 3020-P for a Tekonsha brake controller. You'll need to sever the blue wire exiting the rear of the controller, and attach it to the blue wire you found under the dash. At that point, you should be good to go. Let us know if you have any questions!
Mike L.
11/9/2023
@Nick Depending on the year/make/model vehicle, we might have an install video that you might find helpful...
Neil W.
11/12/2023
Awesome page. I’m trying to replace my 6-pin plug with a 7-pin on my gooseneck trailer. Brakes have worked great with an adapter in the past. When I look at the wiring in the 6-pin, the brake pin is not connected and there isn’t a blue wire. Just a black wire on the 12V center pin. I noticed the page said “Note: In some applications, the black battery power connection and the blue brake controller output connection are reversed.” I’m totally confused how the brakes have been working with the factory brake controller without the brake pin being used. Any help would be appreciated.