Boat trailers, utility trailers, and many standard light-duty trailers use a 4-way wiring harness to connect to the tow vehicle. This connection provides the required running lights, brake lights, and turn signals for the trailer. There’s also an additional pin provided for a ground wire.

Trailer wiring, however, can seem complex for many people but it doesn’t have to be. And, you don’t necessarily have to be an electrician to manage. In this guide, we will walk you through the trailer-wiring process for 4-way plugs and show you how to diagnose wiring problems as well as troubleshoot for issues with your 4-way trailer.

Four Wire Trailer Tail Lights Not Working

Start by tracing the wiring from your tail lights to ensure there are no crimped, damaged, or broken wires that are causing interference with the circuit.

Another reason could be that you do not have the tail lights properly grounded. If the lights have a separate ground wire to attach to the trailer frame it will need to be connected to a suitable point on the trailer frame. The closer to the light the better.

If your light doesn’t have a separate ground wire then it is grounded through the mounting holes and you will have to be sure that the light is mounted directly to the trailer frame and making a good connection.

Use a circuit tester to check the wires and lights to be sure the ground is securely connected and you have power.

If none of the trailer lights work for any function, this may be due to several possible reasons:

• (Powered Converter Only) 12V power wire is not connected to vehicle battery. To fix this, connect 12V power wire directly to the vehicle battery.
• (Plug-In Only) Harness is made for models with a factory tow package, vehicle does not have tow package installed.  Use correct plug-in harness for vehicles without a factory tow package, if available.
• (Plug-In Only) A required fuse or relay is blown or missing. Replace or install fuse or relay as necessary.
• Harness does not have a connection to ground.

To troubleshoot for ground connection, follow these instructions:

On the Towing Vehicle

1. Check the ground area for any paint, corrosion, or build-up.
2. If any is present, thoroughly clean the area until bare metal is visible.
3. If using a factory ground screw, verify that no other ring terminals are stacked below the ground for the wiring harness.
4. If present, move ground for harness to a new location or move it to the bottom of the stack.
5. If ground area is already clean, disconnect the ground wire and attach it to a long piece of wire. Run this wire to the vehicle’s negative battery terminal for testing purposes.
6. If this fixes the problem, the ground wire can be permanently run to the negative battery terminal or moved to a new location with a verified connection to ground.

On the Trailer

1. Make sure that the ground wire (usually white) on the trailer connector is securely attached to the trailer frame.
2. If the trailer tongue folds, place the ground connection on the main body of the trailer frame, behind the tongue hinge.
3. Check the ground area for any paint, corrosion, or build-up.
4. If any is present, thoroughly clean the area until bare metal is visible.
5. Move the ground wire to the trailer frame if it is attached to an aluminum section of the trailer body.
6. Each component should have its own ground for the best performance. This reduces the risk of a ground issue on one component impacting the whole wiring system.

Harness has been overloaded.

To troubleshoot:

1. Check harness instructions for maximum amperage rating. Determine amperage draw of trailer lights and confirm that lights do not draw more power than the harness can handle. View the table below for amperage draws for some of the most commonly used trailer lights.
   1. (Powered Converter Only) If the trailer lights are overloading the harness, it is possible that the unit can be reset if the overload is not extensive.
2. Remove the fuse from the power line and leave it out for 30 minutes before reinstalling it. Then, use a circuit tester to check for function at the 4-Way plug.
3. Do not plug in the trailer before testing the vehicle’s 4-Way plug.
   1.  If after resetting the unit all functions are present, this indicates that the trailer may have a short that is causing an overdraw on the system, or that the trailer lights draw more amperage than the harness is rated to handle.
4. Try removing bulbs from additional clearance lights and connecting the trailer to the vehicle. If the harness continues to work correctly with the bulbs removed, this points to an overdraw from the amount of lights on the trailer.
5. Remove some clearance lights if possible or replace lights with the LED versions which draw much less power.

Turn Signals Not Working

If all trailer lights work except for turn signals, this may be due to the following reasons:

• Harness wires are not connected to vehicle correctly/securely.

• (Plug-In Only) A set of connectors is not seated together properly or is not making a good connection.
• (Plug-In Only) A required fuse or relay is blown or missing.
• (Hard Wire Only) Vehicle has separate turn signal and brake lights, brake wire is not connected.
• Insufficient ground or short to ground on trailer or vehicle side.

To troubleshoot, follow these instructions below:

1. Test for function as follows:

At the 4-Way Plug

• Use a 12V probe-style circuit tester to check for function at the 4-Way plug. Have a helper sit in the front of the vehicle to engage the functions while you test.
  • (Powered Converter Only) Before testing the harness for function, remove the fuse on the power wire for 30 minutes, then reinstall it. You should find this fuse should close to the vehicle battery, contained in a fuse holder. If the powered converter box has engaged its circuit protection feature, this will reset the box unless it was overloaded to the point where the internal connections were damaged.
  • Do not plug a trailer into the 4-Way plug until function has been verified with a circuit tester.
• f any function does not have a correct power reading at the 4-Way plug, test the wiring going into the converter box from the vehicle side.
• If functions are correct at the 4-Way plug, proceed to testing on the trailer.

Behind the Converter Box

• Test to ensure that the signals are going into the converter box from the vehicle side.
• If any function does not have a correct power reading, check for the following:

Plug-in harness:

• Connectors that are not securely seated together or were not plugged in flush.
• Loose wires behind connectors.
• Missing tow package/trailer wiring fuse(s) or relay(s).

Hardwired harness:

• Loose ground connection or weak ground connection.
• Wires not connected to the correct wires on the vehicle.

• Verify correct wire connections. Follow the instructions below:

For a Hardwire Installation,

Make sure that each wire is connected in the correct location. Standard wire colors are listed, but check connections by function if colors vary.

• Brake wire must be grounded with white wire if vehicle has combined turn signals and brake lights.
• (Powered Converter Only) 12V Power wire should be connected directly to the vehicle’s positive battery terminal with the included fuse installed in line. This wire is required for the harness to function.

For a Plug-In Installation,

• (Powered Converter Only) Make sure the 12V power wire is connected to the vehicle battery and that the inline fuse is not blown.
• If the harness is intended for vehicles with a factory tow package, verify that vehicle is equipped with tow package and that all required fuses/relays are installed.
• Contact dealer with VIN to verify vehicle equipment.
• Confirm that the part number on the harness is correct for the year, make, and model of the vehicle.
• Ensure that harness connectors are installed on the correct sides of the vehicle.
• Green wiring goes to the passenger side.
• Yellow wiring goes to the driver’s side.
• Make sure connectors are seated together properly.
  • Disconnect the harness from the vehicle and reconnect it, ensuring connectors click together

• Inspect vehicle and harness connectors for:
  • Bent or loose pins.
  • Loose or damaged wires.
  • Broken locking tabs.

• Check ground connections. Follow the instructions on how to test for ground

If after checking the above issues the problem remains, there could be other causes. To troubleshoot for other possible causes, follow these instructions:

• Inspect the wiring for any damaged areas where copper is visible or any spots where the jacketing on the wire is broken.
  • Repair or replace any damaged wire sections.
  • Pay close attention to areas where wire might run near moving parts.
  • Inspect and replace any butt connectors, splice connectors or direct splices that may have worked loose or are not making a secure connection.
• Check the trailer and vehicle plugs for corrosion.
  • Corrosion will usually be green or white in color.
  • If present, replace the plug or clean it thoroughly with battery terminal cleaner and a wire brush.
• If the trailer lights ground through the mounting hardware, make sure that the light mounting area is clean and paint-free, and that the surface is not made of aluminum.
  • If the mounting surface is made of aluminum, connect a wire to the light stud and ground it to the trailer frame.
• Verify that the bulb in each light is functioning correctly.
  • If damaged, replace the bulb or light assembly as needed

Trailer Running Lights Not Working

If all trailer lights work except for running lights, this may be due to the following reasons:

• Harness wires are not connected to vehicle correctly/securely.

• (Plug-In Only) A set of connectors is not seated together properly or is not making a good connection.
• (Plug-In Only) A required fuse or relay is blown or missing.
• (Hard Wire Only) Vehicle has separate turn signal and brake lights, brake wire is not connected.
• Insufficient ground or short to ground on trailer or vehicle side.

To troubleshoot, follow similar instructions for troubleshooting turn signals not lighting discussed above.

How to Test the Ground on a Four Wire Trailer Plug

To test the ground on your 4-wire trailer plug, follow these instructions below:

On the Towing Vehicle

• Check the ground area for any paint, corrosion, or build-up.
• If any is present, thoroughly clean the area until bare metal is visible.

• If using a factory ground screw, verify that no other ring terminals are stacked below the ground for the wiring harness.
  • If present, move ground for harness to a new location or move it to the bottom of the stack.
  • If ground area is already clean, disconnect the ground wire and attach it to a long piece of wire. Run this wire to the vehicle’s negative battery terminal for testing purposes.
  • If this fixes the problem, the ground wire can be permanently run to the negative battery terminal or moved to a new location with a verified connection to ground.

On the Trailer:

Make sure that the ground wire (usually white) on the trailer connector is securely attached to the trailer frame.

• If the trailer tongue folds, place the ground connection on the main body of the trailer frame, behind the tongue hinge.
• Check the ground area for any paint, corrosion, or build-up.
• If any is present, thoroughly clean the area until bare metal is visible.
• Move the ground wire to the trailer frame if it is attached to an aluminum section of the trailer body.
• Each component should have its own ground for the best performance. This reduces the risk of a ground issue on one component impacting the whole wiring system.

Four Wire Trailer Wiring Diagram Explained

Referring to a wiring diagram for a trailer is helpful during installation. The white wire is your ground wire, which must be cut short and attached to the front of your trailer frame with a screw to power your lights. The remaining wires get installed underneath. The green wire powers the right turn signal, with yellow powering the left turn signal. The brown wire is for the side marker lights and rear brake lights.

How to Connect a Four Wire Trailer Harness

To connect a 4-wire trailer harness, follow these steps below:

Step 1: Connect White Ground to Trailer Frame

Attach the white ground wire directly to a clean, bare section of the trailer frame using a self-tapping screw and ring terminal. For best performance, each component should have its own ground. For instance, each trailer light should be grounded separately along with the trailer frame. This reduces the risk of a ground issue on one component impacting the whole wiring system. You can make these individual ground connections in step 3, when you connect the rest of your wires.

The trailer connector should extend 2-3 feet past the tongue. Make sure there is no paint, corrosion, or buildup present at the ground location. Clean if necessary. If the trailer folds, make the ground connection behind the tongue, on the main body of the trailer frame.

Step 2: Run the Rest of the Wires along Trailer Frame

Run the rest of the wires along the trailer frame to the taillights. Do not allow the wires to catch or chafe against any trailer components. Feed your wires through the hollow trailer frame if possible for the best protection. You can also drill through the frame and use grommets if necessary. Alternatively, you can use zip ties or wire clips to secure loose wires along the frame.

Step 3: Make the Connections to your Trailer Lights

Once you the wires to their respective appropriate locations, it is time to make the connections to your trailer’s lights.

Each light will have 3 wires: one that connects to the main harness’s brown running light wire, one that connects to the turn/brake light wire (typically yellow or green), and a white ground that should be secured to the trailer frame.

Use a crimper to strip back the insulation on the wires. Then, use a butt connector and heat gun to connect the wires. Connect any additional ground wires from your lights.

Typically, the running light circuit (usually the brown wire) is carried on a single wire, so a jumper wire and some additional butt connectors are needed to feed the running lights on the opposite side of the trailer.

Alternatively, you can use a wishbone harness (also known as a Y harness). A wishbone harness is a 4-pole connector with 5 wires that effectively splits the running light circuit into two wires. One wire runs along the driver’s side of the trailer, and the other runs along the passenger side. Wishbone harnesses make the wiring job easier and eliminate the need for splicing in a jumper wire.

How to Test a Four Wire Trailer Harness

You may have connected the trailer to the vehicle, but the turn signals and brake lights do not work. You don’t know if the wiring is bad or you have bad bulbs. One way to troubleshoot the voltage on the connector plug is to use a circuit tester. On 4-way plug, one prong connects to the right turn signal, one to the left, and one to the taillights. One is the ground. You will need a helper to operate the controls of the vehicle while you check the voltages with a circuit tester.

Follow these steps to test a 4-wire trailer harness:

Step 1:

Tell your helper to turn on the ignition while you go to the connector plug. Connect the alligator clip on the circuit tester to a metal part of the vehicle.

Step 2:

Tell your helper to turn on the right turn signal. Place the probe of the circuit tester into the connection that has the green wire. The light on the tester should blink on and off.

Step 3:

Tell your helper to turn on the left turn signal. Place the probe of the circuit tester into the connection that has the yellow wire. The light on the tester should blink on and off.

Step 4:

Tell your helper to shut off the turn signal and step on the brake. Check the green and yellow connections again. The light on the tester should stay on for both.

Step 5:

Tell your helper to release the brake and turn on the lights. Place the probe of the circuit tester into the brown wire connection. The light should stay on.

Tell your helper to turn off the lights and shut off the ignition.

How Do You Wire a Trailer with Four Wires?

You will need a 4-pin wiring kit that is 20 feet or more in length. With the 4-wire system, you have four wires that work with the electrical system of the vehicle via a plug connector. The color code for wiring harnesses can vary between manufacturers, but the general standard for 4-way plugs is as follows:

• Green: Right turn/brake light
• Yellow: Left turn/brake light
• Brown: Tail/running light
• White: Ground wire

When buying wires for trailers, make sure you get the right wire gauge appropriate for a trailer wire 4-way cable. A small gauge trailer wire might not withstand the electric current, and chances are that a short circuit will occur. The minimum recommended gauge of a 4 core trailer wire is 18 AWG for the running lights, turn lights, and brake lights. For the ground wire, the recommended size is 16 AWG.

The first thing you have to do when installing your 4-pin trailer wiring is to make sure that your connector works properly. Make sure that all cables are electrically conductive. Another thing, you should check your vehicle’s owner manual for insights before connecting the 4-wire trailer wire.

Step 1: Connect White Ground to Trailer Frame

Attach the white ground wire directly to a clean, bare section of the trailer frame using a self-tapping screw and ring terminal. For best performance, each component should have its own ground. For instance, each trailer light should be grounded separately along the trailer frame. This reduces the risk of a ground issue on one component impacting the whole wiring system. You can make these individual ground connections in step 3, when you connect the rest of your wires.

The trailer connector should extend 2-3 feet past the tongue. Make sure there is no paint, corrosion, or buildup present at the ground location. Clean if necessary. If the trailer folds, make the ground connection behind the tongue, on the main body of the trailer frame.

Step 2: Run the Rest of the Wires along Trailer Frame

Run the rest of the wires along the trailer frame to the taillights. Do not allow the wires to catch or chafe against any trailer components. Feed your wires through the hollow trailer frame if possible for the best protection. You can also drill through the frame and use grommets if necessary. Alternatively, you can use zip ties or wire clips to secure loose wires along the frame.

Step 3: Make the Connections to your Trailer Lights

Once you the wires to their respective appropriate locations, it is time to make the connections to your trailer’s lights.

Each light will have 3 wires: one that connects to the main harness’s brown running light wire, one that connects to the turn/brake light wire (typically yellow or green), and a white ground that should be secured to the trailer frame.

Use a crimper to strip back the insulation on the wires. Then, use a butt connector and heat gun to connect the wires. Connect any additional ground wires from your lights.

Typically, the running light circuit (usually the brown wire) is carried on a single wire, so a jumper wire and some additional butt connectors are needed to feed the running lights on the opposite side of the trailer.

Alternatively, you can use a wishbone harness (also known as a Y harness). A wishbone harness is a 4-pole connector with 5 wires that effectively splits the running light circuit into two wires. One wire runs along the driver’s side of the trailer, and the other runs along the passenger side. Wishbone harnesses make the wiring job easier and eliminate the need for splicing in a jumper wire.

Vehicle Side

Now that your trailer is hooked up, the next step is to wire your vehicle. If your vehicle already has a 4-way connector, then great! Simply plug the trailer-end connector into the vehicle-end connector, and you are done. If your vehicle does not have a 4-way connector, it is usually pretty simple to add one. Just remember that wiring is a custom component. There are several ways that a wiring harness can connect to your vehicle, and the type of connector you use depends on what is available for your vehicle year, make, and model.

Also be sure to confirm the function of your vehicle wires if you are splicing or clamping wires together. Check your vehicle’s owner manual or use a circuit tester to confirm each wire’s function prior to connecting. You can usually tap into your vehicle’s wiring system with a circuit tester at the rear of the vehicle, behind the taillights. Activate each function (brake lights, turn signal, etc.) in turn and check for a corresponding signal from the circuit tester to make sure you have the right wire.

To wire your vehicle, follow these steps:

Step1: Connect Ground to Vehicle Frame

Attach the white ground wire to a clean, bare metal surface on the vehicle frame. Apply some dielectric grease to help prevent corrosion.

Step 2: Make Vehicle Connections

Your wiring harness will either clamp onto, plug into, or splice into the existing lighting of your vehicle. The type of connection depends on what is available for your vehicle model.

Some general notes and tips:

• Some vehicles require you to route a power wire from the harness to the vehicle battery because the vehicle’s electrical system cannot handle the amperage draw of the trailer lights.
• If the vehicle connector is under the vehicle, use a mounting bracket to attach it to the vehicle. This will help prevent damage that may occur if the connector is left dangling.
• Use a small amount of grease on all electrical connections (the plugs on your automobile and the 4-pole connector itself) to help prevent corrosion.

Clamp-On Style

Some harnesses, clamp onto your vehicle’s wiring without causing feedback, interference, or power draw from your vehicle’s wiring system. Clamp the harness’s sensors to the appropriate vehicle wires, then run the hot lead to your vehicle’s battery to provide the necessary power, since the harness does not draw from the wiring itself.

Plug-In Style

Other vehicle manufacturers essentially “pre-wire” your vehicle so that your wires are easily accessible for connection. T-connectors simply plug into your vehicle’s existing wiring, no cutting or splicing required.

Typically, the plug-in location is near the taillights, underneath the vehicle, or behind the paneling in the back cargo area. You may have to remove your trim access cover, scuff panels, taillights, etc. so as to access the plug.

T-connectors can also be expanded to 5-way, 6-way, or 7-way trailer connectors through the use of a wiring adapter

Splice-In Style

Hardwire kits are not quite as convenient as the other styles, but splicing into your vehicle wiring is actually not a difficult and complex as it may sound.

After confirming your wire functions (using your owner’s manual or circuit tester), connect your wires using one of three methods.

• Soldering

This is the best way to connect wires. Simply use a soldering gun to solder your wires together for the strongest, most reliable connection. Use heat shrink tubing to protect the soldered connection.

• Butt Connectors

If you are not comfortable soldering wires, you can use heat shrink butt connectors and a heat gun.

• Quick Splices

The quickest, easiest way to connect wires is by using a quick splice. Quick splices force a metal piece into two separate wires, thus connecting the circuit. However, quick splices are the least reliable wire connection method.

How Do I Know If My Trailer has a Bad Ground?

If the lights on your trailer work erratically, this is usually a sign you have bad ground.

To diagnose a bad ground, measure the ground potential directly at the offending light bulbs against a known good ground, the trailer connector is best.

To get a good ground, the connections need to be corrosion and oxidation-free. You can use dielectric grease to prevent early corrosion.

Does a 4 Wire Trailer Need to be on the Hitch for Lights to Work?

In some cases, trailer lighting will ground through the hitch ball but that is typically only if the actual ground circuit is insufficient. This is not an ideal setup. It needs to ground through the wiring of the trailer harness.

You can test this by setting the coupler on the ball. If it works, then you know the ground on the trailer needs to be improved. Otherwise, you will get intermitted lighting functions when your trailer hits bumps.

Why is Only One Side of My Trailer Lights Working?

The primary reason behind most trailer wiring issues is poor ground connections. So, this should be the first place to check if only one side of your trailer lights is working.

The lights will have either a ground wire or they will ground through the mounting hardware. Each of the ground connections should be secured to a clean, bare metal surface on the trailer frame or to the negative terminal of a trailer battery if you have one.

If the issue is not the ground connections, then start at the trailer side connector. With your trailer connected to the tow vehicle, use your circuit tester to test the left side stop/turn and running circuits. Probe the wires, past the trailer connector, at different points working your way toward the trailer lights. This will tell you if there is a signal on those circuits and, if there is, where the signal terminates.

If there is no signal in the left stop/turn or running light circuit, then the problem could be in the connector itself. The pins could be bent or damaged. If you did the connector wiring yourself, remember to wire the connector to the socket by function, as opposed to color. Aftermarket wiring can use different colors for corresponding functions.

Where Do You Ground the White Wire on a Trailer?

Attach the white ground wire directly to a clean, bare section of the trailer frame using a self-tapping screw and ring terminal. For best performance, each component should have its own ground. For instance, each trailer light should be grounded separately along with the trailer frame. This reduces the risk of a ground issue on one component impacting the whole wiring system.

Check this to: How to Change Garage Door Opener Frequency

Can You Mix LED and Regular Lights on a Trailer?

You can mix LEDs with incandescent lights on a trailer, though is not recommended. The problem is the amount of power required by each type. LEDs require far less power than incandescent. If one of each were on the same circuit, the draw from the incandescent could damage the LEDs.