Troubleshooting 4 and 5-Way Wiring Installations

Wiring issues can be frustrating and time consuming to fix, especially when you are not sure where to begin troubleshooting. When your trailer lights aren't working, your trailer is not working, and you are losing valuable time and money. Using the following information and testing procedures, you should be able to locate and eliminate the most common issues encountered during or following a 4 or 5-Way wiring harness installation and get back on the road as soon as possible.

Where to Begin

When it comes to troubleshooting a wiring installation, testing is everything. A lighting problem can originate at any point along the wiring on either the tow vehicle or the trailer, so it is important to determine not only what is causing the problem, but where it is located. The question the troubleshooter will find themselves asking is "There are so many things to check. Where do I start?"

Vehicle Wiring System versus Trailer Wiring System Image

Perhaps the most helpful thing you can do to start troubleshooting is to determine whether the problem is on the tow vehicle or on the trailer. When you use your trailer to test, you have no way to know for certain whether the problem stems from an issue with the wiring harness because the trailer's wiring system is still part of the equation. Testing the vehicle without the trailer will allow you to separate the wiring system into manageable sections.

Use the testing procedure described in Method 1 to check for function at the vehicle's 4-Way plug. If all the functions come out correctly at the vehicle plug, you will know that the problem only appears when the trailer's wiring system is connected and you can concentrate your efforts on the trailer.

What Tools Do I Need?

The most helpful tool to use will be a 12V probe-style circuit tester. Click here to see our circuit testers. In addition, you will want to have basic wiring tools on hand. These include electrical tape for repairing connections, a wire stripper for cleaning up wire ends, dielectric grease, and various wiring fasteners like butt connectors, quick splice connectors, or ring terminals, depending on the type of repairs required. Wiring kits that include these items are available.

A trim fastener removal tool can reduce the time needed to access your tail light wiring, particularly in SUV and minivan applications, but a flat-head (slotted) screwdriver will often work just as well. A power drill will be used to install the screws for the ground wires. A 12V battery can also be useful for testing trailer lights in situations where you don't have access to the vehicle or you suspect an issue with a particular trailer light.

Troubleshooting Wiring Tool Checklist Image

Common Wiring Issues

Problem

Possible Issues

Testing Procedures

Trailer lights work except for one function: brake, right turn, left turn, or tail.

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.

Method 1
Method 2
Method 3
Method 4

None of the trailer lights work for any functions.

(Powered Converter Only) 12V power wire is not connected to vehicle battery.
(Plug-In Only) Harness is made for models with a factory tow package, vehicle does not have tow package installed.
(Plug-In Only) A required fuse or relay is blown or missing.
Harness does not have a connection to ground.
Harness has been overloaded.

Connect 12V power wire directly to vehicle battery.
Use correct plug-in harness for vehicles without a factory tow package, if available.
Method 1
Method 2
Method 3
Method 5

Trailer lights worked when the harness was first installed, but eventually stopped working on one or all functions.

Ground location for harness or trailer connector is weak or ground connection has loosened over time.
Harness has been overloaded due to excessive draw from lights or a short in the trailer wiring.

Method 1
Method 3
Method 4
Method 5

Using one turn signal engages the turn signals on both sides of the trailer.

(Hard Wire Only) Vehicle has combined brake and turn signals, brake light wire on harness is not grounded.
Insufficient ground or short to ground on trailer or vehicle side.

Method 1
Method 2
Method 3
Method 4

Turning on head lights in the vehicle causes one or more of the trailer lights to stop working.

Insufficient ground or short to ground on trailer or vehicle side.
Harness is overloaded when tail lights are engaged because the trailer has too many running lights.

Method 1
Method 3
Method 4
Method 5

One or more functions stays turned on, even when the vehicle ignition is off.

(Hard Wire Only) Incorrect connection to vehicle wire.
Insufficient ground or short to ground on trailer or vehicle side.
Harness has been overloaded.
(Powered Converter Only) Trailer has LED lights which can be powered by the low-level residual voltage present at the 4-Way plug.

Method 1
Method 2
Method 3
Method 4
Method 5
LED lights may remain on dimly whenever trailer is connected to vehicle. Disconnect trailer when parked for extended periods.

Harness functions correctly at the vehicle side 4-Way plug until trailer is connected.

Insufficient ground or short to ground on trailer.
Harness is being overloaded when trailer is plugged into vehicle.

Method 3
Method 4
Method 5

(5-Way Only) Trailer reverse lights or reverse lockout function are not functioning.

Fifth wire from harness is not attached to reverse light circuit on towing vehicle.
Insufficient ground or short to ground on trailer.

Attach fifth harness wire to vehicle reverse light circuit.
Method 2
Method 3
Method 4

Basic Testing Procedures

Method 1: Testing for Function

At the 4-Way Plug

Use a 12V probe-style circuit tester to check for function at the 4-Way plug. Click here to see our circuit testers. (Figure 1). 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. This fuse should be located 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.
If 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 (Figure 2).
- 2-Wire vehicle: Green (passenger side) and yellow (driver's side) wires should carry turn signal and brake light functions.
- 3-Wire vehicle: Red wire should carry brake light function, turn signals should be on green and yellow wires.
If any function does not have a correct power reading, look for:
- 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.

Method 2: Verifying Correct Wire Connections

Hardwire Installation

Make sure that each wire is connected in the correct location (Figure 3). 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.
- (5-Way Only) If trailer requires a connection for back-up lights or a reverse lockout feature, ensure that the fifth harness wire (typically blue) is connected directly to the vehicle's reverse light circuit.

How to Verify Correct Wire Connections Diagram

Trailer Wiring Color Code

Connector

Function

Harness Wire Color

Where to Attach - Vehicle Side

Where to Attach - Trailer Side

4-Way and 5-Way

Right Turn

Green

Vehicle's right turn signal

Trailer's right turn signal

4-Way and 5-Way

Left Turn

Yellow

Vehicle's left turn signal

Trailer's left turn signal

4-Way and 5-Way

Ground

White

Vehicle ground point - metal, uncoated, rustproof

Trailer ground point - metal, uncoated, rustproof

4-Way and 5-Way

Tail/Marker

Brown

Vehicle's tail light circuit

Trailer's tail lights

Only 5-Way

Reverse

Blue

Vehicle's reverse light circuit

Trailer's reverse lights or electric lockout solenoid

This chart is a typical guide; wire colors may vary based on manufacturers. Use a circuit tester to verify connections.

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.
- Consult our T-One Fit Guide for more information.
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 (Figure 4).
- 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.

Method 3: Checking Ground Connections

On the Towing Vehicle

Check the ground area for any paint, corrosion, or buildup.
- 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 (Figure 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.
- If this solves the issue, the ground wire can be permanently run to the negative battery terminal or moved to a new location with a verified connection to ground.

Checking Ground Connections on Towing Vehicle Image

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 buildup.
- 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 (Figure 6). This reduces the risk of a ground issue on one component impacting the whole wiring system.

Checking Ground Connections on Trailer Image

Method 4: Finding Other Possible Issues

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 (Figure 7).
- 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.

Method 5: Checking for an Overload Situation

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.
(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.
- 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.
- Do not plug in the trailer before testing the vehicle's 4-Way plug.
If all functions are present after resetting the unit, 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.
- 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.
- Remove some clearance lights if possible or replace lights with the LED versions which draw much less power.

Amperage Draws for Common Trailer Lights

Incandescent Light

Tail Light Circuit

Stop/Turn Circuit

LED Light

Tail Light Circuit

Stop/Turn Circuit

ST70RB

.5 amps

2.4 amps

STL70RB

.054 amps

.362 amps

ST9RB

1 amps

2.5 amps

STL9RB

.062 amps

.259 amps

ST17RB

.9 amps

2.3 amps

STL17RB

.121 amps

.279 amps

MC44RB

.2 amps

N/A

MCL44RB

.054 amps

N/A

MC44AB

.2 amps

N/A

MCL44AB

.054 amps

N/A

For more information, see:

Trailer Wiring Diagrams

Routing the Power Wire for a Trailer Wiring Harness

Updated By: Dani S.

Last Updated: 3/2/2021

Questions and Comments about this Article

Ben W. 8/21/2021

Installed plug and play harness on my truck. Now the green wire is constantly hot on the 4 pin plug. Makes the passenger tail light stay lit like brake is on all the time. Anyone else ran into this? Have a remedy?

Reply from Victoria B. 8/23/2021

What year, make, and model truck do you have? Is the plug and play harness you installed one that has a powered converter box for the trailer lights? If so, test the green wire going into the box and see if it has constant power. It should not - it should only show power when your brake pedal is pressed or the right turn signal is on. If it is showing constant power then there is something going on with the truck's wiring. If it showing power correctly, but the green wire coming out of the converter box has constant power then that is an indication there is a short inside that converter box and it would need to be replaced.

John D. 8/16/2021

2014 Toyota Camry hybrid. All turn signal lights work. Only thing that not comes on are brake or stop lights at night

Reply from Victoria B. 8/23/2021

Have you been able to determine if the issue is on the vehicle wiring harness or if the issue is happening somewhere on the trailer wiring?

Jonathan 8/2/2021

2016 Ford Explorer, plug in connector....worked fine when I installed, now no power to the flat connector. Checked connection - good. Wires from vehicle go from connector inside body - can't really trace them. Not sure where to look next??

Reply from Victoria B. 8/3/2021

I would check your trailer wiring fuse(s) on your Explorer to see if they have blown and need replaced. From what I could find in the online 2016 Explorer owner's manual, fuse location 70 takes care of the trailer tow stop and indicator signals.

Bill C. 7/31/2021

Hi, I've got a 4 way flat connecting to a trailer. I have all functions except brake lights on the trailer. I am puzzled because the brown wire is for brakes and tail lights. I have traced it to the vehicle. The vehicle has brake lights and tail lights combined. LED lights. I've checked the ground so far. Thanks

Reply from Victoria B. 8/3/2021

On a traditional 4-Way trailer connector the functions by wire color are: green - right turn and brake, yellow - left turn and brake, brown - tail light, white - ground. I don't think I've ever run into a brake signal and tail light signal carried on the same wire. If you're sure the issue is on the vehicle side and since you said you've already checked the ground, I would check the trailer wiring fuse locations on your tow vehicle, if your vehicle has them. Since only the brake light wiring is not working, it may be that the trailer brake fuse has blown and needs replaced.

Al A. 7/17/2021

I have trailer brake/running/turn signal that the turn signal is not working. I tested the lead that goes to the light and the tester blinks so apparently there is power to that lead. I put in a new light and have the same issue. Could this be the result of a weak ground or? All other lights work fine although the other rear turn/running/brake light is pretty dim. Thought on what I should do next? Thx!

Reply from Jon G. 7/19/2021

Whenever there is an issue with trailer lights the best 2 things to do are check all of your connections (including grounds) to make sure that they are clean and secure, and to check your wires for any damage.

Reply from Al A. 7/19/2021

@JonG Thx Jon. I’ll clean the grounds today and see if that makes a difference. I suppose it could be the light itself but I put a new light in and had the same result. If cleaning the grounds doesn't work I’ll take the starboard side light and swap sides. I know that light works. Seems I’m always fussing with trailer lights.

Reply from Jon G. 7/19/2021

@AlA That's a good plan. Just know that troubleshooting lights is a very common question here at etrailer so you aren't alone!

Oak 7/1/2021

hello I have a after market dual 7 & 4 pin connector on my car when I check voltage on 7 pin I get 12 v but when I check the 4 pin I get only 4v nothing else and non of my trailer lights work when plugged in I believe this start happening after I accidently dip the dual connector in water while launching a boat, any thoughts or recommendations? (my trailer lights tested good with another vehicle) thanks.

Reply from Jon G. 7/5/2021

Check your fuses, connections, and the plugs themselves to make sure everything has a solid and clean connection. I'm guessing your trailer plugs in via the 4-Way? Are you getting the correct functions and voltages when testing the light functions on the 7-Way?

Reply from Oak 7/5/2021

@JonG Jon thanks for replying to my question so I resolved the issue my converter burn up after the 4 pin got dipped in water.so after I replace it with a new one all my lights work like new.

Reply from Jon G. 7/7/2021

@Oak Awesome! Glad to hear that you were able to get it fixed.

Mike S. 6/17/2021

Just purchased a used Trailmaster trailer. Had a DrawTite hitch installed on my 2018 Nissan Pathfinder. Replaced the trailer side 4 pin connector to accommodate the new vehicle side 4 pin connector. I have lights in all markers and lamps, and left turn signals. I have no right turn signals, and no brake lights in either lamp. This would seem to rule out a grounding issue. ALL bulbs are functioning. I've looked at right side wire (brown and green) and I don't see any exposed wire. All splice connectors seem solid. What am I missing???? Thanx in advance!!!

Reply from Jon G. 6/22/2021

First rule of thumb when it comes to troubleshooting wiring is that it could almost always be a bad ground so I would never rule that out. What I would start of doing is isolate which side of your towing setup the problem is on. Try testing the pins on your vehicle 4-Way and then test your trailer 4-Way by either applying 12V power to it or hooking it up to another vehicle that you know is working properly. Let me know what you find and we can go from there.

Drobertson 6/5/2021

I have a new utility trailer that came with a wiring kit. After inspecting the kit, I find that the end that I would normally expect to have the 4-way connector only has the four colored wires hanging loosely. What is the most logical way to proceed? Do I solder the ends to a connector or is there a type of connector that can be opened and the wires connected inside?

Reply from Jon G. 6/9/2021

You can definitely use a connector like the Hopkins # HM48192 that allows you to just place the wires into the connector but I recommend doing something a little more solid by using heat shrink butt connectors # DW05744-5 and a connector like the Easy-Pull Harness # HM48114 .

Allan H. 5/30/2021

I have a new 12 volt magnetic tow light kit for my 2015 Camry XLE that is not working on my car; but when connected to my neighbour's new 2020 Toyota truck the lights work fine. I connected my tester to my Camry's trailer plug and the indicators light up. My neighbour seems to think that my car lacks the power required for the lights to work?

Reply from Jon G. 6/1/2021

That's what I would say too. Have you been able to get it to work with anything else? It might be a bad ground or damaged wiring that is pulling power from the circuits so you aren't able to get full power back to your magnetic lights.

Kent J. 5/21/2021

Have a brand new small utility trailer. When connecting the 4-pin flat connector to the vehicle (2019 F150) the trailer lights all function correctly. When using the 4-pin flat to 7-pin round adapter and then plugging the round adapter into the truck the left side turn/brake lights do not work on the trailer. Both the flat and round connectors on the F150 are factory equipped options and were not add-ons. I have tried multiple new 7-pin round to 4-pin flat adapters, and even tried another brand new trailer, and the same situation occurs. I have also tried the trailers on different vehicles with no change.

Reply from Jon G. 5/21/2021

You might have a poor ground on the trailer and then a bad fuse or connector on your truck. Have you tried walking through the troubleshooting steps that we show in this article?

Reply from Kent J. 5/21/2021

@JonG I'll double check all the grounds and fuses and report back to you. I just thought it odd with multiple trailers, vehicles, and adapters all doing the same thing.

Monk 5/11/2021

Vehicle has 7 way round connector appears to be wired correctly. Bought adapter to 4way and everything works except brake lights. Looking in the adaptor, there is no terminal for the red brake pin on 7 way to travel into 4 way. How is a 4 way trailer supposed to get it’s brake signal through this adapter?

Reply from Jon G. 5/11/2021

When you say the 7-Way is missing the brake terminal are you talking about for the brakes themselves or the brake light function? The brake light function is normally on the same circuit as the turn signals. Attached is a link to our helpful article about 7-Way wiring. Please note that you'll want to compare per function and not wire color. You might have a fuse that needs to be replaced in your vehicle. You'll want to consult your owner's manual to see what that may be.

Helpful Links

faq-wiring-7-way.aspx

Mike G. 4/8/2021

I have a 7 pin connector i hooked it to my trailer and the right side light came on all the time so i tested the green pin and got 12 volts with my vehicle off i have a converter installed and i checked the ground when i removed it the yellow was also hot all the time is it possible the converter is bad is there any way to test it or bypass the green wire the vehicle lights work fine on the trailer everything works accept the right brake and turn signal

Reply from Jon G. 4/8/2021

Whenever there is a problem with trailer lighting it typically either boils down to a poor ground connection or some faulty wiring. If you checked your vehicle connector and the stop/turn pins were lit up the whole time then the issue is probably on your vehicle and you need to check the wiring. To test the converter you would want to test the wiring going into and coming out of the converter to see if it is the converter or further down the line.