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 and Trailer Wiring Systems

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.

Troubleshooting Tool Checklist

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.

Common Wiring Issues
Problem Possible Issues Testing Procedures
Trailer lights work except for one function: brake, right turn, left turn, or tail.
  1. Harness wires are not connected to vehicle correctly/securely.
  2. (Plug-In Only) A set of connectors is not seated together properly or is not making a good connection.
  3. (Plug-In Only) A required fuse or relay is blown or missing.
  4. (Hard Wire Only) Vehicle has separate turn signal and brake lights, brake wire is not connected.
  5. Insufficient ground or short to ground on trailer or vehicle side.
None of the trailer lights work for any functions.
  1. (Powered Converter Only) 12V power wire is not connected to vehicle battery.
  2. (Plug-In Only) Harness is made for models with a factory tow package, vehicle does not have tow package installed.
  3. (Plug-In Only) A required fuse or relay is blown or missing.
  4. Harness does not have a connection to ground.
  5. Harness has been overloaded.
Trailer lights worked when the harness was first installed, but eventually stopped working on one or all functions.
  1. Ground location for harness or trailer connector is weak or ground connection has loosened over time.
  2. Harness has been overloaded due to excessive draw from lights or a short in the trailer wiring.
Using one turn signal engages the turn signals on both sides of the trailer.
  1. (Hard Wire Only) Vehicle has combined brake and turn signals, brake light wire on harness is not grounded.
  2. Insufficient ground or short to ground on trailer or vehicle side.
Turning on head lights in the vehicle causes one or more of the trailer lights to stop working.
  1. Insufficient ground or short to ground on trailer or vehicle side.
  2. Harness is overloaded when tail lights are engaged because the trailer has too many running lights.
One or more functions stays turned on, even when the vehicle ignition is off.
  1. (Hard Wire Only) Incorrect connection to vehicle wire.
  2. Insufficient ground or short to ground on trailer or vehicle side.
  3. Harness has been overloaded.
  4. (Powered Converter Only) Trailer has LED lights which can be powered by the low-level residual voltage present at the 4-Way plug.
Harness functions correctly at the vehicle side 4-Way plug until trailer is connected.
  1. Insufficient ground or short to ground on trailer.
  2. Harness is being overloaded when trailer is plugged into vehicle.
(5-Way Only) Trailer reverse lights or reverse lockout function are not functioning.
  1. Fifth wire from harness is not attached to reverse light circuit on towing vehicle.
  2. Insufficient ground or short to ground on trailer.

Basic Testing Procedures

Testing the 4-Way Plug

Method 1: Testing for Function

At The 4-Way Plug

Testing Behind the Converter Box

Behind the Converter Box

Verifying Wire Connections

Method 2: Verifying Correct Wire Connections

Hard Wire Installation

Trailer Wiring Color Code
Connector Function Harness Wire Color Where To Attach - Vehicle Side Where To Attach - Trailer Side
5-Way 4-Way Right Turn Green Vehicle's right turn signal Trailer's right turn signal
Left Turn Yellow Vehicle's left turn signal Trailer's left turn signal
Ground White Vehicle ground point - metal, uncoated, rustproof Trailer ground point - metal, uncoated, rustproof
Tail / Marker Brown Vehicle's tail light circuit Trailer's tail lights
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.

T-Connector Installation

Plug-In Installation

Grounding the Harness Separately

Method 3: Checking Ground Connections

On the Towing Vehicle

Trailer Ground Locations

On the Trailer

Fixing Damaged Wiring

Method 4: Finding Other Possible Issues

Method 5: Checking for an Overload Situation

Amperage Draws for Common Trailer Lights
Incandescent Light Tail Light Circuit Stop/Turn Circuit LED Light Tail Light Circuit Stop/Turn Circuit
.5 amps 2.4 amps STL70RB
.054 amps .362 amps
1 amps 2.5 amps STL9RB
.062 amps .259 amps
.9 amps 2.3 amps STL17RB
.121 amps .279 amps
.2 amps N/A MCL44RB
.054 amps N/A
.2 amps N/A MCL44AB
.054 amps N/A

For more information, see:

Updated by: Raymond P.

Last updated: 7/6/18

Questions and Comments about this Article

Janis K.

I paid over 300.00 a year ago to have my trailer lights working again. One year later, the brake lights worked and the left signal light. I took it to one place recently and they told me the problem is with my car. Who do you suggest I go to to find out whats wrong. Just yesterday, I tested the lights, nothing came on. They wanted to charge me 90.00 an hour. I am 72 years old, know nothing about mechanics, etc.

Reply from Chris R.

I don't have any specific recommendations as to where to take the trailer/vehicle other than to a local mechanic, but I can help troubleshoot the problem if you're up to trying (or have a buddy with some tools who can lend a hand). I'd start by testing the connector on your car - using a circuit tester to test each pin while the corresponding light function is powered. This lets you quickly see where the issue is. If you get power at all the appropriate circuits, you know the problem is on the trailer. If you don't show power then the issue is somewhere on the vehicle. If you let me know what you find we can go from there. 59048

Dean B.

Chris,I moved the ground wire to a different location, and it fixed it!Thanks for your help!Dean

Reply from Chris R.

That's great news! 59028

Louie E.

I just rewired my trailer brand new harness and lights everything works turn signals stop etc the problem is when I turn on my lights in the truck everything stops working no running lights no turn signal no stoplight everything ! I turn the lights off and everything works again, any idea of what the issue is? The trailer lights ground off the mounting points I make sure the area was clean to create good ground i also grounded the white wire from the connector i screw it to the frame of the trailer, I tested the lights without hitching the trailer to the truck would that be the problem? do i need to hitch the trailer for the lights to work? I only plugged the trailer to the truck

Reply from Chris R.

You shouldn't need to actually hitch up the trailer for proper light operation (if you did need to, that would actually indicate a weak ground somewhere). With that being said, this does still sound like the symptoms of a bad ground connection. Is your trailer by chance aluminum? What kind of tow vehicle do you have - is it equipped with a factory harness or is it something you installed afterward? I would try to check the ground on the tow vehicle harness as well. Also inspect the connector itself for any debris or corrosion built up inside the pins that needs to be cleaned out. 58960

Dean B.

I installed a trailer wiring kit, and when I hook up an inline LED indicator light, all the functions work lights, brakes and blinkers, but the trailer lights dont work when I hook it up to the trailer. Ive tried it on several trailers that I know the lights work, because we hooked other vehicles up to them. Could it be, that the ground is good enough to run the LED indicator lights, but not a good enough ground to run the trailer lights?

Reply from Chris R.

That's definitely possible. The indicator wouldn't require much power at all to illuminate. You might try to use a multimeter so you can see the actual voltage you're getting on those circuits. If you can tell me what vehicle you have, along with the model wiring kit you used, I can try and provide some troubleshooting steps that are a little more specific. 58900

Amanda S.

Hi!! We just bought a 2000 Bergen horse trailer. It has light issues. When i picked it up i was able to get all lights except on the rear passenger side. I changed the plug and found the majority of the wires were not attached. We were able to get the majority of the lights to work as they should just going off the plug, left signal right signal and tail/markers but there seems to be a short with the brake light wire. It sparks when we apply direct power to it. The connection in the box had melted so it definitely got hot. Where would be the best place to try and find where it is shorting out?

Reply from Chris R.

You've definitely got a short on that brake light wire. Honestly the best thing to do will be to try and inspect this wire the entire length of the trailer, if possible. There's likely an exposed section that's hitting metal frame (or a nearby wire). If the wire is really well hidden and difficult to inspect, you may be better off just cutting it off and running a new one. 58569

Ed A.

Front car Hazard lights flash normal. Rear hazard light left & right signal is going into converter but not out. Converter is combining turn signal and brake 5wire to 4wire or 3wire to 2wire depending on your terminology Rear lights have been removed from car and replaced with combination turn signal/stop LED lights. Rear hazard flasher will flash when brake is applied.

Reply from Chris R.

Can you tell me exactly do you have the Beetle wired up with regards to the converter. How many wires are going into the converter (along with their functions)? 57634

Ed A.

Ive tapped in a 5 wire to 4 wire converter on a 2005 VW Beetle. All tail light functions work except hazard flasher. Power tested positive to converter. Hazard flasher works when brake is applied. Have replace converter once already. What next? Im stumped.

Reply from Chris R.

Are the hazard lights not flashing at all, or is it just that the brake lights are flashing instead (where normally it would be the amber lights). A standard converter doesn't really have the "brains" required to decipher the difference between the hazards and brake lights, so it's going to send the circuit to the brake lights instead. With the converter installed, your brake lights will flash with the hazard lights on. This is completely normal and there's not really a good fix to resolve it. If this isn't what is happening though, please let me know and we'll go from there. 57567

Norm E.

Havent gt a converter yet, but testing my new five wire trailer lights LED, hooking the two lights together, grounding them on battery terminal, then testing each function seperately,each one works properly. Except green wire RT lights uptail light as does the red wire.Will a converter correct this or do I need to attach green wireto the four wire harness on the car, behind the female connector? Very confused.BTW-lights are not yet on trailer, so Im just testing them ona battery before I install them.Thanks much for your time.

Reply from Chris R.

If I understand correctly you are simply testing your new trailer lights by connecting each wire directly to power. Is that right? If so, I'm not sure what would cause the lights to malfunction other than an issue with their wiring or the light itself. A converter wouldn't be needed (at least not yet). Please let me know if I am misunderstanding something or if you can provide a bit more insight as to your current lighting setup, along with the model number of your new assemblies, and I can definitely look into this further. 57300

What Do You Think?

* e-mail address format: [email protected]

I would like an email notification if someone replies to my comment



Sports and Recreation

Trailer Parts


What our customers are saying:

"In these days of customer "no service" I am usually disapointed when doing business with people I don't know. It was refreshing doing business with you. I'm impressed!"

Watkinsville, GA

Popular Vehicles