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



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