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?"
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.
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.
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
Hard Wire 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.
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.
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 (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.
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.
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.
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