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Troubleshooting 4 and 5-Way Wiring Installations

  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

  • Use a 12V probe-style circuit tester to check for function at the 4-Way plug (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.

Testing Behind the Converter Box

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.

Verifying Wire Connections

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.

T-Connector Installation

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.

Grounding the Harness Separately

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.

Trailer Ground Locations

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.

Fixing Damaged Wiring

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
.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

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