Trailer Wiring Troubleshooting

Trailer Wiring Troubleshooting


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Right hand turn signal and brake light Left hand turn signal and brake light Tail and marker lights Ground Aux. power options Electric brakes Backup lights

4- Way plug

5-Way plug

6-Way plug

7-Way plug

With the lights on, when you apply the brakes, all the lights on the trailer go out.

The ground connection is strong enough to provide some lighting functions but not all. This is called a weak ground. When the taillights and brake lights are used at the same time, it creates the maximum amp load of the lights on the trailer. Check the following to restore proper ground:

  1. The connector on the vehicle should have a ground wire secured tightly to a clean surface on the frame. A wire attached to the body or a surface with undercoating or rust can cause a ground problem
  2. The connector on the trailer should have a wire from the connector secured tightly to a clean rust free surface on the trailer.
  3. Each tail lamp assembly at the back of the trailer also must be grounded. This is done in one of two ways. The first is by a separate wire coming from the back of each tail lamp assembly being secured to the frame. The second, and most popular, ground is achieved through the bolts of the tail lamp assembly. In this case, the lamp housing must be attached to the frame of the trailer.  There must be a solid (clean and rust free) electrical connection. 
  4. The last possibility to explore is in the trailer design. If the trailer has a tilting bed, it is possible the ground is not passing through the pivot point resulting in a poor ground. The solution to this is to route a ground wire from the connector at the front of the trailer to each lamp assembly at the back of the trailer bypassing this pivot point.
NOTE: It is not recommended to ground thru the trailer coupler to hitch ball.


Wrong turn signal blinks.
This is usually related to one of three things. The first could be a short somewhere in the system. All wiring and the vehicle connector should be checked and tested.. The second, which is the most common problem, is a weak ground in the system. Refer to "GROUND PROBLEM" above.  Third, the left (flat four yellow) side and right (flat four green) side are connected in reverse.


Always unplug boat trailer when backing trailer into water.
You should always unplug your boat trailer when backing into the water. When the trailer lights are submersed in water, it can create a short causing the vehicle fuses to blow and/or result in damage to your wire harness and vehicle.


Blowing fuses on vehicle.
The first thing to check is that you are using both Part 1 and Part 2 of the system. Both pieces, if a second piece is included with your T-Connector, are crucial for correct operation. The back up light fuse will burn out if part 2 is not used. If Part 2 is being used, the fuse burn out is a direct result of a live wire shorting to ground or an overload of too many lights on the trailer. Check all wires for bare copper or places where a wire could be pinched and calculate the amp load your trailer is pulling. Most vehicles are equipped to handle only standard lighting on trailers; one turn / brake signal on each side and one marker light on each front corner. Check your vehicle owner's manual for specific ratings.


Turn signals are dim and flash rapidly.
When you add trailer lights to a vehicle system, the turn signal amp load basically doubles. Most vehicles have a flasher that controls the blink of a turn signal. The standard flasher provided in many vehicles will not handle this extra load. You should replace your flasher with a heavy duty flasher (8 to 10 amps) and make sure the vehicle engine is running. The flasher will slow down your turn signals and a running engine will increase brightness of the lamps.


Stubborn wiring problems 
Check all wiring insulation as it runs from the front to back of trailer, concentrating on where the wire enters/exits the frame or passes over ends of frame rails or edges.  Road vibration can wear thru the wiring insulation at these points, shorting out the wire. 


Major causes of lighting failure -

Tips to prolong equipment lighting -

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