Common Trailer Light Wiring Problems and Solutions

You will often run into trailer light wiring problems. They can be a huge problem. However, they can be minimized by following a simple maintenance program. Let's look at some examples of problems you may encounter.

Common Trailer Wiring Problems

Problem Possible Issues
Does Not Light Up
  • corrosion in light plug or pigtail
  • ground problem
  • hydraulic lock from too much dielectric grease
  • burnt out light bulb
  • bad spot in wiring harness
  • issue in 7 way plug
  • issue from tractor
All Lights Flicker
  • a small amount of corrosion build up
  • ground problem
  • short in wiring harness
  • loose pins in 7 way
  • loose wire in 7 way
  • problem in tractor
Only Roadside of Trailer Lights Up
  • disconnected wire in 7 way
  • bad cross-over wiring harness
  • loose connection in cross-over harness
Only Curbside of Trailer Lights Up
  • usually you will not encounter this problem
  • check for loose connections at cross-over harness
  • check harness connection at roadside mid-trailer

Sealed Trailer Wiring Harness

The trailer light wiring harness installed in almost any trailer built after 2002 has a sealed, internal-ground, modular wiring harness. This means that, unlike older wiring harnesses, there is no ground located at each terminal such as an older light fixture base. All modular sealed wiring harness ground the wiring harness through the 7-way plug. This is better than the old way because open grounds allow air and moisture to get into the connection. This is the exact recipe for corrosion.

Corrosion- the Destruction of Your Warranty

Most trailer light manufacturers have somewhere between 10 years and lifetime replacement on LED trailer lights and markers. Trailer harness manfacturers usually carry between a 5 and 10 year warranty on trailer wiring harnesses. That is...unless there is corrosion present. Corrosion at any pigtail, harness connection, or 7-way box will void any factory warranty. This means you will have to pay for any trailer light wiring harness, trailer marker, or pigtail replacement parts and labor.The best way to keep your manufacturer's warranty intact is to follow these simple corrosion prevention tips.

What Not to Do to Preserve Your Warranty

Another way to void your trailer wiring warranty is to cut or splice the wiring harness. Wiring harnesses cut or chaffed due to rubbing or dragging will also void the warranty.

It is important to not pierce the trailer light wiring harness like mechanics used to do with non-sealed wiring harnesses. When a sealed wiring harness is pierced it can exhibit what is known as wicking. Moisture, salt, and other chemicals the road commission uses to de-ice roads will actually draw up into wiring harness when they come into contact with the wiring harness. Moisture can wick 3 or more feet to either side of a small hole in a trailer light wiring harness.

Other things to avoid include:

  • washing the trailer with harsh chemicals
    • they can cause the plastic lens to weaken
  • using under-sized wire when replacing damaged or worn wire
  • leaving the lights on when parked in a dock with padded frames
    • the heat created can cause the light lens to melt
    • the high heat can also shorten the life of the light bulb or LED

Repairs to Trailer Wiring, Wiring Harness Diagrams, and Corrosion Prevention

Here are some proper ways to repair trailer wiring, trailer wiring diagrams and some corrosion prevention tips.

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