Trailer Lights Out: Working Backward From the Plug

Lights out on a trailer is a DOT stop waiting to happen and it is usually a small problem. The trick is working through it in an order that finds the fault instead of replacing bulbs that were never bad.

Start at the plug and work back.

Ground first, always

The majority of trailer lighting faults are grounds. A trailer grounds through the plug and through the frame, and both of those corrode. Salt air along the Gulf makes it worse and the panhandle has plenty of it.

Symptoms of a bad ground are strange: lights that dim when you signal, brake lights that come on with the turn signal, markers that glow faintly instead of lighting, or one circuit working through another. Any of that behavior means current is finding a path it should not.

Check the ground pin in the plug, the ground strap at the trailer nose, and the grounds at each light housing.

The plug and the cord

Pull the plug apart and look at the pins. Corrosion, bent pins, a pin pushed back in the housing, or a wire pulled out of a terminal are all common. Dielectric grease keeps moisture out and it is worth using every time you reconnect.

The cord itself takes abuse. Look for chafing where it rubs, cracked insulation, and pinch points behind the cab.

Isolate the trailer from the truck

If you have another trailer or another truck available, swapping tells you which side the problem is on in about a minute. A trailer that works behind a different truck means the fault is in the truck's circuit.

Common failure points on the trailer

• Corroded ground at the light housing, which is the single most common fault
• Water in a sealed light, which corrodes the contacts
• A chafed wire against the frame, which grounds a circuit intermittently
• Splices made with tape instead of proper connectors, which corrode
• A pinched harness where the trailer flexes
• Corrosion inside the junction box at the nose

Intermittent is the hard one

A light that works until you hit a bump is a connection that is nearly broken. Wiggling the harness while somebody watches the lights finds most of these.

Intermittent faults do not get better. They fail completely at the worst time, which is usually at night on an inspection.

Carry spare bulbs, a few connectors and dielectric grease. Half the roadside lighting problems anybody has are fixable in ten minutes with those three things.

LED versus incandescent

LED lights draw far less current and last far longer, and they are also less tolerant of voltage problems and moisture intrusion. An LED light that is flickering usually has a supply or ground issue rather than a failing light.

Mixing LED and incandescent on the same circuit can also produce odd behavior on some systems.

Do not tape it and run

A twisted-and-taped repair on a trailer harness corrodes and fails, and it fails somewhere you cannot see. Proper connectors sealed against moisture are the difference between a repair and a delay.

Testing without a helper

A circuit tester at the plug tells you what the truck is sending before you go looking at the trailer. If the signal is not arriving at the plug, the trailer is not the problem.

From the plug back, a tester at each junction narrows it fast. Working with a helper watching lights is faster if you have one, and you can do it alone with a tester and some patience.

The nose box

The junction box at the trailer nose is where a lot of faults live, because it is where every circuit lands and it is exposed to weather. Corroded terminals, water sitting in the bottom of the box, and connections that were never sealed properly are all common.

Open it and look before you go chasing a harness the length of the trailer.

If you have lights out and you are due for an inspection, or you are sitting somewhere in the dark trying to trace a harness, call (850) 495-0366. Dispatch is answered 24 hours and we come to you with the connectors and the tools to do it right rather than to get you to the next exit.