Trailer Lights and Wiring: Why They Fail and How to Fix Them

Boat trailer lights have a reputation, and they have earned it. Nothing else on the rig is asked to run hot at seventy on the highway and then be dropped into cold salt water a minute later, several times a weekend, for years. That they fail is not a mystery. That they fail the night before a long trip is just timing.

Why do boat trailer lights fail?

Four reasons, and they are usually working together.

• Heat then water. An incandescent globe runs hot. Dunk it and the glass, the lens and the air inside all contract at once; water is drawn in past the seal, and a hot globe hitting cold water simply cracks. This is why the tail lights go first and why the old advice was to unplug the trailer before backing down the ramp.
• Corrosion in the connections. Salt water in a bullet connector, a globe holder or a plug pin builds a green crust that conducts badly and then not at all. It shows up first as a dim light, then a flickering one, then a dead one.
• A bad earth. The single most common fault on any trailer. On a lot of trailers the earth return is through the coupling and the tow ball, and a greasy or rusty ball is a poor conductor. Symptoms are lights that come on dimly when they should not, indicators that flash the wrong lights, or everything working until you press the brake.
• Chafe. Cable run along the chassis rubs through where it passes a spring hanger or a cross-member, and the first you know of it is a blown fuse in the car.

LED units, and why they changed the game

A sealed LED lamp has no globe to crack, no air space to draw water, and draws a fraction of the current, so it runs cool and the pins carry less load. Fitted properly, a set of submersible LED lamps outlasts incandescents many times over and can be backed into the water switched on without a second thought. They are the default now and worth the change on any trailer still running globes. The one thing they are fussy about is the earth: an LED with a poor earth flickers or ghosts, where a globe would just be dim. Sealed LED lamps and the rest of the lighting hardware are in boat trailer parts and accessories, with the Narva marine and 12-volt range in its own Narva collection.

The plug and its pins

Almost every trailer in Australia uses the flat seven-pin plug, and its pin layout is standardised, which makes fault-finding straightforward with a cheap plug tester. Counting across the flat plug: left indicator, reversing, earth, right indicator, electric brakes, stop lamps, tail lamps. The wire colours follow the same order: yellow, black, white, green, blue, red, brown. If a light is doing something odd, the tester on the car socket tells you in seconds whether the fault is on the car side or the trailer side, which is half the job.

Fixing it so it stays fixed

Repairing trailer wiring with twisted joints and insulation tape is why it needs repairing again next season. Done properly:

• Run a dedicated earth wire from the plug to each lamp rather than relying on the chassis, and bolt the earth to clean bright metal at the lamp end.
• Use tinned marine cable, not automotive. Plain copper strands wick salt water up inside the insulation and corrode from the inside out.
• Crimp, then heat-shrink. Adhesive-lined heat shrink over every joint, so the joint is sealed rather than wrapped. Tape holds water in; it does not keep it out.
• Route the cable inside the chassis rail or in split conduit where it crosses anything it can rub on, and secure it every few hundred millimetres so it cannot swing.
• A smear of dielectric grease in the plug pins and lamp connectors keeps the salt out of the one place you cannot heat-shrink.

Cable, connectors and heat shrink for the job are in our Wurth electrical range. Do the whole trailer at once. A trailer with one new lamp on old wiring has not been fixed.

What the law expects

A trailer on the road needs working tail lamps, stop lamps and indicators, a number plate lamp, and reflectors, and the police in the peninsula towns check them. Lights are also the part of a trailer most likely to be picked up in a roadside inspection, because they are the part the inspector can see from the footpath. If the brakes are on your list as well, our guide to trailer brakes covers those, and the two jobs are often done together.

We rewire and re-light boat trailers at Kadina as part of trailer and brake repairs — sealed LED lamps, tinned cable, a proper earth, and a plug that works on the first try. Ring 0412 532 984 before the trip rather than from the side of the road on it.