On a Saturday morning, every boat ramp on Moreton Bay has a queue. Manly, Wynnum Creek, Cleveland, Victoria Point, Scarborough, Cabbage Tree Creek and Bribie are all busy. Every one of those trailers goes into salt water, and salt water is the worst thing that can happen to trailer wiring. This guide explains how trailer wiring works in Australia, why it fails, and how to set up a boat trailer so its lights keep working season after season.
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- Sealed, submersible LED lights. No globes to crack and no gaps for water.
- Tinned marine-grade cable. It resists the corrosion that creeps along bare copper.
- Heat-shrink joins with adhesive lining. Never use twist-and-tape joins.
- A dedicated earth wire back to the plug. Don’t rely on the trailer chassis.
- Rinse with fresh water after every trip.
How Australian trailer plugs work
Most Australian boat and box trailers use a 7-pin flat plug. Caravans usually use a 12-pin flat plug, which adds pins for charging and other circuits. Some older or heavier trailers use a 7-pin large round plug. The 7-pin flat wiring is standardised:
| Pin | Circuit | Usual wire colour |
|---|---|---|
| 1 | Left indicator | Yellow |
| 2 | Reversing lights | Black |
| 3 | Earth (negative) | White |
| 4 | Right indicator | Green |
| 5 | Electric brakes | Blue |
| 6 | Stop lights | Red |
| 7 | Tail and clearance lights | Brown |
Check the pin numbers stamped on the plug itself. Socket and plug are mirror images, so it’s easy to get them crossed when you’re looking at the back of one.
Why boat trailer lights fail
Salt water gets in
When a trailer is backed into the water, water is forced into every gap in the lamp housings, plugs and joins. When it dries, the salt stays behind and keeps drawing in moisture. Corrosion follows. You see it as green crust on the terminals, but the damage also travels inside the insulation, along the copper strands, well away from the visible end.
Hot globes crack
An old-style incandescent tail light that has been on during the drive to the ramp is hot. Dunk it in cold water and the glass can crack, and every launch lets more salt water into the housing. That’s why fully sealed, submersible LED lamps such as the Hella submersible LED rear combination lamp, which run cool and keep the water out, are the better choice. Want your trailer lights upgraded? Get a quote from a local auto electrician.
Bad earths
Many trailers use the chassis as the earth return. Galvanised steel, rust, paint and corrosion all add resistance. The result is dim lights, lights that flicker, or one light feeding back through another: the classic “indicator makes the tail light flash” fault. A dedicated earth wire from each light back to pin 3 fixes most of this.
Chafe and vibration
Wiring rubbing on the frame, or flapping loose, wears through its insulation. Corrugated roads to camp sites and the bumps of a launch ramp make it worse.
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Setting up a boat trailer properly
- Lights. Use sealed, submersible LED trailer lights. Many owners also fit the lights on a removable light board or on raised posts, so they never go under water at all.
- Cable. Run tinned multi-core trailer cable in one continuous length per side if possible, and use conduit where it runs along the frame.
- Joins. Keep joins to a minimum. Where you must join, use crimp connectors with adhesive-lined heat-shrink, or a sealed junction box mounted high on the drawbar.
- Earth. Run a dedicated earth wire to every light, joined at the junction box back to pin 3.
- Plug. Fit a quality plug with a cover. Coat the pins with a little dielectric grease, and check the vehicle socket too.
- Number plate light. It’s required, and it’s often the first light to fail. Make sure it’s sealed as well.
After every trip
- Rinse the whole trailer with fresh water, including inside the lights and the plug if you can reach them.
- Let it dry before covering it or parking it in the garage.
- Check that all lights work before you leave home. It takes 30 seconds with someone watching.
Trailer brakes
Brake rules depend on the trailer’s aggregate trailer mass (ATM). Brakes are required once a trailer is over 750 kg ATM, and the requirements step up again for heavier trailers. Many boat trailers use mechanical override (surge) brakes, which suit salt water better than electric brakes. If your trailer has electric brakes, the vehicle needs a brake controller and the blue wire (pin 5) must be connected. Check the current Queensland rules for your trailer’s mass with the Department of Transport and Main Roads.
When to call an auto electrician
- The vehicle’s towbar socket isn’t working. Modern cars often need a vehicle-specific towbar loom or module so the car’s computer recognises the trailer and doesn’t throw bulb-failure warnings.
- You want a brake controller fitted.
- The trailer’s loom is corroded right through and needs replacing.
- There’s an intermittent fault you can’t track down.
Find a local auto electrician on the Wynnum Manly, Northside or Moreton Bay pages, or in the Brisbane directory.
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