Toubleshooting trailer lights

[Ishmael]

I can't seem to get the lights on my daysailor's trailer to work. My truck has a 6-prong (blades) outlet under the bumper, I got an adaptor to go to the flat 4-wire plug on my trailer lights, and the light set is new. Anybody have any simple, logical troubleshooting advice? Thanks.

[SkiprJohn]

They are a mystery to me but every time I've had trouble with trailer lights it has always been the ground wires that seem to be the culprit. Check for any disconnects or corrosion or thick paint. I've also had to rewire the harness that was already installed on the truck a couple times just to make certain I have compatible parts.
Do you know for certain that your truck side outlet is getting juice, i. e. fuse?
Good luck and I hope some other folks respond to this to help solve the mystery.
JohnL

[scallywag]

Ishmael,
Trailer lights are about as fun as taking an ice shower. I second SkiprJohn. Check the ground. A corroded ground will not let the lights work. Are the trailer bulbs good? Check the lights with a multimeter using the continuity function. If the bulbs all check out, are they the right bulbs for the light sockett. My son did me a favor and replaced one of my bulbs with the wrong type of bulb. It was $150 for the electrician. I initially tied into the turn and brake wires on my car and used the chassis as ground. When the ground is going bad all sorts of things happen, like dimmed turn signals or brake lights. Just some thoughts to consider.

John

[Ex-Calif]

A lot of modern vehicles do not like the trailer wiring to be grounded to the car chasis. Also many vehicles will advise to get a power booster to run the additional load of the trailer lights.

Start with the documentation of the vehicle. There should be a section in the owner manual about trailering.

Then troubleshoot the trailer lighting separately. Trailer lights are set up in many various ways. Primarily you will see one running light circuit that lights up the side lights and one filament of the (usually) two filament tail light bulbs. On occasion you can have dual bulbs in the tail light.

The second circuit is the tail light "bright." This is where things are usually broken down. The bright filament needs to be powered by either the turn signal, separately and intermittently, or the brake light power which is both filaments. So going to the vehicle should be 4 wires minimum. Ground, running, left high and right high.

Ohm out each circuit and as stated check all the grounds. When I wire a trailer there are no grounds to the trailer. Each circuit is a twin wire, tied back at the front and grounded through the vehicle through the plug. The fewer grounds and connections the fewer failure points.

The final piece is to check the mapping of your 6 to 4 adapter. Using the DC function on your meter, map out the outputs from the 6 prong on your vehicle, then plug in the adapter and make sure the output (4-prong) matches what the trailer needs.

[Steve Rust]

I don't know where you live but here in Minnesota most plugs that sit under the bumper will only last a few years. Even with a cap on they fill up with crude and corrode away. I use a van for a tow vehicle and have my plug wire on the inside with a long lead of wires that can reach out under the back door when needed.
Many trailers are pooly wired in the first place so you always have a few failure points. Check for any splices in the wire as they are seldom done in a waterproof manner. Best thing to do if this is an older trailer is to rip out the old wire completely and replace it and don't cut corners by making your splices with electrical tape, wire nuts, or those crimp on splices. I solder my connections, apply a thin coat of sealant then heat shrink over the top.

[Moonchaser2304]

I agree with Steve about totally rewiring the trailer. Auto stores have trailer wiring kits that are only about $60 and get the one with LED tailights. Trailer lights classically are dim and hard to see from behind and the LED tailights are BRIGHT!!
It only takes a couple hours to rewire the whole trailer. Use crimpted barrel connectors, not just wire nuts (or even better, solder like Steve said). You can stick a bunch of reflectors on the sides of the trailer too while you're at it so when car lights hit it the trailer is real visible.

[David M]

Trailer lights are seemingly designed to fail and like everyone else is saying, it is almost always corrosion at the ground..somewhere.

The biggest problem is they design these trailer light systems to be grounded through the trailer ..which of course is just asking for trouble.

I would start from scratch with all new wiring and lights. This time when installing, coat every last bare wire and terminal with something like Tef-Gel to completely stop the metal from oxidizing...including the light bulb terminals. Use heat shrink connectors and heat shrink tubing where possible. Ancor sells the heat shrink connectors. If you can, run dedicated grounding wire instead of grounding everything to the trailer. Use heat shrink ring terminals to connect your ground wire to the terminals on the lights where you connect the lights to the trailer..and plenty of Tef-Gel to cover the metal.

[SkiprJohn]

Didn't you say your light set was new? Anyway, how's it going.
JohnL

[Steve Rust]

I will second David's comment about the dedicated ground wire. That alone fixed the problem on one of my trailers.

[SkiprJohn]

I just had to revive this one. Yesterday I towed the club's West Wight 19 from a neighbors house to get the trailer lights working for a safety check (Hawaii requirement). I got it to my yard and had both turn signal lights working, 4 way flashers working on the left side only and no response when I turned on the headlights for the truck. I worked on the lights for 2 1/2 hours by fixing crushed wires, decorroding ground terminals and redoing connections that I thought might be bad. NO CHANGE!!!
I said a few sailorly things under my breath and decided to tow it to a friends house so that he might work on the boat and fix the lights this coming week. I parked out on the main road in front of the drive where I needed to back down and turned on the 4 way flashers. Voila. Everything worked perfectly. Too late to get the safety inspection. Unbelievable trailer light mystery.
JohnL

[David M]

The designs for boat trailer lights work just fine, provided you don't have to back the trailer into the water. The designers of trailer lights have obviously never had to use their own products. Which is not a whole lot different than men designing bras.

It blows me away how we have all these incredible things for boats that are designed to hold up to the elements and yet the trailer lights are still crap.

Somebody could make a fortune selling trailer lights with a dedicated ground wire.

[SkiprJohn]

I think they do it on purpose. Kind of like making different copper tubing sizes for refrigeration and plumbing and gas and then call them all different names. They do it just to p--- me off.

I must say though that the new LED trailer lights sure are bright.

JohnL

[Brandywine]

just pull it all off and replace it with new light kit... autozone etc less then 40$