Trailer Wiring, Weird Light Behavior

Hello everyone,

I just bought a used cargo trailer (a 6' x 12' enclosed Cargo King), and the lights are acting... strangely. They are pretty dim, and the brake lights work only occasionally. When the turn signal is activated, all of the lights flash, slowly. Here's some more information:

My tow vehicle is a 1979 Chevy C-10 Silverado (pickup). It has a 6-pin round connector. I bought a 6-round to 4-flat adapter (I don't want to post a link here, but it's a Curt 57620 adapter that is easily found on Amazon) that I'm using to connect the trailer to the truck.

The ground wire on the trailer was spliced together and frayed off at the end when I bought it, so this weekend I removed the splice, unscrewed the screw from the frame of the trailer, sanded it down to metal, and reconnected the ground with a new crimp connector and screw. The ground coming from the back of the truck connector seems to be connected solidly to the steel bumper. No change in the behavior of the lights.

I bought a trailer light tester from AutoZone, and the "lights" LED lit up when I turned the lights on. The "left" LED blinked when the left turn signal was turned on. However, when I turned on the right turn signal, the "right" LED stayed dark, and the "lights" LED started blinking.

So that's where I'm stumped. I don't have a great mental grasp on electronics, so my ignorant assumption is that there is either a short somewhere, or the adapter (or connector on the truck) is wired wrong. I know that the PO of the truck used it to tow a 5th wheel camper, so I assumed that the truck's wiring was OK - but perhaps something went pear-shaped between then and now.

For a little more information, I also hooked my father-in-law's Jeep up to the trailer (before I fixed the ground) and the lights didn't really act the way I expected them too, either. But, I understand that I've changed a few variables between then and now.

Any help, or suggestions on systematic methods of testing the truck, trailer, or setup as a whole, would be profoundly appreciated. Again, as I am not well-versed in electronics and wiring, explanations and directions in layman's terms would be all the more helpful.

Thanks again, and take care!
-Brandon

Hello again,

I'd like to provide an update with a little more information about the trailer. I hooked it up and took it for a shakedown drive yesterday, and I want to pass along the behavior of the lights when attached to the truck.

When hooked up and the trucks headlights are turned on, almost all of the trailer lights (tail light and side, rear, and front marker lights) are illuminated. I suspect bad bulbs in the lights that aren't illuminated. When the left turn signal is activated, the left turn signal on the trailer flashes. I should note that the light on the dashboard blinks more slowly than it does without the trailer hooked up.

When the right turn signal is activated, all of the lights on the trailer blink slowly. The right turn signal indicator on the dashboard turns on and does not blink.

When the brakes are applied, all of the lights on the trailer go out. They come back on when you take your foot off the brake.

All lighting functions on the truck (besides the behavior of the turn signal indicators on the dash) behave normally, even with the trailer hooked up.

Hope this information triggers an idea in someone's head.

Thanks again,
Brandon

Alright, wrapping up with the solution.

I brought the truck to my mechanic and he told me that someone had wired the connector all the way up to the front of the truck (to a fuse box or something), and there must have been a short or crossed wire or something along the way. He pulled all the old wiring out and wired the brake lights from the wires at the back of the truck, basically cleaning up the wiring, and now the lights work as good as new.

An hour of mechanic's labor well worth it.

Hope this is helpful for someone in the future.

i would suggest doing a continuity check between right side turn stop wire and same between right side tail and marker lights


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mike childs