How can I troubleshoot the wrong trailer light

[liusigou]

The two trailer lights flash when I turn on the left turn light. But it works normal when I turn on the right turn light.

 

[briangcc]

9v battery to the trailer harness prongs will tell part of the tale.

If your tow vehicle has a wiring adapter in it, chances are good it could be going south. Otherwise, troubleshoot tow vehicle.

If you have a different vehicle to test with that should quickly isolate the issue to trailer or tow vehicle.

 

[sublauxation]

This is why I no longer mess around and just add a junction box to the winch post and run individual wires to each light....but as mentioned above check the tow vehicle fist with a test light to rule it in/out.

 

[dingbat]

Turn signals are powered via separate relay outputs. The only way this can happen is the left turn and tail light wires are switched or the left turn signal is somehow shorted to ground, back feeding the power to the tail light circuit.

With the connector unplugged from the vehicle, there should be no continuity between the brown (tail light) and yellow (left turn signal) wires.

If you have continuity, the problem (short) is in the trailer. If you don't have continuity, the problem is in the tow vehicle.

 

[brodmann]

It's much more complicated than that on a lot of vehicles. With most newer vehicles, the blinkers are amber colored and the brake lights are red. So, the brake lights are wired together. Trailers don't have separate lights for blinkers and brakes. So, on the trailer, one bulb with two elements has to serve as both blinker and brake light. That's why many vehicles have a trailer light adapter. Thise little boxes that connect to the left and right tail light harnesses and then form the trailer light connector have some sort of voo-doo inside of them that makes all of that stuffwork and separates the comonality of the brake lights and sends the signal from the blinkers - which is separated left/right and the signal from the brake lights - which are common to both left and right, and makes it all work like it's supposed to. Like I said.......VooDoo in that box. I seem to have to replace my trailer light adapter about every 3 years. I've ordered from Amazon an picked them up at local auto parts stores and they all seem to be the same and last about the same amount of time. I pull some sort of trailer almost every day for work or play though.

 

[04fxdwgi25]

Or an open / bad ground on one of the lights on the trailer. If one light isn't grounded properly, it usually looks for a ground through any common wiring such as the running / tail lights.

Look for a bad ground on the left tail light.

 

[liusigou]

Afer I replaced all the lights in the back end, it works. Do you know what happened?

 

[bruceb58]

Probably a bad ground somewhere. A sneak path ground path was causing the weird issue.

 

[dingbat]

Afer I replaced all the lights in the back end, it works. Do you know what happened?

I'm guessing you had moisture and or corrosion in the lamp socket which shorted 12v flasher (turn) signal to the tail light circuit energizing the other lights as well.