Only trailer running/clearance lights not working

Start with the easy stuff first.
Pull the ends off the pigtail and make sure the screws are tight and no wires are touching.

 

Unhook the light cord from the trailer and try it again. That will normally isolate it to the truck or the trailer.
If it stops wiggle the ends of the light cord and the rest of it while watching the lights. If they dim or cut out it is likely in the cord.
After you isolate it to the truck or trailer turn off the lights for a couple minutes. Then turn them on and run to look at all the lights. More eyes are better. If you see a light that is out or dimmer than the rest start looking there. Maybe just disconnect it.

 

You have a short to ground on the trailer. The short is causing the circuit breaker to heat up and open. The breaker is resetting when you disconnect the trailer cord and remove the short, the breaker cools and makes contact again. After you find the short, replace the breaker with a new one. They cant take the repeat abuse of a short very long.

 

I see this a bunch with trailers from the north . After 5 Yeats they start getting shorts all over the trailer.
Many times when there is multiple shorts it's time to rewire the trailer but usually the trailers get sold.
Thank you salt.

 

I don’t know what type of trailer your working on, but if it has the sealed preform harness, i’ve Isolated many direct shorts (on tank trailers) by disconnecting sections. With the harnesses I’ve seen, sometimes it’s a little wire pin that locks them, some are just a zip tie.

So if for the 10-20 sec they are on, if you can’t identify a dim or out light. Start looking for disconnects in the harness, unplug it, and see if the forward lights stay on. Usually I find a disconnect mid trailer and at the rear, one for each side in both places.

Good luck, sometimes that’s most of finding shorts

 

If that reefer has the big round or octagon box where you plug the pigtail in, that is where the trouble is a lot of time.