Why would the driver side turn signal on the trailer work, but not the passenger side?

Check your pigtail pin connection. Look for bent/tarnished/damaged/missing pins or pins with grease on them before anything else. Also check the holes as well to make sure the metal hasn't stripped off for example. Next either get a voltage tester and check the pins are actully giving voltage or replace the pigtail.

Odds are good that is the problem. If not it may be a blown fuse though thats unlikely if the truck lights work. Otherwise something shorted in the trailer which can be much harder to track down.

 

Pull your pigtail and meter it while you run your blinkers. If you have juice, it's in the trailer. If not, it's in the truck. Work your way that direction. But my guess is a loose/busted wire at the pigtail, or corrosion inside of one of the 7-ways.

 

On a freightliner, the right and left are two different circuits.
Make sure it isnt a blown fuse. If it has a resetable breaker. lightly touch the outside, if it is hot you have a short

 

Thanks for ck. back in that you got it working, that turn sig. must be working all the time, if you roll up to a DOT insp. and its quit again you will be put out of serv.

 

There is another quick test I have done many times over the years. If you have been doing pre/post trips you should have confidence in the tractor systems working right. If the tractor's lights are working but the trailer is not AND you have been doing the pre/post trip inspections on the rig in the days before you can be 99% sure it's in the trailer. If I am where I can I will report that trailer and go slide under another one.

 

If all it was was the pigtail, the long term fix is either spread the pins on the trailer pins (if not replacing that socket) or replacing the pigtail end on the tractor

Both are annoying and it may be cheaper timewise to just buy a new green cord with the ends already done up

 

Exactly what I do. Every time I grease the truck, I pull both my pigtails and grease both ends with this stuff. It's also great for O-rings and glad hand gaskets, because the silicon grease doesn't eat rubber/nitrile/viton/ect.
https://www.amazon.com/Dow-Corning-Ring-Silicone-Lubricant/dp/B08P5RJVLW