This information is what I have learned over the years working in the trailer industry. It is not meant to be used to teach college kids, although it would probably help them. It is meant to try and help you fix something that is actually not that difficult but we make it harder than it is. I hope this information helps you.
– Allen Roberts

Ok so we’re all hooked up and ready to leave but now the lights on the trailer won’t work. I know they worked the last time I pulled it! No one else has used it. The wife and kids are standing in the yard giving you that look, come on dad just fix it and lets go! So what do you do? Well we’re men so the first thing is we walk around the trailer looking at the lights and we might even kick the tires as we walk by. We know this could ruin our whole trip and we remember the last time we had trouble with 12 volt wiring. It turned into two days of searching for bad wires! I’m going to try and make this simple! Ok so you’ve already cussed at the trailer and the mfg. and probably the dealership that sold it to you. You have a brand new truck and you know it’s perfect, or is it?

  1. The first thing to do is plug in the trailer
  2. Turn on the running (clearance) lights
  3. Make sure truck has key turned on and try blinkers (Not Hazards)
  4. Have someone press the brake

Ok now we should have an idea of which of these circuits is having the problem and we can start to solve the problem. Now you’re going to think I’m crazy but I want you to unplug the trailer! What, is this guy the actual dummy! How can I fix it with the trailer unplugged? The most common mistake people make is when they have trailer light problems they don’t determine where the problem is. Is it the truck or is it the trailer. Now lets say that everything worked except the running (clearance) lights.

  1. Turn on vehicle lights
  2. Use 12volt light tester with ground connected to something metal on the vehicle and check for power on the plug. If it’s a flat black plug with four wires you will have three females and one male prong. The male prong is actually the vehicle ground and the female beside it is the running light wire. Check to see if you have power.
  3. You probably don’t right?

I know what you’re thinking now, how can it be my truck? Well things do happen, you might have pulled another trailer with that brand new truck. Like you wanted to show Bob down the street how much power it had and how easily it could pull anything. That could have been the whole problem, Bob’s 1950 Homemade trailer more than likely had a short in the running lights and it blew your fuse. Bob didn’t tell you that he only pulled the trailer during the day cuz the lights didn’t work but it was OK because the blinkers and brakes worked fine.

Ok so lets fix the problem. Find the owners manual for the truck. I know what you’re thinking now. The lights on my truck work fine! Yes they do, the issue here is the trailer wiring circuits and new vehicles that come from the factory with the tow package (hitch installed) have a separate wiring system just for the trailer. This is actually good because the vehicle lights keep working properly even if there is a problem with the trailer system. Ok so look in the index of the manual and find fuses, turn to that page and start looking for the one labeled TR running lights or it may even say trailer running/clearance. Locate that fuse in the correct fuse box and check to see if it’s good. I said correct fuse box!

That car salesman at the dealership didn’t tell you about the other one did he? Some of the MFG’s put another fuse box under the hood, usually on the driver side fenderwell. Ok so now you found the fuse box, located the correct TRAILER fuse and sure enough it’s bad! Ok so great I don’t have any of these! Read the manual and you will see that there are some replacement or exta fuses just for this instance in the fuse box. Now replace fuse and reconnect trailer and turn on running lights, Yea they work! And the wife thinks you’re a genius and the weekend is saved! If you’re problem was turn signals then simply look for the trailer RT or LT fuse and simply replace if needed.

This will solve the problem of brake lights also, as they are the same fuse. If you go thru all of these simple steps nine out of ten times it solves the problem! If you do these steps and it doesn’t solve the problem then you have actually narrowed it down to the trailer and all you have to do is start at the front of the trailer and follow the wiring till you find the problem. Maybe you loaned the trailer to someone and they drug it thru a ditch and pulled out the wires. The good thing about this is that you have the last laugh! It blew their trailer fuse and they don’t know how to fix it and now you do. If it gets really bad and you couldn’t follow these Simple instructions because you’re just not ready to admit that you’re NOT the greatest truck driver in the world and you know everything about trailers and trucks.

Then you’re exactly like I was when I first started.