How Trailer Wiring Testers Work

Testing Trailer Wiring

The Reese Towpower 74633 4-Way Tester is one of the simpler trailer wiring testers on the market.
The Reese Towpower 74633 4-Way Tester is one of the simpler trailer wiring testers on the market.

So you've hooked up your trailer to your tow vehicle, you've got the tow vehicle's engine running and the lights on the trailer refuse to come on. What do you do next?

The key to detecting a wiring issue is to eliminate possibilities until you can determine the source of the problem. First, you may want to check the lights on your trailer -- the problem may be as simple as a burned-out bulb. If that's not the problem, you'll need to disconnect your trailer's wiring system from your tow vehicle. Next, you'll need to check to make sure your vehicle's lights are in good working order. Test your vehicle's turn signals, brake lights and backup lights to make sure the problem isn't the tow vehicle itself.

If your vehicle's lighting system is working, the next step is to use a trailer wiring tester to check the socket on your tow vehicle. You'll need to make sure your tester fits your tow vehicle's socket. Some testing kits come with multiple attachments, letting you use the same kit to test more than one kind of wiring system.

Plug the tester into your tow vehicle's socket. The tester should have one or more indicators that will alert you if it detects an electric current. Most testers have an indicator for each function. Test each system in turn and check your results. If the tester responded to each system, then the trailer's wiring system is the likely source of the problem. But if one or more of the tests results in no response from the tester, your tow vehicle may be at fault.

If the tester lights up when it shouldn't -- for example, if the left-turn signal indicator lights up even when you haven't engaged the turn signal -- it could indicate that you have a short in your tow vehicle's wiring. You'll need to check the wiring in your tow vehicle to see if there is a point where two or more wires make contact. It's also possible that two or more wires are connected to the wrong connection points.

Some wiring problems are easy to fix. If two wires are connecting to the wrong connection points, it's usually just a matter of using some wire cutters, a wire stripper and a crimper to swap them. Others might require a visit to a mechanic. The important thing to remember is that if the wiring isn't working properly, you can't travel on the road safely. Other drivers could misinterpret your actions if the wrong lights activate on your trailer as you drive.

To learn more about towing trailers and related topics, follow the links below.

Related HowStuffWorks Articles

More Great Links


  • Allen, Mark. "What's Going on Back There?" Popular Mechanics. Oct. 2006. (Oct. 14, 2008)
  • Automotive Accessories Connection. "Trailer Wiring Diagrams." (Oct. 14, 2008)
  • Haystack Hill. "Trailer Wiring." (Oct. 14, 2008)
  • Sunrise RV. "Trailer Wiring." (Oct. 14, 2008)