How Trailer Wiring Testers Work

The Hopkins Plug-In Simple 47185 Multitow 7:4 Adapter fits on a 4-wire flat plug and adapts it to a 7 RV round plug. See more truck pictures.

Correcting problems with electrical wiring can be intimidating, particularly when it involves connecting two large, mobile objects like a tow vehicle and a trailer. It might be easy to detect if something is wrong -- the most obvious indicator is that the trailer's brake or turn signal lights won't light up. But some wiring issues can be harder to detect and narrowing down the specific problem can be even trickier. That's where wiring testers come in.

Wiring testers help you target the specific problems in your tow vehicle or trailer's wiring system. Testers come in a wide range of forms, sizes, capabilities and prices. If the wiring from your tow vehicle is exposed, you may even be able to use a simple working bulb to test the wiring. You could also use a volt meter to measure the voltage from your tow vehicle. On the other end of the spectrum are full wiring tester kits. The most comprehensive testing kits have multiple adapters that can fit practically any wiring setup.


While simple testing methods might work fine on older tow vehicles or trailers, recent vehicles have more sophisticated systems that require specific tools. Current trailer wiring systems can come in several varieties. Four-way flat plugs provide connections for turn signals and taillight or side marker lights (the fourth connection is a ground wire). Five-way flat plugs are similar to 4-way circuits but have an extra wire that allows the trailer to have auxiliary power or brake lights. Six-way rectangular or round plugs have connections allowing for all of the above. Seven-way round plugs are the same as 6-way plugs, with an additional connection that allows for auxiliary power or backup lights

­The plug on your trailer might not fit the one attached to your tow vehicle. If that's the case, you'll need an adapter so that the two systems can connect. If your tow vehicle doesn't have as many wires as your trailer, some of your trailer's systems won't be functional without additional wiring modifications.

Just as you need the right adapter to hook two different systems together, you need the right wiring tester to make sure your tow vehicle is providing power to your trailer when problems arise. It won't do you much good to use a 4-way wiring tester on a 7-way plug. Before testing anything, make sure your equipment matches.

Next, we'll look at how to use a wiring tester safely to make sure you don't have any electrical problems with your trailer.



Testing Trailer Wiring

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 on the next page.


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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)