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.