Hobbies Cars & Motorcycles How to Use a Circuit Tester Share PINTEREST Email Print Songsak Wilairit / EyeEm / Getty Images Cars & Motorcycles Cars How Tos Basics Reviews Classic Cars Corvettes Mustangs Tires & Wheels Motorcycles Used Cars Trucks ATVs & Off Road Public Transportation By Matthew Wright Matthew Wright Matthew Wright has been a freelance writer and editor for over 10 years and an automotive repair professional for three decades specializing in European vintage vehicles. Learn about our Editorial Process Updated on 06/15/18 A test light, sometimes called a test lamp or voltage tester, is a simple but extremely useful electronic tool to check your car's circuits—that is, the presence or absence of electricity to a certain component or piece of equipment. If you are trying to diagnose and troubleshoot an electrical problem, sometimes a test light can help you rule out possible causes much more quickly and easily than a DMM (Digital Multi Meter). It's quick, easy, and versatile. In fact, you'd be hard-pressed to find a more handy gadget to have in your vehicle. You can use it to check any positive circuit, from your cigarette lighter to your headlights and taillights. If the fuse turns out to be good, you can use a circuit tester to trace the wiring path and find out what's gone wrong. If the positive path is intact, you can also use the test light to check the grounding points of the circuit. 01 of 02 Test for Positive Voltage Attach one end to ground and the other end to the positive you want to test. photo by Matt Wright, 2008 It's simple to use a test light to test a positive circuit for voltage. The basic principle is illustrated in this photo. You have a positive power source (in the case of the photo, it's the battery) and you have a ground (any exposed metal that's bolted to the chassis). The test light is the go-between. If you connect one end to the positive power source and the other end to a good ground, it lights up. To test for positive voltage, attach one end to a known ground, and touch the other end to the wire you want to test. If it lights up, you're good. If not, you need to replace or clean the component you just tested. Tips: Before you test a circuit for voltage, be sure your test light is in good order by testing it on the car's battery. The leads of the test light are reversible. It doesn't matter which one goes on the positive and which goes on the ground. Use whichever end makes your job the easiest. Most test lights have a sharp pointed end. You can use this sharp point to pierce the plastic insulation on a wire. This means that you can test the circuit without disconnecting anything. 02 of 02 Test a Ground Circuit Testing for ground is the reverse of a voltage check. photo by Matt Wright, 2008 Your test light circuit tester is great for checking for voltage, but it can also be used to check a ground circuit. If you know that a certain electrical component is getting juice on the positive side, you need to check to see if it has a good grounding point. This is easy. Since you have already established a good positive source, attach one end of the circuit tester to the positive end. Now touch the other end of the tester to the ground wire for this component. If it lights up, you have a good ground and need to check the component further. If you don't get a light, it's time to clean contact points and check the ground path. Luckily, grounds aren't too difficult to re-establish. Usually, all you need to do is ensure that the ground wire is attached to a point that is free of paint, rust, plating, or anything else that might act as an insulator. You can also invest in a handy component known as an engine ground strap.