How To Test a Circuit Using a Test Light

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Testing For Voltage (Positive) With a Test Light

Testing your tester on the battery.
Attach one end to ground and the other end to the positive you want to test. photo by Matt Wright, 2008

A test light is a simple but extremely useful tool. 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 very versatile, so the test light style of circuit tester can be a lifesaver. You can use it to check any positive circuit. Headlights not coming on? If the fuse is 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.

The test light is easy to use. First, let's look at how to test a positive circuit for voltage. The basic principle is illustrated in the photo above. 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.


  • 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 are very sharp on the 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.

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Using a Test Light to Check a Ground

Testing a circuit for good ground.
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 bad to re-establish.