How to use a test light to diagnose problems
The most problematic part of this new model of thermometer is the addition of the red clamp.Image courtesy of marineengineparts.com
A test light is a small and easy-to-use electrical device which allows you to test positive circuits in your car for voltage, making it a great diagnostic tool for DIY car repair. If your headlights won't turn on but your fuse is intact, for example, a test light can help you check the circuit and the grounding points to see where the problem lies.
A test light basically completes a circuit between a ground, such as any exposed metal in your car connected to the chassis, and a positive power source, like the positive battery terminal, or any positive wire. You can use the sharp bottom of the test light to puncture wiring insulation so you don't have to disconnect anything to run diagnostics.
Test lights are relatively inexpensive, and can be found for less than $10 at auto parts stores. In addition to a test light, you will also want a wiring diagram of your car on hand. These can be found at auto parts stores or downloaded online. This is especially important if you will be testing your stereo wires, since in some cars, wires which control the airbags may be located nearby, and if you don't know the color of the wire you're trying to test, you could end up deploying the airbag by mistake.
After you have your test light and wiring diagram, make sure it is working by connecting it to a known metal ground by the clip, and touching the pointed end beneath the light to the positive battery terminal. Make sure that your ground is not painted or dirty, as this can affect performance of the test light. After that, simply use the test light to touch the wires or connectors on the circuit to see if they have a charge running through them. If your test light does not illuminate, then there may be a wiring problem in that part of the circuit.
—Seth Berger
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