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.