Test-Lights

There are two types of test lights used in the automotive industry: A circuit powered test light and a self-powered test light. Both are commonly referred to as 12-Volt Test Lights.

The circuit powered test light depends upon the vehicles battery voltage to light the testers lamp, indicating voltage is present within the circuit being tested.
This test light does not measure voltage in terms of "How much", but indicates whether or not voltage is present. Circuit powered test lights are useful when checking for open circuits.

The self-powered test light contains a small internal battery that supplies voltage to the testers lamp when continuity exists between the test leads. This type of test light is often used when checking for continuity within a circuit. Opens and shorts within a circuit may also be detected using this type of test light.

CAUTION: The self-powered test light should NOT be connected across a 12 volt circuit. This will cause the 1.5 volt bulb to burn out. To verify working condition of the self-powered test light, touch the ends of the test leads together. If the light is lit, the tester is okay.