Chapter 9 Basic Electrical Tests 173 Copyright by Goodheart-Willcox Co., Inc. is touched on diff erent points in the circuit. If the test light glows, the circuit is good up to that point. See Figure 9-12A. As soon as the test light fails to glow, you have found the problem area. Sharp test light probes can be used to puncture wire insulation to check for power. See Figure 9-12B. Th is is use- ful when you cannot access a connector for your tests. Th e test light can also be inserted into the back of a connector so that the probe touches one of the metal terminals in the connector. Th e connector can also be unplugged to check for voltage. Note: Avoid using a test light probe to puncture the insulation on wires in a computer or sensor circuit. Moisture can enter through the hole in the insulation. If the wire is connected to a sensor with a weak or low voltage signal, this can upset the small signal going to the computer, and a computer system malfunction may result. A self-powered test light, or continuity tester, has an internal battery. It uses its own power source to check for a complete electrical path, so the circuit’s power source must be disconnected for testing. Figure 9-13 shows how a self- powered test light can be used to check for continuity. Figure 9-14 shows how a test light can be used to help fi nd shorts. Th e test light is substituted for the fuse in the aff ected circuit. As long as the circuit is complete, the test light glows. Wiggle the wires or disconnect components or wires until the test light goes out. When the test light does not glow, you have found the source of the short. Warning: Test light probes are very sharp. It is easy to jab the probe through your skin when trying to penetrate the wire insulation. Do not place your fingers behind the probe when testing. Figure 9-10. A—Jumper wires can be used to bypass a component or connect power to a circuit for testing. B—A jumper wire provides an easy way to bypass a component or to connect power directly to a component or section of circuit. In this example, if bypassing the switch makes the motor run, the switch is bad. (Snap-on Incorporated) B M Battery Bad Switch Motor Jumper Wire Bypasses Bad Switch Fuse Flexible Insulation Alligator Clips A A B Alligator Clip Test Probe Light – + Battery Start Here If More Than One Component Is Dead Start Here If Only One Component Is Dead To Any Ground Trace Along Circuit Until Open Is Found Figure 9-11. A—A test light provides a way to check for power or voltage in a circuit. B—With the circuit-powered type, the alligator clip is grounded. Then the test probe is touched to the circuit at different locations to find breaks in the circuit. (Snap-on Incorporated)