172 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Many intermittent problems are caused by poor elec- trical connections. For example, two terminals in a connec- tor may be loose. Most of the time, the two terminals make contact and the circuit functions normally. However, when the vehicle hits a bump, the jolt may separate the loose ter- minals, causing the circuit to malfunction temporarily. An intermittent failure can result. Using Testing Devices Th is section of the chapter summarizes the use of elec- trical testing devices, including jumper wires, test lights, and multimeters. To be an eff ective technician, you must know how to use this equipment. Note: When you are looking for shorts or opens, wiggle or move the wires while you are testing. This might move the conductor enough to produce the short. It might also correct the problem temporarily so that you can detect the trouble. Always look for wire damage. Check for cut or smashed wire insulation or loose connections. Any of these could cause a malfunction. Using Jumper Wires A jumper wire is a piece of wire with alligator clips on both ends, as shown in Figure 9-10A. It is commonly used to bypass components or to apply voltage to a component or section of a circuit. Figure 9-10B shows a good example of how a jumper wire can be used to check a switch. You might use the jumper wires to bypass the switch if symp- toms point to an open switch for example, if turning the switch on does not feed current to the load (motor). If attaching the jumper around the switch turns the motor on, the switch is bad. Warning: Never use a jumper wire to bypass a high-resistance component or a fuse or circuit breaker. This could allow excess current and cause an electrical fire. A jumper can also be used to connect voltage directly to the load. If the load begins to work, something is wrong in the circuit leading to the load. If jumping directly to the load does not make it work, the load itself or the ground may be at fault. Using a Test Light A test light provides a quick and easy way of checking a circuit for power or continuity. See Figure 9-11A. Most test lights have a sharp probe for touching conductors and an alligator clip for grounding. As described in Chapter 6, test lights can be either circuit-powered or self-powered. Circuit- powered test lights are more commonly used in auto repair. A circuit-powered test light uses the circuit voltage to illuminate the tester bulb. Figure 9-11B shows how to use a circuit-powered test light to check for power. Th e alligator clip is connected to any good ground. Th en the test probe Figure 9-9. The location of the short in a circuit affects the symp- toms that appear. A—A short after a load cuts power to the sec- ond load. However, the first load still works. Current increases slightly because of less resistance in the circuit. However, cur- rent may not increase enough to blow the fuse because of the resistance of the first load. B—Short before a grounded switch. The load stays on all of the time, even with the switch open. Current bypasses the switch to ground. C—In this wire-to-wire short, the short feeds current across two branches. Even with one switch off, current leaks over to the other branch. Only when both switches are off will the bulbs turn off. Bulb off Bulb has necessary circuit resistance Bulb on A Fuse OK Short after Load Switch open Short to ground before switch Bulb cannot be turned off B Short before Grounded Switch Both bulbs still glow Short between two wires Switch open C Wire-to-Wire Short Switch closed