Chapter 9 Basic Electrical Tests 169 Copyright by Goodheart-Willcox Co., Inc. that cannot be causing the symptoms. Ask yourself the fol- lowing questions: • What could be causing the specifi c symptoms? Picture the parts in the circuit and how they function. Mentally trace through the circuit or use a wiring diagram to fi nd out what wires, connections, and components are in the circuit to fi nd potential trouble sources. • How many components are aff ected? If several com- ponents are not functioning normally, you know that something close to the power source is probably at fault. If only one or two sections of the circuit are faulty, you know to begin your tests at these sections of the circuit. • Is the problem always present, or does it only occur some of the time? If the trouble only occurs some of the time, the conditions causing the problem will have to be simulated. For example, a loose electrical connection might open and close with vibration or movement. By wiggling wires in the circuit, you can simulate the driv- ing conditions and try to make the problem occur. • Is the problem aff ected by heat or cold? If the problem only occurs on a hot day or with the engine warmed to full operating temperature, heat may be activating the problem. Many electronic circuits are greatly aff ected by heat. In fact, too much heat can ruin an electronic component. Th is might tell you which component is at fault. • Is the problem aff ected by moisture? If you discover that the trouble only occurs on a wet day, you again have more information to use when analyzing the source of a problem. Moisture cannot enter a sealed electronic circuit, but it can enter and aff ect wire connections and components exposed to the environment. Th is type of thinking might help you in the troubleshooting process. Narrow Down the Cause Aft er checking the symptoms, you should narrow down the cause to one system, if possible. Th is will allow you to concentrate on the components in that system or cir- cuit. To narrow down the possible problem sources, use the symptoms to visualize what is happening inside the circuit. Use your knowledge of the specifi c system and basic rules of electricity to determine what might be wrong. If you are not familiar with the circuit, use a diagram to trace the circuit with the malfunction. For example, if the right headlight is not working but the left one is, you could narrow down the problem to the circuit feeding current to the right headlight. You have, through logical thought, narrowed down the source of the electrical problem to one section of the circuit. If both headlights failed to function, then anything in the complete headlight circuit could be the cause. Further tests, probably at the power source to the circuit, would be needed to narrow down the cause. Perform the Proper Tests Th e next step is to perform the tests needed to check the specifi c section of the electrical system. If one headlight does not work, you might check the bulb fi rst. If the bulb is good, you might test for power to the bulb socket. Again, you must think of how the circuit operates to select the next most logical test. Use proper testing methods to verify your assumption about what might be wrong in the circuit. If tests prove that your fi rst assumption is not causing the trouble, proceed to the second most logical place to test. When you are testing circuits, remember the following: • If only one component does not work, start your tests at that component. Chances are the problem is in the inoperative component or in the section of the circuit leading to the component. • If more than one component does not work, start your tests at the power source for the circuit. A blown fuse or loose power feed wire could be preventing current into the whole circuit. If you have diffi culty fi nding a problem, refer to the ser- vice manual troubleshooting charts and circuit diagrams. Th ese diagrams are specifi c to the vehicle’s make and model and will give you ideas for fi nding the source of the problem. Always think about the symptoms and problem while you are working. Visualize current through the circuit, as well as each connection, and each component. A new testing method or another potential problem source may occur to you at any time. Make and Verify the Repair Following the steps described in this section should help you determine exactly what the problem is. Aft er you have found the source of the problem, make the repair. Th is might involve replacing a bad component, soldering a bro- ken wire, or repairing a connector. Finally, verify the repair by making sure the circuit is operating normally. If needed, test-drive the vehicle to make sure road shock and vibration do not aff ect the repair. Th en you can be confi dent that the problem will not recur aft er the owner drives away. Types of Circuit Problems Th e four basic types of circuit problems are open cir- cuit, high resistance, intermittent trouble, and short circuit. Th is section of the chapter defi nes these problems, lists their causes, and explains how each can cause diff erent symptoms depending on the circuit confi guration.