168 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. Basic circuit testing skills are essential to today’s auto- motive technicians. Knowing how to analyze circuit prob- lems and use basic test instruments are fundamental skills. Th is chapter introduces basic circuit problems, explains their causes, and summarizes the tests needed to fi nd and correct malfunctions. Basic Circuit Problems Almost all electrical and electronic problems are caused by something in the circuit that increases or decreases cur- rent, resistance, or voltage. For example, a broken wire could stop or decrease current, or a charging system problem could increase output voltage and current. Unfortunately, when a basic circuit problem occurs in a complex circuit, such as a computer control system, it may not seem so simple. For example, a poor electrical connec- tion in a feed wire to a sensor can cause a false signal to return to the computer. Th e computer, reacting to the low sensor voltage, might then alter the operation of the fuel injection system, emission control system, or ignition sys- tem. A technician might think that any of these systems or the computer itself is at fault. Several systems may appear to have a problem. In reality, all of the problem symptoms are caused by a poor electrical connection in one wire. As you can see, it is important to keep an open mind when you are diagnosing electrical problems. If analyzed properly, problems can usually be found and corrected easily. Electrical Diagnosis and Repair Electrical diagnosis involves using a logical sequence of steps to fi nd the source of an electrical problem. Before you begin troubleshooting, you should verify the symptoms and problem to be sure that the customer has described the trouble accurately. Frequently, the vehicle owner or service writer may not completely describe what is wrong with the vehicle. Verifying the problem may give you added informa- tion when determining what to do next. See Figure 9-1. Trial-and-Error Approach In the past, when electrical systems were much less complex, a technician might have been able to get by using trial-and-error tests and part replacements. Older vehicles did not have the large number of complex electronic circuits and components found on late-model vehicles. Only a few tests or part replacements might fi nd the fault. Today, the trial-and-error approach is not a good alter- native. Th is method could take hours or days to fi nd a sim- ple electrical problem. With on-board computers, vehicle sensors, actuators, and the interaction of major circuits, a systematic approach to problem diagnosis is essential. Never use trial and error when trying to fi nd and correct electrical problems. Systematic Approach Th e systematic approach to troubleshooting involves using the theory of system operation and logical thought processes to fi nd the source of the problem quickly. A well- planned procedure must be followed to fi nd the source of a malfunction. Figure 9-2 shows a mnemonic (memory- jogging phrase) that may help you remember the steps in using a systematic approach to troubleshooting. Verify the Symptoms Check the vehicle carefully to fi nd out which compo- nents are not working. Note any unusual noise, blown fuses, or other conditions. Try to determine which parts might be capable of causing these symptoms. Also, think of the parts Figure 9-1. Verify the problem before you begin the trouble- shooting process. Figure 9-2. Use this mnemonic to remember the steps in the troubleshooting process. Veronica Verify the symptoms and problem. Never Narrow down the problem sources. Touches Test to find the cause. Much Make the repair. Voltage Verify the repair.