Chapter 25 Computer System Service 417 Copyright Goodheart-Willcox Co., Inc. not seem like a minor problem. For example, a poor electrical connection in a positive feed wire to a body ground may cause the computer system or one of the systems it controls to lose power. This can result in a shutdown of the fuel injection system, the emissions control system, the ignition system, or the entire vehicle. You might think that any of these systems or the computer itself is at fault. Several systems could appear to have a problem. In reality, it is simply a poor electrical connection in one wire that is causing all the problems. It is important for an automotive technician to remain calm when diagnosing electrical problems. If analyzed properly, problems can usually be found and corrected easily. Locating Computer Problems The most difficult aspect of making computer system repairs is finding the source of the problem. To find the source of computer problems, you must ask yourself the following types of questions: • What could be causing the specific symptoms? Mentally picture the parts in the circuit and how they function. Trace through the circuit while ref- erencing a wiring diagram to find out which wires, connections, and components are in the circuit leading to the potential trouble source. • How many components are affected? If several components are not working, something close to a common power source or ground point is at fault. If only one or two sections of the circuit are faulty, begin your tests at those sections of the circuit. • Is the problem always present or is it intermittent (only occurs under some conditions)? If the problem is intermittent, the conditions causing the problem will have to be simulated. For example, a loose electrical connection could open and close with vibration or movement. You might simulate driving conditions by wiggling wires and connectors in the circuit to make the problem occur. • Is the problem’s occurrence related to heat or cold? If it occurs only on a hot day or when the engine is warmed to full operating temperature, heat is related to the problem’s occurrence. Elec- tronic circuits (transistors in particular) are greatly affected by heat. In fact, too much heat can ruin an electronic component. You can use a heat gun to simulate the heat in an engine compartment. • Is the problem’s occurrence affected by moisture? If the trouble occurs only on wet or humid days, you have information to use when analyzing the source of a problem. In most cases, moisture can- not enter a sealed electronic component, but it may enter and affect the wire connections and any components exposed to the environment. Stress testing refers to the use of heat, cold, or moisture to simulate extreme operating conditions of components, like spark plug wires, explained in a later chapter under spark plug wire leakage. Sensor and Actuator Problems As with other electrical and electronic components, sensors, actuators, and their circuits can develop opens, shorts, or abnormal resistance or voltage values. When your pinpoint tests find a problem, the sensor or actua- tor should be replaced or the circuit repaired. In most cases, a scan tool is used to find the problem circuit and component. A digital multimeter is sometimes used to measure the electrical signal from the component or circuit to verify problems indicated by the scan tool. Then, this value (voltage, resistance, or current) is compared to factory specifications. If the test value is too high or too low, you would know that the sensor or actuator is faulty and must be replaced. The shop manual will also have a wiring diagram, or schematic, for the computer system. The diagram will show the color codes of the wires and the number of connectors that are used to feed signals from the sensors to the computer and from the computer to the actuators. This can be very helpful when servicing any computer system. The following paragraphs discuss the most frequent circuit problems. Poor Electrical Connections Poor electrical connections are the most common cause of electrical-related problems in a computer system. Discussed previously, a wiggle test will help find poor connections and intermittent problems. Always check electrical connections when diagnosing sensors and other electronic components. OBD II scan tools will help you find poor electrical connec- tions, as shown in Figure 25-2. Poor electrical connections can be due to corroded terminals, loose terminal ends, burned terminals, chafed wires, and other problems. Dirt and moisture can get into connectors, causing high resistance. Any of these conditions can prevent a normal sensor signal from returning to the computer. They can also prevent the control current from reaching an actuator.