416 Section 4 Computer Control Technology Copyright Goodheart-Willcox Co., Inc. KISS is an acronym that could help you find the source of performance problems on a computer-con- trolled vehicle. KISS stands for “keep it simple, stupid.” This means you should start your troubleshooting with the simple checks and tests. Then, as the common causes are eliminated as the source of the problem, you will move to more complex tests. It is easy for the untrained person to instantly think “computer problem” when an engine runs rough, fails to start properly, or exhibits some other performance problem. For example, contaminated engine oil can trigger a computer trouble code, Figure 25-1. Fumes from the contaminated oil can be drawn into the engine’s intake manifold from the crankcase. If these fumes are excessively strong, the oxygen sensor could be tricked into signaling a rich air-fuel mixture. The computer would then lean the mixture to compensate for the crankcase fumes. An oxygen sensor trouble code may be produced and, in some cases, an engine perfor- mance problem could result. As this points out, it is critical that you check for conventional or simple problems first. Start checking for computer problems only after all the conventional causes have been ruled out. Electrical Component Damage Semiconductor devices, such as transistors and integrated circuits, are very easy to damage. They can be damaged by static electricity, voltage spikes, heat, and impact shocks. Here are some things to remember when working with semiconductor devices and their wiring: • When welding on any part of a vehicle, disconnect the battery ground and disarm the airbag system. If welding within 12″ (304.8 mm) of any control module, remove the electronic module from the vehicle to avoid heat and radiation damage. • Never disconnect the battery cables while the engine is running. In the past, some technicians would do this to see if the alternator was working. This can destroy or weaken electronic circuits, causing failure in a short period of time. • Do not disconnect or connect wiring, especially the computer wiring, with the ignition key on. This can cause a current surge that can damage the computer. • Make sure you do not reverse the battery cable connections. This can destroy electronic components. • Wear an anti-static wrist strap whenever you handle static-sensitive components (removable PROM chips, for example) to protect them from damage. • Use only high-impedance test lights and meters when checking electronic circuits or their wiring. A conventional test light or meter will draw too much current and destroy electrical components. • Do not disconnect a scan tool from the data link con- nector while the ignition key is on. This could create a voltage spike that can damage the computer. Computer System Circuit Problems Almost all electrical-electronic problems are actu- ally basic circuit problems. A basic circuit problem is caused by a problem in a circuit that increases or decreases current, resistance, or voltage. For example, a broken wire could stop current flow or a charging system problem could decrease output volt- age and current flow. Unfortunately, when a minor problem occurs in a complex circuit like a computer control system, it may Excess fumes Contaminated oil Burned oil PCV system pulls fumes into intake manifold Computer Trouble code out Fumes pulled into combustion chamber and burned Abnormal oxygen sensor voltage Goodheart-Willcox Publisher Figure 25-1. This example shows that a trouble code can be tripped by factors other than a computer system problem. Contaminated engine oil can cause excess fumes to be pulled into the PCV system. When burned in the combustion chambers, the fumes could trick the computer into sensing that something is wrong with the oxygen sensor circuit.