Chapter 14 Troubleshooting Electronic Control System Problems 345 Copyright by Goodheart-Willcox Co., Inc. Scan tool Figure 14-9. Scan tools can be used to retrieve trouble codes and obtain other information about the operation of an electronic transmission or transaxle. OBD II Computer Control Systems Early computer control systems were designed to monitor the operation of computer system parts, such as sensors and output devices. If one of these parts failed, the ECM received an out-of-range electrical reading. The ECM would then set one or more trouble codes and turn on the dashboard-mounted warning light. These early systems are called on-board diagnostics, generation I, or OBD I, systems. OBD I was replaced in 1995-96 by an updated system called OBD II (on-board diagnostics, generation II) systems. The ECM in OBD II systems still detects failed parts, but it also monitors air-fuel ratio changes, engine misfires, tempera- ture changes, and other operating conditions. From these inputs, the ECM can determine that a part is about to fail, or that excessive emissions will be produced in the near future. The ECM will then set one or more trouble codes and turn on the maintenance indicator light (MIL) in the dashboard. OBD I systems react to a problem that is already causing high emission or loss of performance. OBD II systems have the ability to catch potential problems before they begin to cause high emission or loss of performance. To accomplish this, the OBD II system has a misfire monitor, an extra oxy- gen sensor after the converter, and an evaporative emissions monitor. Using Scan Tools If the basic electrical tests do not reveal a problem, the next step is to check the unit using a scan tool. The scan tool is a hand-held electronic device used to retrieve trouble codes and perform other diagnostic procedures. Figure 14-9 shows a typical scan tool. The scan tool can be thought of as a portable computer that can communicate with the vehicle’s on-board computer. While they are expensive, scan tools are becoming common because they save diagnostic time and reduce the chance of misdiagnosis. The scan tool also allows the technician to obtain information directly from the ECM. This information would not be available by any other means. Additionally, many scan tools can be used to reprogram the vehicle’s computer. Caution: On OBD II–equipped vehicles, the proper scan tool must be used to retrieve trouble codes. Never attempt to retrieve trouble codes from an OBD II system by grounding one of the diagnostic connector terminals. Grounding any terminal will damage the ECM. The scan tool must be connected to the system through the diagnostic connector. Always locate the proper diagnos- tic connector. On OBD II vehicles, a 16-pin connector called the data link connector (DLC) is used to access all vehicle computer systems. On older (non-OBD II) vehicles, there may be separate diagnostic connectors for the engine, drive train, anti-lock brakes, air conditioning, and suspension. The power of the modern scan tool increases the amount of information available to the technician. Unfortunately, it also makes it easier to misinterpret the information. For instance, with hundreds of potential trouble codes, it is easy to look up the wrong code. Sometimes the amount of avail- able scan tool information can cause the technician to miss the actual cause of a problem. For example, if the technician concentrates on differences in input and output shaft speeds in all gears (information that is available on modern scan tools), he or she may not notice that the torque converter clutch is not engaging. Much time could be wasted looking for slipping holding members when the actual problem is a defective converter control system. For this reason, you must carefully interpret all trouble codes and other scan tool information before proceeding with diagnosis and repair. Trouble Codes OBD II systems use 5-character alphanumeric codes. Each code contains a letter and a four-digit number. See Figure 14-10. The letter identifies the general system causing the problem. The letter codes cover the four major vehicle subdivisions. They are B (body), C (chassis), P (power train), and U (internal com- puter communications network). The first number indicates whether the code is a standard code, which is assigned by SAE, or a non-uniform code, which is assigned by the vehi- cle manufacturer. The second number indicates the specific system in which the problem is occurring, and the final two numbers indicate which devices or circuits are causing the problem.