Chapter 12 Electronic Control Systems 275 Copyright by Goodheart-Willcox Co., Inc. Scan tool lead Data link connector Figure 12-15. In late-model vehicles, data link connectors are always located under the dashboard. In older vehicles, the data link connectors were located in various places, depending on the manufacturer. input sensors, to the computer, to the output devices, to the vehicle system, and back to the input sensors. For example, the speed sensor tells the ECM that a certain speed has been reached. The throttle position sen- sor informs the ECM that throttle opening is not changing. The ECM will then decide to energize or de-energize the necessary solenoid(s) to shift into the next higher gear. If the throttle position sensor tells the ECM that the throttle is being opened quickly, the ECM will decide to operate the necessary solenoid(s) to shift into a lower gear. These two basic inputs are affected by inputs from the other sensors. The ECM operates many control loops at the same time. The ECM can also compensate for a failed sensor by substituting other sensor inputs for those from the faulty component. For example, if the throttle position sensor fails, some ECMs can temporarily operate the transmission or transaxle by substituting the input of the MAP or MAF sensor for the input of the TPS. The ECM will also store information about the problem in memory, so the techni- cian can access it when diagnosing system malfunctions. ECM Diagnostic Outputs In addition to operating the output devices, the ECM provides a diagnostic output to the technician. Internal ECM processes store diagnostic information. The external parts of the diagnostic output are: • A dashboard-mounted warning light that illuminates when a problem is detected. This light is generally referred to as a Maintenance Indicator Light, or MIL. Other terms for this warning light were used on older (OBD I) vehicles. • A data link connector, or DLC, which is used to retrieve information about system problems. This con- nector can be located in several places, depending on the vehicle year and model. It allows the technician to directly access the information stored in ECM memory. All OBD II systems use standardized 16-pin connec- tors. The connectors used in OBD I systems may vary from one manufacturer to another. When a system defect occurs, the ECM stores informa- tion about the defect in memory. On older (OBD I) systems, the trouble codes identify components or systems that have failed completely. On OBD II systems, the ECM can store information about developing problems, out of range sen- sors, and systems that seem to be losing their effectiveness. Information about defects is stored in the form of a trouble code. On vehicles up to 1995, trouble codes are 2- or 3-digit numbers that correspond to a specific defect. On vehicles with OBD II systems, the trouble code consists of a 5-character, alphanumeric code. To determine whether trouble codes are present in the ECM’s memory, the technician must use an electronic test device called a scan tool. Attaching the scan tool to the data link connector, Figure 12-15, causes the ECM to enter the self-diagnostic mode. The ECM will then release stored trouble codes to the scan tool. Use of scan tools will be dis- cussed in detail in Chapter 14. Output Solenoids Solenoids are the primary output devices used in elec- tronic transmissions and transaxles. Solenoids are installed in the valve body. Solenoids can be on or off for long periods of time, or they can be pulsed (turned on and off rapidly) to provide a duty cycle (a ratio of on to off). The ECM drivers operate the output solenoids. Drivers are special ECM circuits that consist of power transistors and related elec- tronic devices. The power transistors in the driver circuits can handle the high current flows needed to operate the solenoids. The following sections explain the various functions of the solenoids used in late-model automatic trans missions and transaxles. Note that not every electronic transmission or transaxle has all these components. Line Pressure Control As discussed in Chapter 9, a valve and spring assembly controls the line pressure. This assembly is usually called the main pressure regulator. Spring-operated main pressure regulator valves are used on all older automatic transmis- sions and on some newer models. Pressure from the throttle valve enters the spring side of the valve and assists pressure developed by spring tension. This is a simple and generally effective way of controlling pressure in response to load.