268 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. Introduction All modern transmissions and transaxles are fully or partially controlled by an on-board computer. To service these units, the technician must understand the electronic parts and circuits involved in their operation. This chapter covers the components and operating principles of elec- tronic transmissions and transaxles. Studying this chapter will give you the knowledge needed to understand the electronic transmission and transaxle troubleshooting and service chapters. Development of Electronic Controls Electronic control of transmissions and transaxles began in the early 1980s. Transmissions and transaxles manufactured during this period used an on-board com- puter to control the converter lockup clutch. Later, some transmission shifts were made by electrical solenoids controlled by the computer. Pressure and temperature switches, input and output speed sensors, and fluid level sensors were also installed to monitor transmission operation. Since the 1990s, almost all transmissions and transax- les have been computer controlled. In addition to shifts and converter clutch apply, the computer sometimes controls line pressures and detent operation. The computer can also use solenoids to control the initial application pressures of some clutches and bands to improve shift feel. Electronic versus Hydraulic Controls Electronic controls are more precise than hydraulic controls. They can take engine, vehicle, and atmospheric conditions into account. They can also precisely control the pressures to the holding members, resulting in smooth shifts that reduce holding member wear. Precise control of shifts reduces transmission fluid overheating, lengthens transmission/transaxle life, and eliminates the need to adjust linkage. Overall vehicle benefits from precisely controlled shifts are better mileage and performance, and decreased emissions. A vehicle with electronic transmission/transaxle controls also operates more smoothly, with shifts that are barely perceptible. All automatic transmissions and transaxles, whether electronically or hydraulically controlled, have simi- larities. They all use a torque converter, hydraulic pump, planetary gears, clutches and bands, and manual linkage. The main difference is the hydraulic control system. On older transmissions, as you have learned, the shift valves were operated by governor pressure and throttle pressure. When governor pressure rose above throttle pressure, the shift valve moved to the upshifted position. When throttle pressure overcame governor pressure, the shift valve moved to the downshifted position. A completely electronic transmission does not have governor and throttle valves. In their place are electrical solenoids that control shifts and pressures, and sensors that monitor transmission, engine, and vehicle conditions. An on-board computer processes the inputs from the sensors and controls the operation of the output solenoids to pro- duce the proper pressures and shift points. Electronic Control System Components To fully understand how electronic transmissions and transaxles operate, you must be familiar with the parts that make up the electronic control system. The computer receives inputs from sensors and issues output commands, which are based on these inputs, to the solenoids. The sequence of operation of the computer and related compo- nents is referred to as a control loop. Control loop operation was discussed briefly in Chapter 7. The following sections discuss the construction and operation of the input sensors, computer, and output solenoids used in electronic transmis- sion control. Later in this chapter, we will explain how these parts work together to form a control loop that operates the transmission. Note: Vehicle computers are called by many names, depending on the manufacturer. In this chapter, as well as the chapters that follow, the transmission control computer will be referred to as the ECM. Older computer control systems are often referred to as OBD I systems. This is an abbreviation for onboard diagnostics, generation 1. Computer control systems in 1996 and later vehicles are called OBD II systems, standing for onboard diagnostics, generation 2. OBD II systems can be identified by their use of a standardized 16-pin diagnostic connector. Input Sensors Input sensors provide information to the ECM. The ECM will make decisions based on this information and issue output commands to the transmission. Some input sensors are located in the transmission and monitor trans- mission operation. Other sensors are primarily used to monitor engine operation. Information from these sensors