340 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. ECM or engine sensor defect Output solenoid defect Input sensor defect Internal hydraulic or mechanical problem Wiring harness or connector defect Battery or charging system defect Figure 14-1. Possible trouble spots on an electronically controlled transaxle. Remember that conventional problems, such as worn clutches or internal leaks, are just as likely as electronic component failure. (General Motors) Introduction To diagnose problems on electronically controlled transmissions and transaxles, the technician must be able to proceed logically, without randomly replacing expen- sive parts. This chapter covers the logical processes that must be used to diagnose electronic control systems. It also covers diagnosis of electronically controlled automatic transmission and transaxle components. Information on trouble code retrieval and other uses of scan tools is also presented. Electronic Transmission and Transaxle Problems Many of the problems that occur in an electronically controlled transmission or transaxle are the same as those that occur in hydraulically controlled transmissions. Other problems are unique to the electronic control system. The following section covers common electronic control system problems. Figure 14-1 shows the possible sources of elec- tronic control system problems in a modern transaxle. Many computer control system problems are called pattern failures. A pattern failure is a problem that is common to a certain type of vehicle. The experienced technician will learn to spot pattern failures and quickly determine the defective part. The chart in Figure 14-2 shows some common electronic control system failures that occur in one manufacturer’s automatic transmissions and transaxles. Limp-In Mode If a major problem occurs in the computer control system, the ECM will place the system in limp-in mode. When the system is in the limp-in mode, the ECM ignores most input sensor readings and operates the engine and drive train output devices based on internal settings. The transmission or transaxle solenoids are energized in a way that gives the unit only one or two forward gears. Whenever the system goes into limp-in mode, the ECM will illuminate the dashboard maintenance indicator light, or MIL. On older vehicles, the MIL is called the service engine soon light. An illuminated MIL is always an indication that the computer control system has a problem and that trouble codes should be retrieved. Input Sensor Problems In many cases, the transmission or transaxle reacts to problems in various vehicle sensors. Some sensors are located on the valve body, Figure 14-3. Other sensors pri- marily monitor engine operation and are installed on the engine. Faulty input sensors are the most common cause of electronic transmission or transaxle problems. A defective throttle position sensor (TPS) can cause improper shifting or erratic application of the converter lockup clutch. A failed TPS can also cause rough upshifts or a bump on closed throttle downshifts. Speed sensors can cause problems by sending incor- rect engine or output shaft information to the ECM. This usually affects shift speeds. A speed sensor located inside the case can collect metal filings that affect the signal. Some