276 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. Pressure failsafe valve Main regulator valve Converter regulator valve Feed pressure Exhaust Coil Electronic pressure control solenoid Armature Spool valve Feed pressure Output pressure Figure 12-16. This solenoid controls line pressure by controlling the assist pressure behind the main pressure regulator. On this particular transaxle, pump pressure reaches the solenoid through the converter pressure regulator. (Ford) However, it does not provide the precise control needed for maximum fuel economy and shift feel. Many modern transmissions control the line pressure through an electronically controlled pressure regulator. Electronically controlled pressure regulators are operated by a solenoid controlled by the ECM. The solenoid is pulsed to create a duty cycle. The duty cycle creates a precisely controlled pressure leak at the main pressure regulator. This controlled leak accurately modifies pressure. See Figure 12-16. In other systems, the solenoid controls pres- sure by controlling the output of the vane pump. Operation of a pump output control solenoid is shown in Figure 12-17. Converter Lockup Clutch Control The basic operation of torque converter lockup clutches was covered in Chapter 8. Remember that on hydraulically operated units, the converter clutch was engaged by the ECM. Some older units used a pressure or speed switch to engage the converter clutch. On an electronically controlled transmission or transaxle, the ECM operates the lockup clutch control solenoid. The ECM makes the decision to apply the clutch based on inputs from the engine and drive train sensors. On some vehicles, an additional solenoid con- trols clutch apply pressure. This reduces roughness when the clutch is applied. The ECM varies the solenoid duty cycle based on sensor inputs. See Figure 12-18. On other vehicles, the main apply solenoid itself can provide cushioning based on an ECM-supplied duty cycle. Shift Control The basic principle of electronic shift control is the replacement of certain hydraulic components with ECM- operated solenoids that control fluid flow. On some trans- missions and transaxles, energizing the solenoid opens and closes passages to the holding members, Figure 12-19. On other systems, the solenoid affects hydraulic pressure on one side of a shift valve, causing it to move. See Figure12-20. In some transmissions and transaxles, the solenoids are directly connected to check balls that act as shift valves, Figure 12-21. Energizing the solenoids causes the check balls to move and either open or close pressure passages leading to the holding members. On a few transmissions, the solenoids are mechanically connected to the shift valves. Energizing the solenoids directly operates the shift valves. Refer to Figure 12-22. On other transmissions, the valve body contains conventional shift valves. The solenoids indirectly move the shift valves by closing or opening pressure passages. Opening a passage causes a pressure drop, while closing the passage causes pressure to increase. Any pressure change on one side of a valve causes it to move toward the side with the lowest pressure, Figure 12-23A. When the solenoid closes the same passage, pressure equalizes on both sides of the valve. A spring then moves the valve back to its original position, Figure 12-23B. As with hydraulically controlled transmissions, pressure from another valve may be used to assist the spring. Once moved by the solenoids, the shift valves perform the same job that they do in a hydraulically operated transmis- sion. The shift valves direct oil pressure to the various hold- ing members through passages in the valve body and case.