298 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. Note: When the shift selector is in the drive position instead of overdrive, the forward clutch is applied to provide engine braking. Solenoid #3 will be de-energized in this position. When solenoid #3 is energized, there is no longer any pressure directed to the forward clutch control valve. The forward clutch control valve moves to its original posi- tion, and oil pressure is removed from the forward clutch piston. This disengages the forward clutch. With the direct and intermediate clutches applied, and the forward clutch released, the transmission is in third gear. Overdrive, Fourth Gear In fourth gear, the ECM energizes solenoid #1. Solenoids #2 and #3 remain in the same positions as in third gear. See Figure 12-45. Energizing solenoid #1 moves the 1-2 shift valve back to the upshifted position. Oil from the 1-2 shift valve is delivered to the upshift side of the 3-4 shift valve, causing it to move to the upshifted position. The 3-4 shift valve directs oil to the overdrive servo. Pressure to the overdrive servo causes it to apply the overdrive band, plac- ing the transaxle in fourth gear. Some overdrive servo oil is diverted to the 3-4 accumulator to cushion the shift. In this circuit diagram, the converter clutch has been applied. Oil from the bypass clutch control solenoid moves the bypass clutch control valve to the left, cutting off con- verter release pressure. This allows the pressure on the apply side of the converter clutch plate to push it into engagement with the converter cover. Partial Electronic Shift Controls Some transmissions and transaxles use a combination of electronic and hydraulic shift controls. In most cases, these are older three-speed designs that have been rede- signed to upgrade them to four speeds. A conventional shift valve acted on by the throttle and governor valves controls the 1-2 and 2-3 shifts. The 3-4 shift is controlled by a sole- noid operated by the vehicle’s computer. The transmission oil pressure diagrams illustrated in the next two figures are those of an updated 4-speed ver- sion of a once-common 3-speed transmission. The major differences are on the right side of the illustration. Note the presence of the overdrive clutch, 3-4 accumulator, and three valves related to the 3-4 shift. The 3-4 shift valve is held in the downshifted position by a spring. Note that the first two shifts (1-2 and 2-3) are completed hydraulically by the action of the governor and throttle valves on the shift valves. On later versions of this trans- mission, the governor valve has been replaced by a governor pressure solenoid like the one shown in Figure 12-28. The final shift (3-4) is performed by the solenoid. This solenoid operates conventional shift valves by opening or closing pressure passages. Also, note that the transmission contains Refer to Figures 12-42 through 12-45 as you read the following paragraphs. You may want to compare the operation of this hydraulic system with the operation of the hydraulic system on an older version of this transaxle shown in Figures 11-23 through 11-28. For easier understanding of the solenoid action, the operation of some valves is not covered. Note: The solenoid marked VFS TV SOL is the main pressure regulator control solenoid. It adjusts line pressure to match throttle opening and engine load in all gears. Overdrive, First Gear In first gear, Figure 12-42, solenoid #1 is de-energized. Oil flows through the solenoid to the 1-2 shift valve, holding it in the downshifted position. Solenoid #2 is energized, and no oil can flow through it. Solenoid #3 is de-energized, and oil flows through the solenoid to the forward clutch control valve and the 3-2 shift timing valve. This pressure moves the forward clutch control valve to the right. Oil pressure flows through the forward clutch control valve, through the 3-4 shift valve, and into the forward clutch apply piston. Some fluid also travels to the neutral-drive (ND) accumulator to cushion the appli- cation of the forward clutch. With all shift valves in the downshifted position and the forward clutch applied, the low roller clutch locks up and the transaxle is in first gear. Overdrive, Second Gear To shift the transaxle into second gear, the ECM ener- gizes solenoid #1. Solenoid #1 opens, removing the pressure from the upshift passage to the 1-2 shift valve. The 1-2 shift valve moves to the left, opening a line oil pressure passage to the intermediate clutch shuttle valve. The intermediate clutch shuttle valve sends oil pressure to the intermediate clutch and 1-2 accumulator. This action is illustrated in Figure 12-43. Applying the inter mediate clutch causes the low roller clutch to unlock and shifts the transaxle into second gear. Note that the pressure first trav- els through the 1-2 capacity modulator (1-2 cap mod). This device further controls shift feel. Overdrive, Third Gear To shift into third gear, the ECM de-energizes solenoids #1and #2, and energizes solenoid #3, Figure 12-44. When solenoid #3 is energized, the 2-3 shift valve moves back to the first gear position. This will allow oil to flow to the direct clutch. When solenoid #2 is de-energized, oil flows to the 2-3 shift valve, moving it to the upshifted position. Line oil passes through the 1-2 shift valve to the direct clutch. This oil is also sent to the intermediate clutch shuttle valve to ensure that the intermediate clutch stays applied even though the 1-2 shift valve has returned to the first gear position.