292 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. to the downshifted position. This cuts off the oil to 3-4 relay and 4-3 sequence valves. Spring pressure moves the 3-4 relay and 4-3 sequence valves to their original positions, exhaust- ing the oil in the servo apply chamber. The transmission returns to third gear. To shift from third to second, the ECM re-energizes the 2-3 solenoid, Figure 12-37. The 1-2 solenoid remains de-energized. With the 2-3 solenoid back on, AFL pressure dead ends at the solenoid and pushes the 2-3 shuttle and 2-3 shift valves to the downshifted position. Moving the 2-3 valves cuts off the oil supply to the 3-4 clutch and the 2-4 servo release chamber. This places the transmission in second gear. To more accurately match the 3-2 shift timing and feel to engine and road conditions, the 3-2 downshift control solenoid (CONT SOL in the illustration) is pulsed to vary the pressure to the 3-2 control valve. This varies the speed and timing of the 3-4 clutch release and the 2-4 band apply. The 3-2 downshift valve and #8 check ball and orifice also help create a downshift that matches vehicle speed and power output. Manual Third When the shift lever is moved to the manual third gear position, Figure 12-38, the manual valve directs oil, called D3 oil, to the back of the 3-4 shift valve to keep it from upshifting to third, and also opens a normally closed (NC) pressure switch on the pressure switch assembly. The pressure switch tells the ECM that manual third has been selected, and the ECM ensures that the solenoid sequence that is necessary to obtain fourth gear (1-2 solenoid on, 2-3 solenoid off) cannot be obtained. Therefore, fourth gear is prevented both hydraulically and electrically. Operation in manual third is similar to operation in Drive. Starting from rest, the transmission will shift through first, second, and third. However, the transmission will not shift into fourth. As with the nonelectronic version of this transmission, oil pressure is delivered through the 2-3 shift and 4-3 sequence valves to the overrun clutch. The overrun clutch is applied to keep the input roller clutch from overrunning. This provides engine braking for descending steep grades. Oil passes through an orifice and check ball assembly (#6) to ensure a smooth overrun clutch application. The converter clutch can be on or off, depend- ing on the inputs to the ECM. Manual Second Selecting manual second causes the transmission to operate in second gear only, no matter what the road speed or throttle opening. When the manual valve is moved to the manual second gear position, Figure 12-39, the valve directs oil, called D2 oil, to the back of the 2-3 shift valve to keep it from upshifting to third. Oil from the manual valve also opens a normally closed (NC) pressure switch on the pressure switch assembly. The pressure switch tells the ECM that manual second has been selected, and the ECM ensures that the 1-2 shift solenoid will not be energized. The transmission will stay in second gear under all conditions. The overrun clutch remains applied for engine braking. The converter clutch may be applied in this gear. Manual First In manual first, Figure 12-40, the manual valve sends pressure to a normally open switch in the pressure switch assembly. This switch is closed by oil pressure and tells the ECM that manual first has been selected. The ECM uses this input to ensure that both the 1-2 and 2-3 solenoids are on, holding the transmission in first gear. The manual valve also sends pressure to the center of the 1-2 shift valve, where it is directed to the low-overrun valve and into the low-reverse clutch apply piston. The low-reverse and low- overrun clutches prevent roller clutch overrunning and provide for engine braking. If manual first is selected at high vehicle speed, the transmission will not downshift to first until speed is reduced. Reverse When the transmission is placed in reverse, Figure 12-41, oil is directed to the reverse input clutch pis- ton and both pressure areas of the low and reverse clutch piston. To raise line pressures in reverse, reverse oil is directed to the reverse input clutch piston through the boost valve of the main pressure regulator. This oil assists spring pressure in pushing on the main pressure regulator, raising line pressure. Reverse oil is also directed to a normally open switch on the pressure switch assembly. This switch tells the ECM that the transmission is in reverse. Note that AFL pressure is still being produced. AFL pressure may be used by the pressure control solenoid to further raise line pressures if the vehicle is heavily acceler- ated in reverse. For smooth reverse engagement, oil sent to the reverse input clutch piston must pass through an orifice and check ball (#3). If the engine speed is above idle, reverse pressure will be higher than normal, opening the reverse abuse valve. When the reverse abuse valve is open, oil pressure bypasses the orifice and check ball assembly, applying the clutch much more quickly and firmly. Electronic Transaxle with Three Shift Solenoids The following sections explain how a common electronically controlled transaxle uses three multi-shift solenoids to operate through four forward gears and the application of the converter lockup clutch. The action of the solenoids and hydraulic system is similar to that of the electronically controlled transmission described previously.