Chapter 12 Electronic Control Systems 279 Copyright by Goodheart-Willcox Co., Inc. 3-4 shift valve D3 Sig A 3-4 shift valve D3 Sig A A B Figure 12-23. Study the effect of pressure on this 3-4 shift valve. A—Pressure from the solenoid moves the 3-4 shift valve against spring pressure, allowing pressure to flow through other pas- sages. B—When pressure is removed, the spring moves the valve to its original position. (General Motors) Solenoid Valve Spring Figure 12-22. This solenoid is directly connected to the valve. When the solenoid opens, it moves the valve against spring pressure, opening the valve. When the solenoid is de-energized, the spring returns the valve to its original position. (General Motors) present on one side of the valve. These solenoids usually exhaust pressure directly to the oil pan or sump. A few dedicated solenoids operate by moving the valve through linkage or by directly moving the valve. Multi-Shift Solenoids In transmissions with multi-shift solenoids, the sole- noids work directly on some shift valves and work through the valves and related oil passages to control other shift valves. The solenoids are arranged with the shift valves and oil passages so that they move the shift valves, depending on whether the solenoids are energized or de-energized. Figure 12-25 shows a schematic of a four-speed trans mission with two multi-shift solenoids. Shift solenoids have two pos- sible positions: energized (on) and de-energized (off). When two multi-shift solenoids are used together, the possible sequences are shown in Figure 12-26. The on-off sequence of the solenoids controls the appli- cation of the clutches and bands to produce various gears. An explanation of how the solenoids operate to provide four forward speeds on a common rear-wheel drive automatic transmission and front-wheel drive automatic transaxle will be presented later in this chapter. Shift Cushioning On hydraulically controlled transmissions, shifts are cushioned (shift feel improved) through the use of accumulators, calibrated orifices, and check balls. On many electronic transmissions and transaxles, shift feel is controlled by a pulsed solenoid. The solenoid restricts fluid flow according to ECM commands that are based on sensor inputs. The solenoid is pulsed to cause a soft apply under light throttle. When the throttle opening is large or the vehicle is heavily loaded, the solenoid is pulsed to allow maximum pressure to quickly reach the holding members. Figure 12-27 shows a typical pressure control solenoid. Note that it is installed between the shift valve and the holding member. Most electronically controlled transmissions and transaxles still rely to some extent on accumulators, orifices, and check balls. Governor Pressure Solenoids On some partial electronic transmissions, governor and throttle pressure move the shift valves. The governor valve, however, has been replaced by a solenoid controlled by the power train ECM. The ECM receives inputs from speed sensors and a governor pressure sensor. Based on these inputs, it varies the solenoid duty cycle to send the correct governor pressure to the shift valves. Figure 12-28 shows the solenoid and pressure sensor installed on the valve body. Ex 4th sig 3-4 sig Ex 4th sig 3-4 sig