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Automatic Transmission and Transaxle, Page 1 of 3

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The automobile has become so sophisticated and the automatic transmissions so reliable; that automatic transmissions are the most popular option, or are even standard on many models. Over 85% of all new vehicles are ordered with an automatic transmission. All the driver has to do is start the engine, select a gear and operate the accelerator and brakes. It may not be as much fun as shifting gears, but it is far more efficient if you haul heavy loads or pull a trailer.

The automatic transmission anticipates the engines needs and selects gears in response to various inputs (engine vacuum, road speed, throttle position, etc.) to maintain the best application of power. The operations usually performed by the clutch and manual transmission are accomplished automatically, through the use of the fluid coupling, which allows a very slight, controlled slippage between the engine and transmission. Tiny hydraulic valves control the application of different gear ratios on demand by the driver (position of the accelerator pedal), or in a preset response to engine conditions and road speed.

How the automatic transmission works
See Figures 1, 2 and 3

The automatic transmission allows engine torque and power to be transmitted to the drive wheels within a narrow range of engine operating speeds. The transmission will allow the engine to turn fast enough to produce plenty of power and torque at very low speeds, while keeping it at a sensible rpm at high vehicle speeds.

The transmission uses a light fluid as the medium for the transmission of power. This fluid also operates the hydraulic control circuits and acts as a lubricant. Because the transmission fluid performs all of these three functions, trouble within the unit can easily travel from one part to another.

The automatic transmission operates on a principle that fluids cannot be compressed, and that when put into motion, will cause a similar reaction upon any resisting force. To understand this law of fluids, think of two fans placed opposite each other. If one fan is turned on, it will begin to turn the opposite fan blades. This principle is applied to the operation of the fluid coupling and torque converter by using driving and driven members in place of fan blades.

Every type of automatic transmission has two sections. The front section contains the fluid coupling or torque converter and takes the place of the driver operated clutch. The rear section contains the valve body assembly and the hydraulically controlled gear units, which take the place of the manually shifted standard transmission.

Figure 1 Basic components of a automatic transmission.
Basic components of a automatic transmission

Figure 2 Cutaway view of a typical 3-speed automatic transmission showing the basic components. Click on picture to enlarge view

Figure 3 Cutaway view of a typical 3-speed automatic transaxle showing the basic components. Click on picture to enlarge view

Electronic transmission controls
See Figure 4

Numerous changes have occurred in transaxles and transmissions in the last decade. The demand for lighter, smaller and more fuel efficient vehicles has resulted in the use of electronics to control both the engine and transmission to achieve the fuel efficient results that are required by law. The transaxle/transmission assembly is a part of the electronic controls, by sending signals of vehicle speed to an on-board computer which, in turn, relates these signals, along with others from the engine assembly, to determine gear selection for the best performance.

Sensors are used for engine and road speeds, engine load, gear selector lever position, and the kickdown switch operation. In addition, the driving program, set by the factory, is used to send signals to the microcomputer to determine the optimum gear selection, according to a preset program. The shifting is accomplished by solenoid valves in the hydraulic system. The electronics also control the modulated hydraulic pressure during shifting, along with regulating engine torque to provide smooth shifts between gear ratio changes. This type of system can be designed for different driving programs, such as giving the operator the choice of operating the vehicle for either economy or performance.

The transmission's sensors also let the operator of the vehicle know if there are any problems with the system. If the transmission control computer detects a problem it will store a trouble code in memory and it will light or flash a transmission warning lamp (or engine service light) on the dash to alert the operator something is wrong. Using the proper scan tools or techniques, a technician can retrieve the code (depending on the manufacturer) in order to help diagnose the trouble. To get a better understanding of engine and transmission trouble codes, see a repair manual for your year/make/model car.

Figure 4 Electronic controlled transmissions use solenoids for gear selection with microcomputer control.
Electronic controlled transmissions use solenoids for gear selection with microcomputer control.

Brake shift interlock system

As a safety feature on some vehicles the transmission will not allow the operator to shift into drive or start the car until they place their foot onto the brake pedal. This system is usually controlled by a cable from the brake pedal to the transmission or the transmission computer and sensors. On some vehicles the driver will hear a clicking which is the sensor operation.

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©1998 W. G. Nichols - Chilton's Easy Car Care