Automatic Transmissions

The automatic transmission was introduced as early as the 1930s in automobiles and used extensively in military equipment in the 1940s. In a way it has been a revolution to the automobile industry over the years. An automatic transmission is defined as a transmission which does not require the driver to manually shift progressively through the gears. In the last issue of Diesel Tech magazine, standard transmissions were discussed and dissected. In this Tech Corner column we will review the extensive parts utilized in most automatic transmissions and then review automatics which have been used in the diesel pickup trucks throughout the years.

The automatic transmission can be outwardly perceived to many as a very complex mechanism—both mechanically and hydraulically. There are so many different types of automatics which have been manufactured over the years as well in various arrangements. These arrangements may include rear-wheel drive transmissions, front-wheel drive transaxles, and also all-wheel drive transmission/transaxles. The truth is, almost all automatic transmissions are very similar and utilize many of the same internal components to function.

Common Factors

Let’s discuss first the laundry list of similarities in all automatic transmissions and how each part contributes to the workings of the tranny. These parts will include—but not be limited to—torque convertors, valve body/solenoid body, driven shafts, planetary gears and clutches.

Transmissions in general have the sole responsibility of transferring power from the engine to the axles. The coupling device between the engine and transmission first begins with the flex plate. The engine will have a flex plate, which will be bolted to the crankshaft. This part is similar to a flywheel with a manual transmission. It houses the ring gear for the starter and the outside of it is bolted to the torque convertor. Stock flex plates are designed to flex in order to soak up torsional vibration from a reciprocating piston engine as well as shock from the transmission during up shifts and down shifts. Aftermarket flex plates have high strength and integrity, but are also thicker and more rigid. The flex plate is bolted directly to a fluid coupling device called a torque convertor. The torque convertor utilizes several internal components in order to function. These components include an impellor or pump, a stator, and a turbine, and in some applications a lock-up clutch. The impellor is what the engine is turning when it is running. The stator is splined into a stationary set of splines, which are bolted directly to the front of the transmission. The stator does not move. It is placed between the impellor and turbine and directs the flow of oil and controls the torque convertors efficiency. Its shape is directly related to how fluid flows from the driven impellor to the turbine, which drives the transmission input shaft.

Convertors

Most aftermarket convertor’s characteristics such as stall and torque multiplying capabilities are directly related to the design of the internal stator, impellor and turbine. Most convertors have an efficiency rating of around 70 to 95 percent. Efficiency refers to the amount of driven power from the engine, which is being transferred to the transmission through the torque convertor. Ironically enough, the more efficiency loss a convertor has, the more heat it will build.  It is impossible to have a torque convertor achieve a 100 percent efficiency rating—unless it is a lock up torque convertor. Lock up torque convertors are exactly like a non lock-up convertor until fluid fills the convertor clutch circuit and the clutch is applied. Then the torque convertor impellor is locked solid to the turbine. There is virtually no energy loss through fluid coupling and close to 100 percent of the engine’s power is transferred to the internals of the transmission, much like a clutch in a manual transmission. The power from the convertor is diverted into an input shaft, which supplies the intermediate and output shafts with power and speed. The torque convertor also usually has some kind of drive flange on its driven hub, which drives the transmission pump.

Valve Body

As the input shaft turns, the question is raised, “How does the input shaft transfer power to the output shaft as well as distinguish speed supplied to the shafts?” All this is controlled with the valve body. The valve body becomes the brains of the transmission. Simply put, the valve body directs fluid under pressure to the individual clutch circuits. The valve body contains a series of passageways which have valves to allow fluid to release or engage certain clutches. The older valve bodies are just that: they have a component which houses most of the valves and each valve is moved when hydraulic pressure overcomes spring tension. This is why when a shift kit is installed in the older transmissions it involves different springs and check balls to enhance shift points and characteristics of the transmission. The valve bodies are usually found in the sump of the transmission, but part of them may be found by the pump of the transmission as well. Since the enhancement of electronically managed powertrain on newer vehicles, automatic transmissions have changed some.

Instead of having hydraulically controlled valve bodies, now automatic transmissions are controlled by solenoid bodies or valve/solenoid body hybrids. With hydraulic valve bodies, sheer hydraulic force controls shift points, now solenoids—which are electronically controlled through the PCM or TCM—control all the shifts made in the transmission. So instead of putting a shift kit in these transmissions, you just need to electronically tune them through a tuner, chip or laptop. In the end both the solenoid body and valve body are only performing two functions: the delivery and timing of oil to designated clutch circuits. When the valve body directs fluid flow from the pump to a clutch pack or packs, the clutches are mated directly or indirectly to one of the three shafts found in the transmission through planetary gear sets. Planetary gears receive an input and increase or decrease the speed as an output. Most transmissions have a series of several planetary gear sets necessary to be capable of several forward speeds. The clutches found inside the housing of an automatic transmission are wet clutches. This means they are bathed in oil.  Clutches will be found in clutch drums in a pack. Each clutch pack will have a certain number of friction plates as well as steel plates. The type of material, number of friction/steel plates, and pressure applied to a clutch piston all contribute to the holding capacity of a certain clutch pack. Oil is so vital in clutch life in a transmission. The oil serves several purposes such as lubrication, an additive package carrier and heat dissipation. The cooler the transmission oil can be kept when operating, the longer it will last. This is why most transmissions will be equipped with an external oil cooler.   

 About The Author

Levi Perkins is the head instructor for the diesel program at the College of Southern Idaho located in Twin Falls, Idaho. The diesel program has had 100 percent placement in the last three years and provides future diesel mechanics and engineers a bright future in the diesel industry. For more information visit www.csi.edu or call 208-733-9554.