An automatic transmission has many seals and gaskets to control the flow of hydraulic fluid and to keep it from leaking out.  There are two main external seals: the front seal and the rear seal. The front seal seals the point where the torque converter mounts to the transmission case. This seal allows fluid to freely move from the converter to the transmission but keeps the fluid from leaking out.  The rear seal keeps fluid from leaking past the output shaft.

A seal is usually made of rubber (similar to the rubber in a windshield wiper blade) and is used to keep oil from leaking past a moving part such as a spinning shaft. In some cases, the rubber is assisted by a spring that holds the rubber in close contact with the spinning shaft.

A gasket is a type of seal used to seal two stationary parts that are fastened together. Some common gasket materials are: paper, cork, rubber, silicone and soft metal.

Aside from the main seals, there are also a number of other seals and gaskets that vary from transmission to transmission. A common example is the rubber O-ring that seals the shaft for the shift control lever.  This is the shaft that you move when you manipulate the gear shifter.  Another example that is common to most transmissions is the oil pan gasket.  In fact, seals are required anywhere that a device needs to pass through the transmission case with each one being a potential source for leaks.

On automatic transmissions, the  torque converter takes the place of the clutch found on standard shift vehicles.  It is there to allow the engine to continue running  when the vehicle comes to a stop.  The principle behind a torque converter is like taking a fan that is plugged into the wall and blowing air into another fan which is unplugged.  If you grab the blade on the unplugged fan, you are able to hold it from turning but as soon as you let go, it will begin to speed up until it comes close to the speed of the powered fan.  The difference with a torque converter is that instead of using air, it uses oil or transmission fluid, to be more precise.

A torque converter is a large doughnut shaped device (10″ to 15″ in diameter) that is mounted between the engine and the transmission.  It consists of three internal elements that work together to transmit power to the transmission.  The three elements of the torque converter are the Pump,  the Turbine, and the Stator.  The pump is mounted directly to the converter housing which in turn is bolted directly to the engine’s crankshaft and turns at engine speed.  The turbine is inside the housing and is connected directly to the input shaft of the transmission  providing power to move the vehicle.  The stator is mounted to a one-way clutch so that it can spin freely in one direction but not in the other. Each of the three elements have fins mounted in them to precisely direct the flow of oil through the converter

With the engine running, transmission fluid is pulled into the pump section and is pushed outward by centrifugal force until it reaches the turbine section which starts it turning.  The fluid continues in a circular motion back towards the center of the turbine where it enters the stator. If the turbine is moving considerably slower than the pump, the fluid will make contact with the front of the stator fins which push the stator into the one way clutch and prevent it from turning. With the stator stopped, the fluid is directed by the stator fins to re-enter the pump at a “helping” angle providing a torque increase.   As the speed of the turbine catches up with the pump, the fluid starts hitting the stator blades on the back-side causing the stator to turn in the same direction as the pump and turbine.  As the speed increases, all three elements begin to turn at approximately the same speed.

Since the ’80s, in order to improve fuel economy, torque converters have been equipped with a lockup clutch (not shown) which locks the turbine to the pump as the vehicle speed reaches approximately 45 – 50 MPH.  This lockup is controlled by computer and usually won’t engage unless the transmission is in 3rd or 4th gear.Use your business checks and upgrade your office automobiles for secure driving.

Automatic transmissions contain many gears in various combinations.  In a manual transmission, gears slide along shafts as you move the shift lever from one position to another, engaging various sized gears as required in order to provide the correct gear ratio. In an automatic transmission, however, the gears are never physically moved and are always engaged to the same gears.  This is accomplished through the use of planetary gear sets.

The basic planetary gear set consists of a sun gear, a ring gear and two or more planet gears, all remaining in constant mesh.  The planet gears are connected to each other through a common carrier which allows the gears to spin on shafts called “pinions” which are attached to the carrier .

One example of a way that this system can be used is by connecting the ring gear to the input shaft coming from the engine, connecting the planet carrier to the output shaft, and locking the sun gear so that it can’t move.  In this scenario, when we turn the ring gear, the planets will “walk” along the sun gear (which is held stationary) causing the planet carrier to turn the output shaft in the same direction as the input shaft but at a slower speed causing gear reduction (similar to a car in first gear).

If we unlock the sun gear and lock any two elements together, this will cause all three elements to turn at the same speed so that the output shaft will turn at the same rate of speed as the input shaft. This is like a car that is in third or high gear. Another way that we can use a Planetary gear set is by locking the planet carrier from moving, then applying power to the ring gear which will cause the sun gear to turn in the opposite direction giving us reverse gear.

The illustration on the right shows how the simple system described above would look in an actual transmission. The input shaft is connected to the ring gear (Blue), The Output shaft is connected to the planet carrier (Green) which is also connected to a “Multi-disk” clutch pack. The sun gear is connected to a drum (yellow) which is also connected to the other half of the clutch pack.  Surrounding the outside of the drum is a band (red) that can be tightened around the drum when required to prevent the drum with the attached sun gear from turning.

The clutch pack is used, in this instance, to lock the planet carrier with the sun gear forcing both to turn at the same speed. If both the clutch pack and the band were released, the system would be in neutral.  Turning the input shaft would turn the planet gears against the sun gear, but since nothing is holding the sun gear, it will just spin free and have no effect on the output shaft. To place the unit in first gear, the band is applied to hold the sun gear from moving.  To shift from first to high gear, the band is released and the clutch is applied causing the output shaft to turn at the same speed as the input shaft.

Many more combinations are possible using two or more planetary sets connected in various ways to provide the different forward speeds and reverse that are found in modern automatic transmissions.

Some of the clever gear arrangements found in four and now, five, six and even seven and eight-speed automatics are complex enough to make a technically astute lay person’s head spin trying to understand the flow of power through the transmission as it shifts from first gear through top gear while the vehicle accelerates to highway speed.  On modern vehicles (mid ’80s to the present), the vehicle’s computer monitors and controls these shifts so that they are almost imperceptible.

Clutch Packs

A clutch pack consists of alternating disks that fit inside a clutch drum. Half of the disks are steel and have splines that fit into groves on the inside of the drum.  The other half have a friction material bonded to their surface and have splines on the inside edge that fit groves on the outer surface of the adjoining hub.  There is a piston inside the drum that is activated by oil pressure at the appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one.

The modern automatic transmission consists of many components and systems that are designed to work together in a symphony of clever mechanical, hydraulic and electrical technology that has evolved over the years into what many mechanically inclined individuals consider to be an art form.  We try to use simple, generic explanations where possible to describe these systems but, due to the complexity of some of these components, you may have to use some mental gymnastics to visualize their operation.

The main components that make up an automatic transmission include:

• Planetary Gear Sets which are the mechanical systems that provide the various forward gear ratios as well as reverse.
• The Hydraulic System which uses a special transmission fluid sent under pressure by an Oil Pump through the Valve Body to control the Clutches and the Bands in order to control the planetary gear sets.
• Seals and Gaskets are used to keep the oil where it is supposed to be and prevent it from leaking out.
• The Torque Converter which acts like a clutch to allow the vehicle to come to a stop in gear while the engine is still running.
• The Governor and the Modulator or Throttle Cable that monitor speed and throttle position in order to determine when to shift.
• On newer vehicles, shift points are controlled by Computer which directs electrical solenoids to shift oil flow to the appropriate component at the right instant.