One of the indications of a transmission problem is delayed engagement, when there is a long delay between the moment you shift the shifter into “D” (Drive) or “R” (Reverse) and the moment the transmission kicks in.
It’s easier to note delayed engagement after a car was sitting for a while: With the transmission in “P” (Park) start the engine, and wait until the engine rpm has reduced to normal level (650 – 850 rpm).
With your foot holding down the brake pedal, shift to the “D” (Drive) position. Almost immediately the transmission should engage – it feels like the car wants to creep forward. This should happen very smoothly, without a strong jerk or clunk.
Shift to “N” (Neutral), and the transmission should disengage. Now, still holding the brakes, shift to the “R” (Reverse) position. Again, the transmission kicks in almost immediately – you will feel the car wants to creep backward. This also should be very smooth, without a jerk or clunk.
Now, still holding the brake pedal down, try to shift from D to R and back. There should be no strong jerk or clunk.
If there is a notable long delay (more than 1 seconds) between the moment you shift and the moment the transmission kicks in, such a transmission might be either too worn or has some problem, avoid this car.
If you feel a strong jerk or clunk while shifting, the car may have a transmission problem, avoid such a car.

Now it’s time to test drive the car.
With the shifter in “D” (Drive) position drive gently, with smooth and gradual acceleration. Until the vehicle reaches a speed of 30-37 mph (50-60 km/h) or you should feel the gears shifting at least twice (from first to second, and from second to third gear).
All shifts should be done very smoothly, without jerks or slipping.
You should be able to feel when the transmission shifts by the slight change in the engine tone or change in engine rpm. If the transmission is extremely worn it may shift with quite a strong jerk, shudder or a delay (especially from first to second gear).
Driving at a speed of 25-30 mph (40-50 km/h) if you press down the accelerator pedal for a few seconds, you should feel downshifting to the lower gear, if the automatic transmission works properly.
The next step: check overdrive.
While driving at 60-70 km/h or 35-45 mph on a level road, without using the accelerator, switch overdrive ON. You should feel an upshifting to the next speed. Switch it to “OFF,” and you should feel a downshifting.
Another thing that may indicate the transmission problem is the slipping. When the transmission is excessively worn it may slip – which means you press the accelerator, the engine rpm increases but the speed remains the same.

If during the drive test you feel any problem such as transmission seems to slipping or shifts with a jerk or shudder or if the transmission got stuck in some gear, or has trouble shifting into a particular gear (for example, from second to third), avoid buying such a car.
Test-drive the car as long as possible. Often the transmission may work well when it’s cold but when it’s warmed up it starts giving troubles or visa versa. So, it’s better to spend more time checking the transmission than later fixing it endlessly. Normally there should be no shudder, no noises or any kind of strong jerks at any speed and at any engine temperatures while any shifting. If the salesperson tells you that the jerks or shudder or any other abnormal transmission behavior is “normal” for this car or it’s just because the car is cold or anything alike, don’t trust them. If the “check engine” and/or a flashing overdrive light comes on while driving, have the problem assessed with your mechanic before buying a car.

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.

The valve body is the control center of the automatic transmission.  It contains a maze of channels and passages that direct hydraulic fluid to the numerous valves which then activate the appropriate clutch pack or band servo to smoothly shift to the appropriate gear for each driving situation.  Each of the many valves in the valve body has a specific purpose and is named for that function. For example the 2-3 shift valve activates the 2nd gear to 3rd gear up-shift or the 3-2 shift timing valve which determines when a downshift should occur.

The most important valve, and the one that you have direct control over is the manual valve.  The manual valve is directly connected to the gear shift handle and covers and uncovers various passages depending on what position the gear shift is placed in.  When you place the gear shift in Drive, for instance, the manual valve directs fluid to the clutch pack(s) that activates 1st gear. it also sets up to monitor vehicle speed and throttle position so that it can determine the optimal time and the force for the 1 – 2 shift.  On computer controlled transmissions, you will also have electrical solenoids that are mounted in the valve body  to direct fluid to the appropriate clutch packs or bands under computer control to more precisely control shift points.

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.