9.7 Automatic Transmissions

The automatic transmission, like the manual transmission, is designed to match the load requirements of the vehicle to the power and speed range of the engine (Figure 9-15). The automatic transmission, however, does this automatically depending on throttle position, vehicle speed, and the position of the control lever. Automatic transmissions are built in models that have two, three, or four-forward speeds and in some that are equipped with overdrive. Operator control is limited to the selection of the gear range by moving a control lever.

Figure 9-15 — Automatic transmission.

The automatic transmission is coupled to the engine through a torque converter. The torque converter is used with an automatic transmission because it does not have to be manually disengaged by the operator each time the vehicle is stopped. Because the automatic transmission shifts without any interruption of engine torque application, the cushioning effect of the fluid coupling within the torque converter is desirable.

Because the automatic transmission shifts gear ratios independent of the operator, it must do so without the operator releasing the throttle. The automatic transmission does this by using planetary gearsets whose elements are locked and released in various combinations that produce the required forward and reverse gear ratios. The locking of the planetary gearset elements is done through the use of hydraulically actuated multiple-disc clutches and brake bands. The valve body controls the hydraulic pressure that actuates these locking devices. The valve body can be thought of as a hydraulic computer that receives signals that indicate vehicle speed, throttle position, and gearset lever position. Based on this information, the valve body sends hydraulic pressure to the correct locking devices.

The parts of the automatic transmission are as follows:

Torque Converters

The torque converter is a fluid clutch that performs the same basic function as a manual transmission dry friction clutch (Figure 9-16). It provides a means of uncoupling the engine for stopping the vehicle in gear. It also provides a means of coupling the engine for acceleration.

Figure 9-16 — Torque converter.

A torque converter has four basic parts:

  1. Outer housing—normally made of two pieces of steel welded together in a doughnut shape, housing the impeller, stator, and turbine. The housing is filled with transmission fluid.
  2. Impeller—driving member that produces oil movement inside the converter whenever the engine is running. The impeller is also called the converter pump.
  3. Turbine—a driven fan splined to the input shaft of the automatic transmission. Placed in front of the stator and impeller in the housing. The turbine is not fastened to the impeller but is free to turn independently. Oil is the only connection between the two.
  4. Stator—designed to improve oil circulation inside the torque converter. Increases efficiency and torque by causing the oil to swirl around the inside of the housing.

The primary action of the torque converter results from the action of the impeller passing oil at an angle into the blades of the turbine. The oil pushes against the faces of the turbine vanes, causing the turbine to rotate in the same direction as the impeller (Figure 9-17). With the engine idling, the impeller spins slowly. Only a small amount of oil is thrown into the stator and turbine.

Figure 9-17 — Torque converter operation.

Not enough force is developed inside the torque converter to spin the turbine. The vehicle remains stationary with the transmission in gear.

During acceleration, the engine crankshaft, the converter housing, and the impeller begin to move faster. More oil is thrown out by centrifugal force, turning the turbine. As a result, the transmission input shaft and vehicle starts to move, but with some slippage.

At cruising speeds, the impeller and turbine spin at almost the same speed with very little slippage. When the impeller is spun fast enough, centrifugal force throws oil out hard enough to almost lock the impeller and turbine. After the oil has imparted its force to the turbine, the oil follows the contour of the turbine shell and blades so that it leaves the center section of the turbine spinning counterclockwise.

Because the turbine has absorbed the force required to reverse the direction of the clockwise spinning of the oil, it now has greater force than is being delivered by the engine. The process of multiplying engine torque has begun.

Torque multiplication refers to the ability of a torque converter to increase the amount of engine torque applied to the transmission input shaft. Torque multiplication occurs when the impeller is spinning faster than the turbine. For example, if the engine is accelerated quickly, the engine and impeller rpm might increase rapidly while the turbine is almost stationary. This is known as stall speed. Stall speed of a torque converter occurs when the impeller is at maximum speed without rotation of the turbine. This condition causes the transmission fluid to be thrown off the stator vanes at tremendous speeds. The greatest torque multiplication occurs at stall speed.

When the turbine speed nears impeller speed, torque multiplication drops off. Torque is increased in the converter by sacrificing motion. The turbine spins slower than the impeller during torque multiplication.

If the counterclockwise oil were allowed to continue to the center section of the impeller, the oil would strike the blades of the pump in a direction that would hinder its rotation and cancel any gains in torque. To prevent this, you can add a stator assembly.

The stator is located between the pump and the turbine and is mounted on a one-way clutch that allows it to rotate clockwise but not counterclockwise (Figure 9-16). The purpose of the stator is to redirect the oil returning from the turbine and change its rotation back to that of the impeller. Stator action is only needed when the impeller and turbine are turning at different speeds. The one-way clutch locks the stator when the impeller is turning faster than the turbine. This causes the stator to route oil flow over the impeller vanes properly. Then, when turbine speed almost equals impeller speed, the stator can freewheel on its shaft so not to obstruct flow.

Even at normal highway speeds, there is a certain amount of slippage in the torque converter. Another type of torque converter that is common on modern vehicles is the lockup torque converter. The lockup torque converter provides increased fuel economy and increased transmission life through the elimination of heat caused by torque converter slippage. A typical lockup mechanism consists of a hydraulic piston, torsion springs, and clutch friction material.

In lower gears, the converter clutch is released. The torque converter operates normally, allowing slippage and torque multiplication. However, when shifted into high or direct drive, transmission fluid is channeled to the converter piston. The converter piston pushes the friction discs together, locking the turbine and impeller. The crankshaft is able to drive the transmission input shaft directly, without slippage. The torsion springs assist to dampen engine power pulses entering the drive train.

Planetary Gearsets

A planetary gearset consists of three members--sun gear, ring gear, and planetary carrier, which hold the planetary gears in proper relation with the sun and ring gear (Figure 9-18). The planetary gears are free to rotate on their own axis while they "walk" around the sun gear or inside the ring gear.

Figure 9-18 — Planetary gearset.

By holding or releasing the components of a planetary gearset, it is possible to do the following:

Figure 9-19 shows the simplest application of planetary gears in a transmission. With the application shown, two forward speeds and neutral are possible. High gear or direct drive is shown. The clutch is holding the planet carrier to the input shaft, causing the carrier and sun gear to rotate as a single unit. With the clutch released, all gears are free to rotate and no power is transmitted to the output shaft. In neutral, the planetary carrier remains stationary while the pinion gears rotate on their axis and turn the ring gear. Should the brake be engaged on the ring gear, the sun gear causes the planetary gears to walk around the inside of the ring gear and forces the planet carrier to rotate in the same direction as the sun gear, but at a slower speed (low gear). To provide additional speed ranges or a reverse, you must add other planetary gearsets to this transmission.

Figure 9-19 — Planetary gearset operation.

A compound planetary gearset combines two planetary units into one housing or ring gear. It may have two sun gears or a long sun gear to operate two sets of planetary gears. A compound planetary gearset is used to provide more forward gear ratios than a simple planetary gearset.

Clutches and Bands

Automatic transmission clutches and bands are friction devices that drive or lock planetary gearsets members. They are used to cause the gearset to transfer power.

Multiple-Disc Clutch

The multiple-disc clutch is used to transmit torque by locking elements of the planetary gearsets to rotating members within the transmission. In some cases, the multiple-disc clutch is also used to lock a planetary gearset element to the transmission case so it can act as a reactionary member. The multiple-disc clutch is made up of the following components (Figure 9-20):

Figure 9-20 — Multiple-disc clutch.

The operation of the multiple-disc clutch is as follows (Figure 10-21):

Figure 10-21 — Multiple-disc clutch operation.

Overrunning Clutch

An overrunning clutch is used in automatic transmissions to lock a planetary gearset to the transmission case so that it can act as a reactionary member. The overrunning clutch for the planetary gears is similar to the one in a torque converter stator or an electric starting motor drive gear. A planetary gearset overrunning clutch consists of an inner race, a set of springs, rollers, and an outer race.

Operation of the overrunning clutch is very simple to understand. When driven in one direction, rollers lock between ramps on the inner and outer race, allowing both races to turn. This action can be used to stop movement of the planetary member, for example. When turned in the other direction, rollers walk off the ramps, and the races are free to turn independently.

Brake Band

The brake band is used to lock a planetary gearset element to the transmission case so that the element can act as a reactionary member. The brake band is made up of the following elements (Figure 9-22):

Figure 9-22 — Brake band.

The operation of the brake band is as follows (Figure 9-23):

Figure 9-23 — Brake band operation.

In the applied circuit of a clutch or band, an accumulator is used to cushion initial application. It temporarily absorbs some of the hydraulic pressure to cause slower movement of the applied piston.

Hydraulic System

The hydraulic system of an automatic transmission serves four basic purposes:

The hydraulic system for an automatic transmission typically consists of a pump, pressure regulator, manual valve, vacuum modulator valve, governor valve, shift valves, kick down valve, and a valve body.


The typical hydraulic pump is an internal-external rotor or gear-type pump. Located in the front of the transmission case, it is keyed to the torque converter hub so that it is driven by the engine. As the torque converter spins the oil pump, transmission fluid is drawn into the pump from the transmission pan. The pump compresses the oil and forces it to the pressure regulator. The pump has several basic functions:

Pressure Regulator

The pressure regulator limits the maximum amount of oil pressure developed by the oil pump. It is a spring-loaded valve that routes excess pump pressure out of the hydraulic system, assuring proper transmission operation.

Manual Valve

A manual valve located in the valve body is operated by the driver through the shift linkage (Figure9-24). This valve allows the operator to select park, neutral, reverse, or different drive ranges. When the shift lever is moved, the shift linkage moves the manual valve. As a result, the valve routes hydraulic fluid throughout the transmission to the correct places.

Figure 9-24 — Manual valve operation.

When the operator selects overdrive, drive, or second, the transmission takes over, shifting automatically to meet driver conditions. When the selector is placed in low and reverse, the transmission is locked into the selected gear.

Vacuum Modulator Valve

The vacuum modulator valve is a diaphragm device that uses engine manifold vacuum to indicate engine load to the shift valve (Figure 9-25). As engine vacuum (load) rises and falls, it moves the diaphragm inside the modulator. This in turn moves the rod and hydraulic valve to change throttle control pressure in the transmission. In this way, the vacuum modulator can match transmission shift points to engine loads.

Figure 9-25 — Vacuum modulator valve.

Governor Valve

The governor valve senses engine speed (transmission output shaft speed) to help control gear shifting (Figure 9-26). The vacuum modulator and governor work together to determine the shift points. The governor gear is meshed with a gear on the transmission output shaft. Whenever the vehicle and output shaft are moving, the centrifugal weights rotate. When the output shaft and weights are spinning slowly, the weights are held in by the governor springs, causing low-pressure output, and the transmission remains in low gear. As the engine speeds increases, the weights are thrown out further and governor pressure increases, moving the shift valve and causing the transmission to shift into higher gear.

Figure 9-26 — Governor valve.

Shift Valves

The shift valves are simple balance type spool valves that select between low and high gear when the manual valve is in drive. Using control pressure (oil pressure from the regulator, governor, vacuum modulator, and manual valves), they operate the bands, servos, and gearsets. Oil pressure from the other transmission valves acts on each end of the shift valve. In this way, the shift valve is sensitive to engine load (vacuum modulator valve oil pressure), engine speed (governor valve oil pressure), and gearshift position (manual valve oil pressure). These valves move according to the forces and keep the transmission shifted into the correct gear ratio for the driving conditions.

Kickdown Valve

The kickdown valve causes the transmission to shift into a lower gear during fast acceleration. A rod or cable links the carburetor or fuel injection throttle body to a lever on the transmission. When the operator depresses the throttle, the lever moves the kickdown valve. This action causes hydraulic pressure to override normal shift control pressure and the transmission downshifts.

Valve Body

The valve body is a very complicated hydraulic system (Figure 9-27). It contains hydraulic valves used in an automatic transmission, such as the pressure regulator, shift valves, and manual valves. The valve body bolts to the bottom of the transmission case and is housed in the transmission pan. A filter or screen is attached to the bottom of the valve body. Passages in the valve body route fluid from the pump to the valves and then into the transmission case. Passages in the transmission case carry fluid to other hydraulic components.

Figure 9-27 — Valve body.

Automatic Transmission Service

Automatic transmission service can be easily divided into the following areas: preventive maintenance, troubleshooting, and major overhaul. Before you perform maintenance or repair on an automatic transmission, consult the maintenance manual for instructions and proper specifications. As a floor mechanic, however, your area of greatest concern is preventive maintenance. Preventive maintenance includes the following:

Checking the Fluid

The operator is responsible for first echelon (operator’s) maintenance. The operator should be trained not only to know to look for the proper fluid level but also to know how to look for discoloration of the fluid and debris on the dipstick.

Fluid levels in automatic transmissions are almost always checked at operating temperature. This is important to know since the level of the fluid may vary as much as three quarters of an inch between hot and cold.

The fluid should be either reddish or clear. The color varies due to the type of fluid. (For example, construction equipment using OE-10 will be clear). A burnt smell or brown coloration of the fluid is a sign of overheated oil from extra heavy use or slipping bands or clutch packs. The vehicle should be sent to the shop for further inspection.



Not all transmission fluids are the same. Before you add fluid, check the manufacturer’s recommendations first. The use of the wrong fluid will lead to early internal parts failure and costly overhaul.

Overfilling the transmission can result in the fluid foaming and the fluid being driven out through the vent tube. The air that is trapped in the fluid is drawn into the hydraulic system by the pump and distributed to all parts of the transmission. This situation will cause air to be in the transmission in place of fluid and in turn cause slow application and burning of clutch plates and facings. Slippage occurs, heat results, and failure of the transmission follows.

Another possible, but remote, problem is water, indicated by the fluid having a "milky" appearance. A damaged fluid cooling tube in the radiator (automotive) or a damaged oil cooler (construction) could be the problem. The remedy is simple. Pressure-test the suspected components and perform any required repairs. After repairs have been performed, flush and refill the transmission with clean, fresh fluid.

Linkage and Band Adjustment

The types of linkages found on an automatic transmission are gearshift selection and throttle kickdown. The system can be a cable or a series of rods and levers. These systems do not normally present a problem, and preventive maintenance usually involves only a visual inspection and lubrication of the pivot points of linkages or the cable. When adjusting these linkages, you should strictly adhere to the manufacturer’s specifications.

If an automatic transmission is being used in severe service, the manufacturer may suggest periodic band adjustment. Always adjust lockup bands to the manufacturer’s specifications. Adjust the bands by loosening the locknut and tightening down the adjusting screw to a specified value. Back off the band adjusting screw with a specified number of turns and tighten down the locking nut.



Not all bands are adjustable. Always check the manufacturer’s service manual before any servicing of the transmission.

Fluid Replacement

Perform fluid replacement according to the manufacturer’s recommendations. These recommendations vary considerably for different makes and models. Before you change automatic transmission fluid, always read the service manual first.

Severe service includes the following: hot and dusty conditions, constant stop- and-go driving (taxi service), trailer towing, constant heavy hauling, and around-the- clock operations (contingency). Any CESE operating in these conditions should have its automatic transmission fluid and filter changed on a regular schedule, based on the manufacturer's specifications for severe service. Ensure the vehicle is on level ground or a lift, and let the oil drain into a proper catchment device.

The draining of the transmission can be accomplished in one of the following three ways:

  1. Removing the drain plug.
  2. Loosening the dipstick tube.
  3. Removing the oil pan.



Oil drained from automatic transmissions contains heavy metals and is considered hazardous waste and should be disposed of according to local instructions.

Once the oil is drained, remove the pan completely for cleaning by paying close attention to any debris in the bottom of the pan. The presence of a high amount of metal particles may indicate serious internal problems. Clean the pan, and set it aside.

All automatic transmissions have a filter or screen attached to the valve body. The screen is cleanable, whereas the filter is a disposable type and should always be replaced when removed. These are retained in different ways: retaining screws, metal retaining clamps, or O-rings made of neoprene. Clean the screen with solvent and use low-pressure air to blow-dry it. Do not use rags to wipe the screen dry, as they tend to leave lint behind that will be ingested into the hydraulic system of the transmission. If the screen is damaged or is abnormally hard to clean, replace it.

Draining the oil from the pan of the transmission does not remove all of the oil—draining the oil from the torque converter completes the process. To do this, remove the torque converter cover and remove the drain plug, if so equipped. For a torque converter with- out a drain plug, special draining instructions may be found in the manufacturer’s service manual. Before performing this operation, clear it with your shop supervisor.

Refilling the Transmission

Reinstall the transmission oil pan, the oil plug, and the fill tube. Fill the transmission with the fluid prescribed by the manufacturer to the proper level. With the brakes applied, start the engine and let it idle for a couple of minutes. Move the gear selector through all gear ranges several times, allowing the fluid to flow through the entire hydraulic system to release any trapped air. Return the selector lever to park or neutral and recheck the fluid level. Bring the fluid to the proper level. Run the vehicle until operating temperature is reached, checking for leaks. Also, recheck the fluid and adjust the level as necessary.



Overfilling an automatic transmission will cause foaming of the fluid. This condition prevents the internal working parts from being properly lubricated, causing slow actuation of the clutches and bands. Eventually, burning of the clutches and bands results. Do NOT overfill an automatic transmission.

Electronic Systems

Speed Sensors

The vehicle speed sensor is a device that is mounted on the output shaft of the transmission or transaxle. This device tells the electronic control module (ECM) how fast the vehicle is moving. It consists of a wheel with teeth around it and a magnetic pickup. The wheel can either be attached to the output shaft or be gear driven off the output shaft. As the wheel is turned, it induces an alternating current (AC) in the magnetic pickup. The ECM uses this information to calculate how fast the vehicle is moving.

Shift Solenoids

Electronic transmissions utilize shift solenoids to control when the transmission will shift from one gear to the next. The solenoid affects hydraulic pressure on one side of a shift valve, causing it to move. In some transmissions this solenoid is connected directly to a check ball that acts as a shift valve. Energizing the shift solenoid causes the check ball to move and either open or close pressure passages leading to the holding members.

The ECM works with the shift solenoids, either receiving or sending input to tell the solenoid to operate or hold. If the speed is appropriate for an upshift, however, the throttle position sensor tells the ECM it is wide open; under a heavy load, for example, the ECM may hold the shift solenoid from operation until the throttle is changed.

Test Your Knowledge

7. In a torque converter, what component is known as the converter pump?

A. Stator
B. Impeller
C. Turbine
D. Drive fan

8. The condition that exists when the impeller of a torque converter is at maximum speed and the turbine is almost stationary is known by what term?

A. Torque speed
B. Engine speed
C. Acceleration speed
D. Stall speed

9. What type of torque converter eliminates the heat caused by torque converter slippage, which results in increased fuel economy and transmission life?

A. Antislip
B. Hydraulic
C. Direct
D. Lockup

10. What gear is the center gear in a planetary gearset?

A. Planet pinion
B. Ring
C. Sun
D. Planetary carrier

11. What component of a multiple-disc clutch is used to distribute application pressure equally on the surfaces of the clutch discs and plates?

A. Clutch hub
B. Pressure plate
C. Clutch drum
D. Clutch springs