NT’s PRIMER ON AUTOMATIC TRANSMISSIONS

• What is a transmission and what does it do?
• How does an automatic transmission work?
• What goes wrong when my transmission no longer works right?

What Is a Transmission?

Every motor vehicle has a transmission. The transmission is between the engine and final drive gear in the front or rear (or both) axles that turn the wheels. Some vehicles have standard (manual) transmissions wherein the driver has to push in a clutch and shift gears manually. Most vehicles on the road today have some form of automatic transmission – the subject of this introductory paper.

Transmissions make possible the matching of the engine’s power and speed (revolutions per minute or rpms) to different road speeds of the vehicle. Lower gear ratios are used to get the vehicle moving or overcome large loads such as a steep hill or heavy vehicle weight. As the vehicle’s speed picks up, the transmission shifts to higher ratios where the engine turns more slowly, thus becoming quieter and consuming less fuel.

What Is a Torque Converter?

In an automatic transmission, the clutch used with a standard (manual) transmission is replaced by a torque converter. The torque converter is a fluid coupling that always slips a bit. Similar to two electric fans placed face-to-face, the “fan” driven by the engine causes the other fan to turn. This allows the engine to be continuously coupled to the transmission, yet remain running when the vehicle is at rest. Torque converters also have the advantage of being able to multiply engine torque to achieve the effect of a lower gear ratio for starting the vehicle moving. Some torque converters even have a lockup clutch to in effect “lock” the “fans” together to eliminate slippage at cruising speeds and increase the vehicle’s fuel mileage.

How Does an Automatic Transmission Work?

Early automatic transmissions started out as a combination of mechanical gears and hydraulic systems. In order to increase the performance and fuel efficiency of the vehicle, automatic transmissions have steadily progressed from two or three speeds initially to seven speeds in today’s most sophisticated passenger vehicles. In the mid-90’s, electronics in the form of computer controls were added to automatic transmissions. Today’s automatic transmissions are now highly complicated devices that are electronically linked to computer-based engine management systems and a variety of sensors in the vehicle in order to perform optimally. This is a major reason they require sophisticated computer-based test equipment to troubleshoot, and are time-consuming and expensive to rebuild.

The heart of an automatic transmission is one or more sets of planetary gears, made up of a ring gear, a set of small planetary gears, and a sun gear. By holding a different one of these three gears still relative to the other two, the transmission gear ratio can be changed. Different clutches and bands are used to hold these parts still or “lock” them together to obtain the gear ratio and forward or reverse rotation as desired.

Every automatic transmission has its own oil pump to generate fluid under pressure that lubricates the transmission’s moving parts and engages various clutches and bands at the right moment. Much of the transmission’s shifting is controlled by valves moving in the valve body to direct high oil pressure to engage the proper clutch or band. Clutches or bands can also be controlled by electronic solenoids that open or close oil passageways as necessary.

What Goes Wrong with an Automatic Transmission?

Good maintenance will help ensure the longest possible automatic transmission life before it requires an overhaul. Many people do not realize that an automatic transmission contains its own fluid filter that must be changed along with the transmission fluid at least every 15,000 miles. (This change interval may vary with vehicle make and model – check your vehicle’s owner’s manual for the manufacturer’s recommendation.)

Here in Texas heat and stress from towing heavy loads (in addition to drag racing or abusive driving) are the enemies of long automatic transmission life. Transmission fluid breaks down with time, mileage, and heat, clutch and band facing material wears away, metal parts wear from turning millions of revolutions or moving thousands of times. In addition, if the transmission fluid becomes low due to an oil seal or pan gasket leaking or poor maintenance, this accelerates wear of all internal parts. Occasionally, metal parts just break – due to lack of lubricant, heavy loads, or even poor design or manufacture.

No one can predict how long any automatic transmission will last before needing a rebuild. Some last 100,000 miles or more, others only last 40,000 miles. It depends on the manufacturer’s design, the vehicles use and how hard or easy it is driven, and whether good maintenance practices are followed.

You can be sure that NT will service and rebuild your transmission with only the best parts and original manufacturer-recommended lubricants. It is very much in our interest to have you as a satisfied customer that will recommend our services to others.

For more information on how automatic and standard transmissions, torque converters, clutches, and gears work, please refer to the Web site: howstuffworks.com. In addition, be sure to note our wall display of parts we normally replace during a rebuild. You can even see many of the parts mentioned in this paper in the cut-away transmission model and torque converter we have in our Austin lobby. Please note that this transmission is a simple 20-year old transmission, not indicative of the complexity of automatic transmissions found in today’s vehicles.

We hope that this section answers many of your questions and makes you more comfortable with the work we may perform on your vehicle. Any of our management staff will be happy to answer further questions you may have. Our goal is to calm your anxieties regarding what may be a very expensive vehicle repair, assure you that your vehicle is in the hands of highly professional technicians, and to be certain that you are a very satisfied customer. That will preserve our reputation as Austin’s finest transmission shop. We hope it will also encourage you to recommend us to others in need of such service. Thank you for your patronage.