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How Automatic Transmissions Work

Introduction to How an Automatic Transmission Works
Planetary Gearsets & Gears
The Clutches and Bands
The Hydraulic System and How It Shifts

The Hydraulic System
The automatic transmission in your car has to do numerous tasks. You may not realize how many different ways it operates. For instance, here are some of the features of an automatic transmission:

• If the car is in overdrive (on a four-speed transmission), the transmission will automatically select the gear based on vehicle speed and throttle pedal position.
• If you accelerate gently, shifts will occur at lower speeds than if you accelerate at full throttle.
• If you floor the gas pedal, the transmission will downshift to the next lower gear.
• If you move the shift selector to a lower gear, the transmission will downshift unless the car is going too fast for that gear. If the car is going too fast, it will wait until the car slows down and then downshift.
• If you put the transmission in second gear, it will never downshift or upshift out of second, even from a complete stop, unless you move the shift lever.

Figure 15.

You've probably seen something that looks like Figure 15 before. It is really the brain of the automatic transmission, managing all of these functions and more. The passageways you can see route fluid to all the different components in the transmission. Passageways molded into the metal are an efficient way to route fluid; without them, many hoses would be needed to connect the various parts of the transmission. First, we'll discuss the key components of the hydraulic system; then we'll see how they work together.

The Pump
Automatic transmissions have a neat pump, called a gear pump. The pump is usually located in the cover of the transmission. It draws fluid from a sump in the bottom of the transmission and feeds it to the hydraulic system. It also feeds the transmission cooler and the torque converter.

Figure 16. Gear pump from an automatic transmission.

The inner gear of the pump hooks up to the housing of the torque converter, so it spins at the same speed as the engine. The outer gear is turned by the inner gear, and as the gears rotate, fluid is drawn up from the sump on one side of the crescent and forced out into the hydraulic system on the other side.

The Governor
The governor is a clever valve that tells the transmission how fast the car is going. It is connected to the output, so the faster the car moves, the faster the governor spins. Inside the governor is a springloaded valve that opens in proportion to how fast the governor is spinning -- the faster the governor spins, the more the valve opens. Fluid from the pump is fed to the governor through the output shaft.

The faster the car goes, the more the governor valve opens and the higher the pressure of the fluid it lets through.

Figure 17. The governor.

Throttle Valve or Modulator
To shift properly, the automatic transmission has to know how hard the engine is working. There are two different ways that this is done. Some cars have a simple cable linkage connected to a throttle valve in the transmission. The further the gas pedal is pressed, the more pressure is put on the throttle valve. Other cars use a vacuum modulator to apply pressure to the throttle valve. The modulator senses the manifold pressure, which drops when the engine is under a greater load.

Manual Valve
The manual valve is what the shift lever hooks up to. Depending on which gear is selected, the manual valve feeds hydraulic circuits that inhibit certain gears. For instance, if the shift lever is in third gear, it feeds a circuit that prevents overdrive from engaging.

Shift Valves
Shift valves supply hydraulic pressure to the clutches and bands to engage each gear. The valve body of the transmission contains several shift valves. The shift valve determines when to shift from one gear to the next. For instance, the 1 to 2 shift valve determines when to shift from first to second gear. The shift valve is pressurized with fluid from the governor on one side, and the throttle valve on the other. They are supplied with fluid by the pump, and they route that fluid to one of two circuits to control which gear the car runs in.

Figure 18. The shift circuit.

The shift valve will delay a shift if the car is accelerating quickly. If the car accelerates gently, the shift will occur at a lower speed. Let's discuss what happens when the car accelerates gently.

As car speed increases, the pressure from the governor builds. This forces the shift valve over until the first gear circuit is closed, and the second gear circuit opens. Since the car is accelerating at light throttle, the throttle valve does not apply much pressure against the shift valve.

When the car accelerates quickly, the throttle valve applies more pressure against the shift valve. This means that the pressure from the governor has to be higher (and therefore the vehicle speed has to be faster) before the shift valve moves over far enough to engage second gear.

Each shift valve responds to a particular pressure range; so when the car is going faster, the 2 to 3 shift valve will take over, because the pressure from the governor is high enough to trigger that valve.

Electronic Controls
Electronically controlled transmissions, which appear on some newer cars, still use hydraulics to actuate the clutches and bands, but each hydraulic circuit is controlled by an electric solenoid. This simplifies the plumbing on the transmission and allows for more advanced control schemes.

In the last section we saw some of the control strategies that mechanically controlled transmissions use. Electronically controlled transmissions have even more elaborate control schemes. In addition to monitoring vehicle speed and throttle position, the transmission controller can monitor the engine speed, if the brake pedal is being pressed, and even the anti-lock braking system.

Using this information and an advanced control strategy based on fuzzy logic -- a method of programming control systems using human-type reasoning -- electronically controlled transmissions can do things like:

• Downshift automatically when going downhill, to control speed and reduce wear on the brakes.
• Upshift when braking on a slippery surface to reduce the braking torque applied by the engine.
• Inhibit the upshift when going into a turn on a winding road.

Let's talk about that last feature -- inhibiting the upshift when going into a turn on a winding road. Let's say you're driving on an uphill, winding mountain road. When you are driving on the straight sections of the road, the transmission shifts into second gear to give you enough acceleration and hill-climbing power. When you come to a curve you slow down, taking your foot off the gas pedal and possibly applying the brake. Most transmissions will upshift to third gear, or even overdrive, when you take your foot off the gas. Then when you accelerate out of the curve, they will downshift again. But if you were driving a manual transmission car, you would probably leave the car in the same gear the whole time. Some automatic transmissions with advanced control systems can detect this situation after you have gone around a couple of the curves, and "learn" not to upshift again. - Source howstuffworks.com

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Table of Contents:

Introduction to How an Automatic Transmission Works
Planetary Gearsets & Gears
The Clutches and Bands
The Hydraulic System and How It Shifts

• Automatic Transmissions, how do they work?
• Torque Converters... What? What are those?
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