Car Automatic Transmission Basics

A car automatic transmission executes gear changes automatically, so many drivers do not have a good sense of what this component is and what it does. But understanding how a car automatic transmission works can make you a better driver; give you a better appreciation of what your classic car can do; and lastly when something goes wrong with your vintage car’s automatic transmission you will have a basic understanding to trouble shoot your car.

Why Does a Car Need a Transmission?

The engine of your vehicle creates rotational power. To move the car, we need to transfer that rotational power to the wheels. That is what the car’s drivetrain — which the transmission is a part of — does.

But here is the problem: an engine can only spin within a certain speed to operate efficiently. If it spins too low, you would not be able to get the car moving from a standstill; if it spins too fast, the engine can self-destruct.

What we need is some way to multiply the power produced by the engine when it is needed (starting from a standstill, going up a hill, etc.), but also decrease the amount of power sent from the engine when it is not needed (going downhill, going really fast, slamming on the brakes).

Thus, you have the car transmission.

The transmission ensures that your engine spins at an optimal rate (neither too slow nor too fast) while simultaneously providing your wheels with the right amount of power they need to move and stop the car, no matter the situation you find yourself in. It sits between the engine and the rest of the drivetrain and sort of acts like a power switchboard for the car.

How Car Automatic Transmissions Works

When you drive a vehicle with an automatic transmission, putting the car in Drive activates the system. By pushing on the accelerator pedal, the engine’s crankshaft begins rotating more rapidly and this, in turn, leads to more pressure within the torque converter. As the car moves forward, the transmission’s hydraulics and electronics sense its speed in relation to engine speed, and the transmission changes gears automatically in response.

In the classic automatic transmission, hydraulics within the transmission sense those speed changes. That hydraulics then activate other hydraulics to change gears in the planetary gearset. Today, electronics supplement that sensing process, and the automatic transmission works in concert with the car’s other systems.

Parts of Car Automatic Transmission

So now, you should have a basic understanding of a transmission’s purpose: it ensures that your engine spins at an optimal rate (neither too slow nor too fast), while simultaneously providing your wheels with the right amount of power to move and stop the car, no matter the situation.

Let us look at the parts that allow this to happen in the case of the automatic transmission:

Key Components of Car Automatic Transmissions

An automatic transmission does something that is simple to describe in words. It changes gears without direction from the driver, aside from choosing Drive, Reverse, Neutral, and Park. But getting that to happen involves many components that must work in harmony in a variety of conditions over and over and over again. Here are the key elements:

Torque Converter

A torque converter was a fundamental breakthrough leading to the development of a practical automatic transmission.

A torque converter performs the function that a mechanical clutch does in a manual transmission. It enables the engine connection to and disconnection from the driveline as needed to facilitate the changing of gears.

The use of fluid dynamics can transfer torque from the engine when needed by the car to move forward, but it will essentially soak up that torque when moving forward is not desired.

This capability enables you to leave your car in Drive while you have your foot firmly on the brake when stopped at a stoplight. When you release the brake, the torque converter enables the car to creep forward.

Planetary Gears

If your car had a torque converter and a single forward gear, your forward speed would be severely limited by the engine’s operating range and torque production ability. That is why gear-changing is necessary.

An automatic transmission uses a different type of gearset than a manual transmission vehicle. It uses a set of planetary gears to create several gear ratios as the automatic transmission’s hydraulics controls them.

A planetary gear consists of three components:

A sun gear. Sits at the center of the planetary gear set.
The planet gears/pinions and their carrier. Three or four smaller gears that surround the sun gear and are in constant mesh with the sun gear. The planet gears (or pinions) are mounted and supported by the carrier. Each one of the planet gears spins on their own separate shafts that are connected to the carrier. Planet gears not only spin, but they also orbit the sun gear.
The ring gear. The ring gear is the outer gear and has internal teeth. The ring gear surrounds the rest of the gear set, and its teeth are in constant mesh with the planet gears.

A single planetary gear set can achieve reverse drive and five levels of forward drive. It all depends on which of the three components of the gear set is moving or held stationary.

Transmission Brake Bands and Clutches

Brake bands and clutches actuate the changing of gears in the planetary gearset. Brake bands tighten to hold a particular gear stationary or loosen to enable that gear to spin. And it is the combination of stationary and spinning gears in the gearset that produces each individual gear ratio.

The clutches in the transmission have a similar function, helping to determine the particular gear ratio the planetary gearset is creating at that moment. To go from one gear ratio to another involves the tightening and loosening of clutches and bands. This is directed by hydraulics, electronics, or a combination of both.

Car Automatic Transmissions: Hydraulics, Pumps and the Governor

Transmission Hydraulics

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.

You have probably seen something that looks like this 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 will discuss the key components of the hydraulic system; then we’ll see how they work together.

The Automatic Transmission 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.

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 spring-loaded 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.

Automatic Transmissions: Valves and Modulators

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 increases when the engine is under a greater load.

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 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.

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.

When You Put the Car in Park

It may seem like a simple thing to lock the transmission and keep it from spinning, but there are some complex requirements for this mechanism. First, you must be able to disengage it when the car is on a hill (the weight of the car is resting on the mechanism). Second, you must be able to engage the mechanism even if the lever does not line up with the gear. Third, once engaged, something must prevent the lever from popping up and disengaging.

The parking-brake mechanism engages the teeth on the output to hold the car still. This is the section of the transmission that hooks up to the drive shaft — so if this part cannot spin, the car can’t move.

Finally

That, broadly speaking, is how an automatic transmission works. There are sensors and valves that regulate and modify things, but that is the basic gist of the operation of a car automatic transmission. So, the next time you drive your car, you should be aware that millions of calculations go into the gear that your car’s automatic transmission chose at that moment. Your classic car is doing quite a bit of work every second to make driving much easier for you.