What Controls the Shifting in an Automatic Transmission?

The invention of the automatic transmission (AT) system has improved our driving experience. The system has lessened the number of things a driver has to learn to drive the car. Modern versions of AT vehicles have significantly optimized fuel consumption. You no longer experience lugging engines. But what controls the shifting in an automatic transmission? How does the system know when to shift? Let’s answer these questions and more by first differentiating AT vehicles from manual transmission (MT) cars. Then let’s take a look at the different parts of the automatic transmission, and work our way to how mechanical engineers put these parts together. Finally, let’s understand what controls the system.

Difference Between Automatic Transmission and Manual Transmission

The first automobiles developed only had two forward-direction gears and one reverse gear. They had two forward-direction gears and one reverse gear. The vehicle’s engine and pedals work together to move the car. One of these pedals is called the clutch. The driver changes gears by stepping on the clutch pedal. Carmakers use this principle and improve their vehicles.

Difference Between Automatic Transmission and Manual Transmission

Meanwhile, AT vehicles change gears based on engine speed and data from different sensors. These cars typically have no clutch pedal. In addition, there is no gear shift. Once you put the transmission into drive, gear shifting becomes automatic.

The mechanical parts that make up this AT system are ingenious. Automobile engineers have spent decades perfecting these components. To understand this system better, let’s look at its parts.

Let’s Know: How To Clean Transmission?

The Parts of an Automatic Transmission System

When you break apart and peer inside an automatic transmission, you find different components laid down in a compact space. Some of the things you will see are:

  • The planetary gearset
  • Bands to lock parts of the gearset
  • A collection of wet-plate clutches that also lock other sections of the gearset
  • A hydraulic system that controls the bands and clutches
  • A powerful gear pump that moves transmission fluid around

The main component here is the planetary gear set. This gear set is about the size of an average melon. It creates all the different gear ratios the transmission can produce. All the other components help the planetary gearset perform its function.

The planetary gear set contains three primary components. These are the sun gear, the planet gears and carriers, and the ring gear. They can either be the input or the output in a transmission cycle. The exact configuration determines what gear ratio to use. This mechanical principle determines when to shift gears. But how does the system choose which arrangement to use? How does an automatic transmission select gears?

How Does an Automatic Transmission Know When to Shift?

Automatic transmission systems use a Transmission Control Module (TCM) to adjust speed, change gear, and engage the clutch. The TCM determines whether the clutch is engaged or disengaged, reads the vehicle’s speed, and executes an order based on the driver’s feedback and data from various sensors. Some TCM modules have an internal memory that stores and recalls driving habits and then apply that data to optimize your transmission’s performance.

What Controls the Shifting in an Automatic Transmission

Transmission Sensors and Modulators

The TCM makes its decisions based on sensor data from the engine and other vehicle parts. This wide array of sensors helps the TCM regulate your car’s transmission. Here is a list of the most important ones:

  • Vehicle Speed Sensor and Differential Speed Sensor: These components measure your car’s speed. The Throttle Position Sensor measures the position of the gas pedal (the driver’s feedback) and tells the TCM how open is the air intake valve, controlling the amount of air flowing into the engine.
  • Mass Airflow Sensor: This sensor measures how much air is flowing into the engine. The sensor uses this information to control air and fuel ratios and calculate engine load.
  • The Intake Air Temperature Sensor: It determines air temperature while entering the engine’s air intake valve. This process helps determine and control a car engine’s air/fuel ratio.
  • Powertrain Control Module (PCM): This component times the gear shift and determines when to engage the clutch. It works closely with the TCM to control the AT system.
  • The Transmission Range Sensor: This part tells the PCM the position of the shifter. The PCM uses this data to control which gears to engage or disengage.
  • Manifold Absolute Pressure Sensor: It measures manifold pressure, engine load, and altitude.

Now that we have learned about the different parts of an automatic transmission system and the various control modules and sensors that regulate gear ratios and shift timing, let’s put everything together and find out how all these components work side by side.

How Does an Automatic Transmission Select Gears?

Your car’s automatic transmission performs many tasks, and the components that make up the system operate in many ways. Here are some ways your automatic transmission and its parts control your vehicle:

  • The automatic transmission will select the gear if you turn on your car’s overdrive switch. This will be based on the speed of your vehicle and the position of the throttle/gas pedal.
  • When you step on your gas pedal and accelerate gently, your automatic transmission shifts gear during lower speeds.
  • When you floor the throttle pedal, your transmission shifts to the next lower gear
  • If you move the shift selector to a lower gear, your car’s transmission will shift down to a lower gear. However, it will first determine if your vehicle moves too fast for that gear. If your car is moving too fast, the transmission waits until sensors tell it that the car is slowing down and ready to downshift.
  • When you position the shift selector to second gear, your car’s transmission will never shift down to lower or higher gear. This is unless you move the shift selector to a different shift.

The Hydraulic System

The hydraulic system is considered the brain that manages all the functions above in a mechanical automatic transmission system. The passageways of the hydraulic system route fluid to all the different parts of the transmission. A gear pump in the transmission casing draws transmission fluid from a sump. The sump is a shared space where transmission fluid is stored. It then feeds it to the hydraulic system, transmission cooler, and torque converter. The transmission cooler acts as a heat exchanger. Here, transmission fluid flows and dissipates heat, cooling down the transmission fluid. The torque converter transmits torque or rotational force from the engine to a rotating load. The torque converter connects the power source (car engine) to the load (transmission, wheels).

During operation, the inner gear of the pump connects to the housing of the torque converter. It spins at the same speed as the car’s engine. This innermost gear then turns an outermost gear. As these two gears rotate, the gear pump draws transmission fluid from the sump into one side of the transmission cover. It then forces it into the hydraulic system from the other end.

Let’s study the other components that work closely with the hydraulic system: the governor and the shift circuit.

The Governor

The governor is a valve that tells the transmission how fast your vehicle moves. The valve connects to the output. In this case, the faster the car goes, the quicker the governor rotates. Inside the governor is a spring-triggered valve that opens proportionally to how fast the governor turns. The faster the governor rotates, the more this valve opens. Transmission fluid from the pump feeds to the governor through the output shaft. The more this governor valve opens, the higher the pressure of transmission fluid it allows through.

The Shift Circuit

The shift circuit is a set of components: speed, pressure, and other sensors shift selector, valves, clutches, and bands. These components make the transmission shift appropriately.

The automatic transmission needs to know how hard the engine is working for your car to shift correctly. Several sensors tell your car’s transmission system of this information, as discussed earlier. Some vehicles have cable links connected to a throttle valve and sensor. The more you step on the throttle/gas pedal, the more pressure it puts on the throttle valve. The throttle sensor measures this pressure. Other cars have a vacuum modulator that applies pressure to the throttle valve. The modulator detects and measures the manifold pressure, which increases when the engine increases its load.

Meanwhile, the shift selector hooks up to the manual valve. The manual valve feeds transmission fluid to the hydraulic circuits based on what gear is currently selected. It tells it what gears to use and inhibit. For example, suppose the shift selector is in third gear. In that case, the manual valve fills the circuit with only enough fluid and pressure that prevents the transmission from engaging the gears that will take the vehicle to its highest speed. This is also known as overdrive. But how does this circuit do this?

Shift Valves, Clutches, and Bands

Shift valves feed hydraulic pressure to the clutches and bands that engage each gear. The transmission system has several shift valves that determine when to shift from one gear to the next. For example, the two and three shift valves determine when to shift from second gear to third gear. Transmission fluid pressurizes the shift valve from the governor on one side and the throttle valve on the other. The gear pump supplies these fluids to the shift valve. These valves route that fluid to one of two circuits to determine which gear the car should run in.

Smooth Shifting

The shift valve delays shifting if the car is accelerating too fast. If the vehicle moves or accelerates gently, the gear shift happens at a slower speed. How does this happen?

Smooth gear Shifting

As your car’s speed increases, the pressure from the governor valve builds. This pressure forces the shift valve open until the first gear circuit is closed and the second gear circuit opens. Since the car is accelerating gently, the throttle valve does not apply too much pressure on the shift valve.

Meanwhile, what happens when your car accelerates too fast? The throttle valve applies more pressure against the shift valve. This is because you floored the gas pedal (driver’s feedback). Therefore the governor must have a higher pressure. Furthermore, your vehicle’s speed must be faster before the shift valve moves far enough to enable the second gear.

Each shift valve reacts to a specific pressure range. In this case, when the vehicle is moving faster, the two-to-three shift valve takes over because the pressure from the governor valve is high enough to push that valve.

Are Automatic Transmissions Computer Controlled?

Modern automatic transmissions use computers to identify the engine’s ideal operating points. Miniature computers act like control units that enable the complex operations of various types of transmissions. The control unit shifts transmissions automatically using programs stored in its chips. In many modern vehicles, the driver also can shift to another gear with switches on the steering wheel. This prevents shifting errors.

Other Types of Automatic Transmissions

Electronically-Controlled Transmissions (ECT)

ECTs still use hydraulics to regulate clutches and bands. However, this time, an electric solenoid controls each hydraulic circuit. This configuration simplifies the fluid routing in the transmission. It also enables a more advanced control scheme. Besides monitoring vehicle speed and throttle position, the transmission controller can now monitor engine speed. In some modern vehicles, the state of the anti-lock braking system. Using information from the advanced sensors and advanced control strategy based on fuzzy logic, ECTs can perform things like:

  • Automatically shift the transmission to a lower gear when the car moves downhill to control speed and reduce wear and tear on the brakes.
  • Shift the gear to a higher gear when braking on a slippery surface to reduce the braking torque that the car’s engine applies
  • Prevent shifting to a higher gear when you’re turning the car towards a winding road

Let’s further discuss that last feature. This is, preventing the upshift when your car turns on a winding road.

Suppose you are driving uphill on a winding elevated roadway. When you go on the linear sections of the route, the electronic transmission shifts to second gear. This gives you enough uphill-climbing power. You slow down when you reach a curve. Afterward, you take your foot off the throttle pedal and perhaps apply a gentle break. Your transmission will shift up to third gear when you take your foot off the gas pedal. Next, when you accelerate out of the curve, your transmission downshifts again to the second gear.

Transmission systems with more advanced control units capably detect and remember these situations after several repetitions and learn not to shift gears up again.

Continuously Variable Transmission (CVT)

CVT does not use gears like the standard automatic transmission, and it does not have any fixed shifting point, which removes shifting response delays. Using a CVT-enabled vehicle, you can accelerate your car without interruptions because the driving force is always available. The transmission does not move back and forth between fixed shifting points. This provides extra comfort for drivers and passengers.

Automated Manual Transmission (AMT)

Also known as robotized transmission, AMT combines the best features of automatic and manual transmissions. The clutch remains open when the car is idle and only uses energy when closed. This type of transmission, therefore, saves fuel and reduces CO2 emissions.


Automatic transmission employs mechanical and electronic-controlled systems to determine when to shift gears smoothly. This automated system comprises the planetary gear set, bands, and clutches that lock onto this gear set, a hydraulic system, and a powerful gear pump. A transmission control module, digital or purely mechanical, controls all these components. Various sensors and modulators measure different factors affecting the vehicle, the engine, and the transmission system. These include car speed, engine load, valve pressure, fluid temperature, etc. These sensors and modulators provide data to the control unit, which uses the information to select which gears to engage and at what speeds to engage the gears. With advanced miniature computers learning how to optimize engine performance and control the automatic transmission system, the driver and passenger will experience a smooth ride throughout.