What Does a Transmission Do?
Whether you drive a car with an automatic or manual transmission, the transmission's purpose is the same: convert rotational power from the engine into torque that spins the wheels. You can think of torque simply as a force that rotates or turns things.
Since engines can only maintain efficiency within a narrow window of rotations per minute (RPM), the transmission is needed to multiply engine power and direct it to the wheels. The transmission makes sure your engine stays at an optimal rotation while transferring the necessary torque to move your car across a wide range of speeds.
Transmissions accomplish this by using a variable set of gear ratios to significantly change torque output to the wheels from only narrow changes in engine rotation. Without a transmission, a car would be limited to only a single gear ratio, and the engine would nearly destroy itself trying to drive at high speeds.
In an automatic transmission, shifting gears is managed without driver input by a transmission control module (TCM) and torque converter. On manual transmissions, the driver must operate a clutch pedal and stick shift to change gears. Shifting gears in the transmission is what allows your car to increase or decrease speed while driving.
Overview of Automatic Transmission Parts & Fluid
Before getting into the specifics of how everything works, it's best to have a basic understanding of the different parts found inside a car's transmission. The transmission is usually located beside, behind, or partially beneath the engine on front-wheel-drive cars. It connects the engine to the rest of the drivetrain, which powers the wheels.
In this case, we cover only the main parts of an automatic transmission as it is by far the most common transmission type in the United States — in fact, only about 1.2% of cars sold in 2019 had manual transmissions.
Transmission Fluid
Transmission fluid is a type of oil found in both manual and automatic transmissions. Its job is to cool and lubricate the gears for seamless gear shifting. In automatic transmissions, transmission fluid is also the source of hydraulic pressure required to shift gears.
Torque Converter
The torque converter is the connecting point between the engine and transmission. Using hydraulic power provided by the transmission fluid, the torque converter performs two crucial tasks: it multiplies engine torque and transfers it to the transmission input shaft.
The torque converter also allows for smooth gear shifting. In order to shift gears, the transmission must temporarily disengage from the engine. The torque converter keeps the engine running while it's disconnected from the transmission. In this way, the torque converter performs the same function as the clutch on a manual transmission.
Oil Pump
The oil pump transfers transmission fluid from the oil pan into the valve body, where it is distributed throughout the rest of the system.
Planetary Gear Set
The planetary gear set is an example of the automatic transmission's incredible technological ingenuity and one of the components that makes it completely different from manual transmissions. Most automatic transmissions have multiple planetary gear sets that combine to create the various gear ratios needed to shift gears.
Each planetary gear set consists of a sun gear at the center, surrounded by three or more planetary gears and their carrier, all encapsulated within a ring gear. The torque output from the transmission will change depending on which components of each planetary gear set are used as the input gear, output gear, or are being held stationary.
Clutch Packs
A clutch pack is a series of compressible friction discs on the planetary gear sets. Clutch packs use oil pressure from the transmission fluid to compress by varying degrees. When the pack compresses, the clutch becomes engaged and either initiates movement in the gear set or holds it stationary.
Brake Band
The brake band works together with the clutch packs to lock certain parts of each planetary gear into place. The brake band is tightened or loosened as needed by the hydraulic pressure system.
Output Shaft
The output shaft is the last stop in the transmission before power is sent to the wheels. Once all of the internal mechanisms have switched to the appropriate gear ratios, the output shaft transfers the resulting torque to the driveshaft, which in turn sends power to the wheels.
Oil Pan
Similar to the oil pan beneath your engine, the transmission oil pan is a reservoir for excess transmission fluid. The oil pump draws fluid from the oil pan before sending it to the valve body.
Valve Body
The valve body is like the brain of your transmission. It uses various sensors and input from the transmission control module to regulate how much transmission fluid is sent to the system. The changes in hydraulic fluid pressure delivered by the valve body determine which parts of the transmission operate and when.
How Does an Automatic Transmission Work?
As soon as you put your Nissan in drive, the transmission engages. Putting the vehicle in drive signals to the transmission control module (TCM) that the vehicle is ready to move into first gear. In response, the TCM signals the oil pump to start transferring transmission fluid to the valve body. The valve body then directs the fluid to the necessary components. In this case, fluid is sent to the torque converter and clutch packs to shift into first gear.
The hydraulic pressure sent to the torque converter causes its turbine to spin, which spins the central shaft of the transmission. The rotating central shaft sets the first planetary gear set into motion. The output shaft then transfers this rotational energy from the single planetary gear set into the rest of the drivetrain and onto the wheels.
As you begin to pick up speed, the transmission will automatically shift into higher gears. To smoothly transition to other gears, the valve body redirects fluid away from the torque converter to prevent excessive force on the gears as they realign.
Once you've moved past first gear, the brake band or clutch packs will activate to either lock certain parts of each planetary gear set into place or put them in motion. The specific gear ratio within the transmission will depend on which parts of each planetary gear set (i.e. the sun gear, planetary gears, and ring gear) are stationary or moving.
The gear ratio is what determines the amount of torque that is then sent through the output shaft and eventually to the wheels. In higher gears, less torque is required to keep the car moving. In contrast, lower gears create a high torque to help the vehicle accelerate.
A transmission has many moving parts, and it can be difficult to wrap your head around the process. Luckily, the best part of an automatic transmission is that you don't have to think about how it works while driving — it just works! This is the biggest advantage automatic transmission vehicles have over manual transmissions.
No clutch pedal and no manual gear shift make automatic cars easier to learn and drive, especially in stop-and-go traffic or on hills. On the flip side, the many intricate parts in an automatic transmission could result in complicated and costly repairs.
How Does a Manual Transmission Work?
Compared to an automatic transmission, manual transmissions are quite simple. They typically have fewer parts, require less maintenance, and are usually more durable over a car's lifespan. In the past, manual transmission vehicles also had better fuel economy — and this idea is still pervasive today. However, as automatic transmissions have evolved to add more gears and rely less on the torque converter, many automatics have overtaken their manual counterparts in terms of fuel efficiency.
The basic principles of how each type of car transmission works are the same, but they use very different mechanisms to achieve torque conversion. For example, manual transmissions do not have planetary gear sets, but they still use the power of gear ratios to increase or decrease the amount of torque sent to the wheels.
Instead of using complex planetary gears, a manual transmission simply uses gear pairs equal to the number of gear speeds the vehicle is meant to drive in. Typically cars have between four and six gear speeds, though some high-performance vehicles have as many as eight.
Typically, a manual transmission comprises three shafts: the input, intermediate, and output shafts. The input shaft, which is connected to the engine, consists of a single clutch pack where an automatic transmission's torque converter would be. This clutch pack is what a driver engages and disengages with the clutch pedal when shifting gears.
When engaged, the input shaft rotates the intermediate shaft holding the drive gears. In turn, the drive gears on the intermediate shaft spin their corresponding driven gears on the output shaft. To achieve higher gears, hubs and sleeve gears on the output shaft move into place to engage new gears and gear ratios.
Transitioning between gears with a manual transmission is usually not as smooth as on an automatic transmission, but manual transmissions do allow for greater control of the car's speed and gear changes. While the ride may not be as smooth on a manual, the simpler mechanism means manual transmissions can be easier and less expensive to repair.
Signs of Transmission Failure
Typically, transmissions are pretty durable and can go a long time without needing major repairs. That being said, it's not uncommon to experience problems, especially if you've been neglecting routine transmission maintenance such as fluid exchanges. If you're experiencing any of these common transmission problems, it may be time to check the transmission fluid and bring your Nissan in for an inspection.
Transmission Fluid Leak
One of the most common causes of transmission trouble is low fluid levels from a leak. Without the proper amount of oil running throughout the system, your transmission is likely to overheat easily, wear down quickly, and perform inefficiently.
A leak can occur anywhere, but will most frequently be found in the oil pan. You can check if the leaking fluid is transmission oil by placing a piece of cardboard under the leak and checking its color. Transmission fluid is usually bright red in color, though it may be dark brown or black if burnt or excessively contaminated. Seek immediate repairs if this is the case.
Slipping Gears
Gears slipping without warning may signify that your Nissan's gears are significantly worn down and no longer meshing properly. While driving, you may notice your car lurching or changing speed as the gears slip. You may also be experiencing gear slippage if the vehicle suddenly feels underpowered, lacks acceleration, or unexpectedly enters neutral. Immediately bring your Nissan in for repairs, as this poses a serious safety risk.
Trouble Shifting Gears
Trouble shifting gears is a telltale sign of transmission trouble, but many things could be causing the issue. The problem may be mechanical, hydraulic, or even electrical. Still, if the gears feel clunky, make audible grinding or whining sounds, or are simply delayed in their shifting, it's best to have the transmission inspected.
Burning Smell
While many parts of your car could potentially cause a burning smell, the smell of burning transmission fluid is characteristically strong — closer to burning rubber than oil. A burning smell from your transmission likely means the transmission fluid is overheating.
Transmission fluid that hasn't been replaced in a long time becomes sludge-like and contaminated with metal debris from gears and other components, causing it to overheat. Not only does old transmission fluid like this lose its ability to cool and lubricate, but it can also wreak havoc on the internal parts of your transmission. This outcome can easily be avoided by routinely changing your transmission fluid.
Check Engine Light
The dreaded check engine light can indicate any number of problems with your vehicle. It may be something as simple as a loose gas cap or something more serious like transmission failure.
Either way, it's best to get the error code checked by a professional as a transmission sensor may be setting off the warning light, especially if you're experiencing any of the above symptoms at the same time.
How Long Does a Transmission Last?
An automatic transmission's lifespan varies according to many factors — namely, how much maintenance it receives, the type of transmission fluid it uses, how hot it gets, and the particular driving conditions it's used in. On average, you can expect an automatic transmission to last somewhere between 150,000 to 200,000 miles, or roughly seven years.
Well-maintained transmissions may last significantly longer, whereas people who don't service their transmissions or put their transmissions under heavy stress from rough driving or heavy towing can expect the components to wear out much sooner.
You can help your transmission last longer by regularly servicing it and practicing good driving habits. Start by changing the transmission fluid and filters approximately every 20,000 to 30,000 miles. Check your fluid levels at least once a month by pulling out the dipstick with the red handle on your transmission.
Consider using synthetic transmission fluid or even transmission cooler if your transmission tends to get hot from heavy use. Be sure to check your owner's manual for the recommended type of transmission fluid and maintenance intervals.
Finally, go easy on your transmission by practicing good driving habits such as letting the engine warm-up before driving and not shifting gears while in motion.
Keep Your Transmission Running Smoothly
Your Nissan's transmission is critical! Ensure it maintains its performance with genuine Nissan transmission and driveline parts. Shop genuine Nissan transmission parts online or at your nearest authorized service center.