This is the second installment of our new Back to the Basics column. Last month we unveiled our vision for this column and how we hope to impact the training needs of our industry. Future installments will introduce more complex aspects, but we want to start at the most basic level. This article introduces the fundamentals of transmissions – features and functionality.
Whether you’re a veteran technician or new to our industry, you have something in common; you’re curious about how things work and why they work. Some would call this curiosity, and others would call it a fundamental necessity to becoming a great technician. In this issue of Back to Basics, we’ll journey into the most basic fundamentals of the transmission using some of today’s common transmissions as examples.
The transmission is the drivetrain component that transfers engine power, also known as torque, to the wheels to move the vehicle. It achieves this by providing multiple forward gear ratios and one reverse gear ratio. These gear ratios enable the engine to efficiently accelerate the vehicle, handle heavy loads, achieve a wide range of forward speeds, and reverse the vehicle.
The transmission is designed with different gear ratios because the vehicle requires substantial energy to get in motion, whereas maintaining the vehicle’s momentum at higher speeds requires less energy. By providing various gear ratios or torque multiplying ratios, the transmission provides optimal performance and fuel efficiency across all vehicle operating ranges.
The automatic transmission enables the vehicle to go into gear, move, slow down, and stop without stalling the engine, eliminating the necessity of a manual clutch pedal. The torque converter makes this possible, as it acts as a fluid coupling device, separating the engine’s torque from the transmission – similar to the clutch in manual transmissions.
The transmission gears can also function as brakes in certain gear ranges. Engine compression braking occurs by resisting the vehicle’s tendency to go faster than the engine speed allows. For instance, when going down a steep slope, the vehicle will attempt to accelerate without pressing the gas pedal. However, the transmission prevents the vehicle from coasting freely if the gear selector is in the correct Manual Gear range position to do so. Instead, the engine compression will slow down the vehicle, effectively providing a braking effect – engine braking.
The transmission delivers smooth acceleration while automatically selecting the optimal gear ratio to promote effortless towing heavy loads, and easier negotiation of hills and inclines like when you’re towing a trailer to your favorite camping spot. The transmission enables the vehicle to reverse its direction simply by moving the lever to the “R” position. Through the torque converter, it also allows the vehicle to stop with the engine running without disengaging the transmission or stalling the engine. As mentioned above, engine braking slows the vehicle from any speed or for descending hills safely without needing to overuse and overheat the vehicle’s brakes.
The benefits of an automatic transmission go beyond convenience for the driver. They also significantly contribute to a safer driving experience. For instance, eliminating the manually operated clutch allows for smoother vehicle operation. A major advantage is the ease of starting and stopping, even on hills. It automatically uses the appropriate gear ratio for the given driving condition without manually shifting. 
The Park (P) position provides a transmission lock to prevent the vehicle from rolling forward or backward. The driver can always keep both hands on the steering wheel, unlike with a manual transmission, which requires the driver to continuously work the manual shift lever to change gears. Another often overlooked benefit of an automatic transmission is that it makes it easier for drivers with physical limitations.
Now let’s look at some examples of transmissions you might already know about, at least from the driver’s seat. For instance, the Hydra-Matic automatic transmissions you might be accustomed to are the 4T and 4L series. We’ll talk about both the front-wheel and rear-wheel drive vehicle applications. While front-wheel and rear-wheel drive transmissions serve the same purpose, there are two key distinctions.
Firstly, the transmissions are mounted in the vehicle in distinctly different configurations relative to the engine. Secondly, the internal transmission designs are modified to adapt to the specific mounting position and the axle and wheels to which power is transferred – front or rear.
FRONT WHEEL DRIVE (FWD) “TRANSAXLE”
The Front-Wheel Drive (FWD) transmission (Figure 1) is connected to the engine’s flywheel at the rear of the engine, just like in a rear-wheel drive vehicle. However, the engine is mounted sideways (transversely) in the vehicle with FWD transmissions. This design allows for two drive axles – one connected to each front drive wheel, providing increased traction in wet and snowy conditions.
Unlike a Rear-Wheel Drive (RWD) transmission, the FWD design combines the differential and final drive assembly within the transmission case. This integration means that the FWD transmission is commonly referred to as a “Transaxle” because it functions as part of the drive axle.
REAR WHEEL DRIVE (RWD) “TRANSMISSION”
The RWD transmission (Figure 2) is positioned at the back of the engine and has a single output shaft that connects to the vehicle’s drive shaft. This drive shaft transfers power from the transmission to the rear axle and drive wheels through a differential at the rear of the vehicle. The RWD transmission design offers balanced weight distribution from the front to the rear of the vehicle, making it ideal for towing trailers.
The Hydra-Matic transmission and transaxle product designation system uses a combination of numbers and letters to indicate specific features. You can use this for almost every transmission in the world (Figure 3). For example, let’s look at the 4L80-E transmission. The first character in the name represents the number of forward gear ratios in the transmission. In this case, the “4” indicates four forward gear ratios.
The second character is a letter that indicates the transmission and engine mounting orientation. “L” signifies a longitudinally mounted transmission as in rear-wheel drive vehicles. “T” indicates a transversely mounted transmission as in front-wheel drive vehicles. If the letter “M” is also present, it means the unit is a manual transmission or transaxle and likewise, either longitudinally or transversely mounted.
The third and fourth characters signify the series designator, which indicates the transmission’s torque capacity. A higher number in the series indicates a greater torque capacity. For example, the “80” series has a higher torque capacity than the “40” or “60” series. The fifth character represents any key features incorporated in the transmission. For example, the letter “E” indicates the presence of electronic controls. This isn’t used as much with newer transmissions because all transmissions are now equipped with electronics.
Using this classification system, the Hydra-Matic 4L80-E is a 4-speed transmission longitudinally mounted (rear-wheel drive) and belongs to the 80 (high capacity) series. It also features electronic controls.
The Hydra-Matic automatic transmission offers seven gear range selections for flexible operation, with the driver selecting the desired range. However, three-speed transmissions have six positions on the shift quadrant due to the absence of an Overdrive position (Figure 4).
Overall, the Hydra-Matic automatic transmission offers a range of options to suit different driving conditions and optimize performance. The selector positions provide the following features.
- When in the Park position, the transmission prevents the vehicle from rolling and the engine can be started. However, it’s also important to engage the parking brake as an additional safety precaution. The Park position should only be selected when the vehicle is at a complete stop, as the output shaft is mechanically locked within the transmission case.
- The Reverse range allows the vehicle to operate in a backward direction.
- The Neutral position enables the engine to start and run without driving the vehicle, but it will still roll if the parking brake is not engaged.
- The Overdrive range offers maximum efficiency and fuel economy for normal driving conditions. However, it should not be used when pulling heavy loads or driving on hilly terrain.
- A lower manual gear selection is recommended for optimal efficiency in these situations.
- The Manual Third range provides three gear ratios and prevents the transmission from shifting into fourth gear. It is suitable for towing, driving on hilly terrain, or navigating congested city traffic. The automatic shifting is similar to the Overdrive range, and third gear also offers “Engine Compression Braking” to slow the vehicle when the throttle is released.
- The Manual Second range typically has two gear ratios, First and Second, and excludes Third and Fourth gear. It also provides “engine compression braking” for added control. In Manual First, the engine compression braking is maximized, and Second gear is available as a safety feature at higher speeds to protect the transmission.
Okay, so we covered the fundamentals of the modern era of automatic transmissions. As we go through these articles, we’ll bring you from the beginning to the present day and show you that you can fix darn near every transmission out there with a basic understanding of how a transmission works.
