Automatic Transmission Operation
Introduction
Automatic transmissions have a fascinating history. One of the main reasons they were developed is because drivers wanted a way to operate a vehicle without the need to shift gears manually. The earliest designs were based on planetary gear sets—these traditional automatics were the first to offer self-shifting capability. From that point on, automatics became a system that many service professionals viewed with caution, largely because of their complexity compared to manual transmissions. Even so, automatics provide a clear advantage: they allow for smoother power delivery and higher towing capacity than manuals. The purpose of this chapter is to help you gain a solid understanding of planetary gear–type automatics so that you can confidently perform both basic maintenance and advanced repairs.
The engine and transmission work together as the core of the powertrain. They do not operate isolated or independent from each other. Today’s cars and trucks rely on multiple interconnected systems linked through vehicle communication networks. Anti-lock brakes, stability control, body controls, climate systems, and the powertrain all share information which makes transportation safer and more efficient for drivers. Because of this level of integration, diagnosing a problem is not always straightforward. An issue in one module or sensor often affects the performance of another system.
With that in mind, technicians must learn a priority order when dealing with diagnostic trouble codes (DTCs). Whenever a U code (communication fault) is present, it must be addressed before working on any P codes (powertrain issues). For example, if a scan reveals both a U1000 “loss of communication” code and a P0104 “MAP sensor voltage high,” the U code takes priority. Similarly, if both an engine performance code like P0104 and a transmission-related code such as P0741 (torque converter clutch failure) are found, the drivability issue should be repaired before addressing the transmission concern. In short, transmission-related DTCs are diagnosed only after U codes and drivability codes have been addressed.
As just indicated, network-related issues, identified as U codes, are always diagnosed first because they can cause a vehicle to behave in strange and unpredictable ways. When a sensor that shares information across the network fails, it can send incorrect data to multiple modules. This often leads to misleading symptoms and wasted time if the network problem isn’t corrected first. For this reason, U codes take top priority in diagnostics.
Next in order are engine performance codes, which must be resolved before looking at transmission codes. The transmission control module (TCM) depends on accurate signals from engine sensors to manage transmission shifting. If the engine itself is not running properly, the TCM cannot make correct decisions about shift timing or line pressure,which is essential for “good” shifts to occur. For example, if one cylinder is misfiring due to low compression or a failed ignition coil, the result is inconsistent intake manifold pressure. The MAP sensor sends irregular pressure signals to the TCM (transmission control module) which has a difficult time interpreting the unstable readings which causes erratic or abnormal shift patterns.
Consider a cylinder with a leaking intake or exhaust valve. Every compression stroke sends positive pressure pulses into the intake manifold. These distorted pressure signals confuse the TCM, which inturn respons by sending incorrect fluid pressure to the transmission’s clutches. The outcome is often harsh or poorly timed shifts, such as a jarring 1–2 upshift.
This is why transmission DTCs are third in priority. Until the network and engine systems are delivering reliable information, a real transmission issue cannot be properly diagnosed.
Fail-safe mode
Automatic transmissions are designed with a back-up operating strategy called fail-safe mode, sometimes referred to as “limp mode.” When this happens, the transmission is limited to just one forward gear and reverse. Fail-safe is triggered by two main conditions:
Loss of power to the TCM.
On many automatics, if the transmission control module loses electrical power, the transmission reverts to hydraulic control only. In this state, shifting stops, leaving the transmission “stuck” in a single forward gear. The actual gear varies by design. Some transmissions default to to 2nd, 3rd, 4th, or even 6th gear depending on how the transmission was engineered. Creating this default gear is a challenge for engineers, since it has to balance drivability with the limits of hydraulic control. Drivers often describe the vehicle as sluggish or low on power when fail safe occurs.Protective default triggered by the TCM.
Sometimes the TCM detects a problem that could cause damage to the transmission, such as a clutch control issue. In these cases, the module deliberately places the system into fail-safe mode to protect internal components. Transmission and software engineers work together to predict these failure scenarios and program the module to respond in a way that avoids further damage.
The TCM itself manages the solenoids that control hydraulic fluid flow to the transmission’s clutches. These clutches either hold or drive members of the planetary gear sets, which is how gear ratios are created. To make accurate decisions, the TCM relies on some critical sensor inputs.
The three high-priority inputs are:
MAP (manifold absolute pressure) or MAF (mass air flow/volume).
Vehicles may use one or both of these two sensors. Either way, the goal is to determine intake manifold pressure and airflow, which reflects engine load. Keep in mind that many service manuals use the term “load” as a term for “intake manifold pressure.”WSS (wheel speed sensor) or VSS (vehicle speed sensor).
These sensors, report vehicle speed to the network. A VSS was usually located on the transmission case near the output shaft or near one of the front CV axle shafts on a FWD vehicle. WSS or wheel speed sensors are more the standard since about 2010 model year since all vehicles today have stability control / traction control and ABS systems. As you likely learned when you studied ABS systems there is a WSS at each wheel on today’s vehicles.
Also referred to as the APP (accelerator pedal position sensor) in some vehicles, this input tells the TCM the exact throttle angle. With this information, the TCM calculates how much hydraulic pressure to apply to the clutches. A sudden throttle increase also signals the TCM to downshift for better acceleration. In addition, TPS/APP values help determine when to apply or release the torque converter clutch (TCC) inside the converter.
Other shift Inputs
Think of shift timing like riding a bike; you don’t move into a higher gear until your speed is up and the load on your legs feels lighter. Things are much the same for the TCM; it watches vehicle speed and engine load to decide the right moment to shift the transmission.
Secondary / other shift inputs don’t directly trigger shifts but they fine-tune how and when shifts occur:
CKP (crank position): Helps calculate shift timing and controls TCC apply/release by reporting engine RPM.
ECT & IAT (engine coolant temp & intake air temp): Prevent TCC engagement and delay shift timing until the engine and transmission are warmed up, according to factory programming.
TFT (transmission fluid temp): This impacts both TCC application and shift timing. Best performance happens once the fluid is warm.
Transmission Inputs as stated from various car manufacturers
To help understand transmission inputs more fully, it is helpful to look at statements from two different manufacturers’ technical training publications. The first statement comes from the Hydra-Matic 6 Speed RWD Technician’s Guide, produced by General Motors:
“Electrical signals from various sensors provide information to the TCM about vehicle speed, throttle position, engine coolant temperature, transmission fluid temperature, gear range selector position, engine speed, converter turbine speed, engine load and operating mode.”
In other words, the TCM uses this information to determine the precise moment to either upshift or downshift and how much fluid pressure to send to the hydraulic clutches. These inputs also control when to apply and when to release the torque converter clutch (TCC).
The second statement is from a Mercedes transmission publication, the Mercedes 722.9 or NAG2 (New Automatic Gearbox, generation 2):
“Primary transmission inputs are:
Engine RPM
Engine load
Throttle position
Vehicle speed
Coolant temperature
Shift mode or shift lever position”
Even though the wording is slightly different, these two statements show us that GM and Mercedes rely on essentially the same sensors, just like most other manufacturers do.
Inputs Review
To review, both of the manufacturers listed above require the following inputs:
Engine load – determined by a MAP or MAF sensor.
VSS (Vehicle Speed Sensor) – tells the TCM how fast the vehicle is moving.
TPS (Throttle Position Sensor) – tells the TCM throttle angle and acceleration demand.
CKP (Crankshaft Position Sensor) – provides engine RPM information.
TRS (Transmission Range Selector) – communicates gear range selection.
Fluid temperatures – both transmission and engine coolant.
With these inputs, the TCM can calculate shift timing, hydraulic pressure, and torque converter clutch operation.
TRS – Transmission Range Switch
The TRS is a particularly important input. This switch allows the starter motor to engage only in Park or Neutral. It also turns on the reverse lights when Reverse is selected and tells the TCM to direct hydraulic fluid to the reverse clutches. When Drive is selected, the TRS informs the TCM so it can apply the correct clutches for 1st gear and prepare for all transmission upshifts.
When the selector is moved to Manual 1st, the TRS tells the TCM to hold the transmission in 1st gear only. However, if the engine RPM becomes dangerously high, the TCM will override and forces a shift into 2nd to protect the engine. Manual gear selection is especially useful when descending long hills because it provides engine braking, reducing the need to rely solely on the brakes.
Other Names for TRS
Manufacturers often use different terms for similar components:
TRS – Transmission Range Switch (Ford)
MPLS – Multi Lever Position Switch (GM)
Manual Shift Shaft Position Switch (GM)
Inhibitor Switch (Subaru)
Ford uses the TRS designation for its 10R80 transmission. This switch is located inside the transmission rather than outside. The TRS allows starter motor engagement in Park/Neutral, activates reverse lights, and signals gear range selection to the TCM.
Transmission Models
Automatic transmissions and transaxles are identified by model numbers. These designations tell us about speed count, drive type, and torque handling capability.
GM Turbo Hydra-Matic 6L90
The “6” means 6 forward speeds.
“L” indicates a longitudinal / rear-wheel-drive design.
“90” refers to a torque rating of 900 Newton-meters, or about 660 lb-ft of torque.
This rating tells us how much torque the transmission can reliably handle from the engine.
Ford 8F35
An 8-speed front-wheel-drive transaxle.
Designed to handle 350 Newton-meters of torque.
ZF 8HP
A highly popular 8-speed transmission used by Chrysler, Ram, Jaguar, Jeep, BMW, Audi, Volkswagen, Land Rover, and others.
“8HP” stands for 8 speed, Hydraulic converter, Planetary gear set.
ZF is short for Zahnradfabrik Friedrichshafen, which translates from German as Cogwheel Factory.
Chrysler 68RFE
“6” = 6 forward speeds.
“8” = relative torque rating of 8 (on a 0–9 scale, though exact torque values aren’t always published).
“R” = rear-wheel drive.
“FE” = Fully Electronic.
Commonly paired with the 6.7L Cummins diesel in Ram 2500 trucks (2007–2021).
Why More Speeds?
Since 2016, the industry standard has moved toward 8, 9, and 10-speed automatics. The reason is simple: more forward ranges allow the engine to stay in its most efficient RPM band. This improves fuel economy, performance, and emissions.
The more gears you have, the easier it is to select the right gear for the terrain and riding conditions. In a vehicle, more gears allow smoother acceleration, lower highway RPMs, and better use of the engine’s power curve.
While CVTs (Continuously Variable Transmissions) accomplish this task more effectively because they offer infinite ratios, CVTs’ cannot match the towing strength and durability of traditional automatics. That’s why manufacturers continue to invest heavily in multi-speed planetary gear based automatics.
Vehicle Platform
Let’s take a closer look at what is meant by a transmission/vehicle platform.
A front-wheel drive (FWD) platform is where the engine and transaxle sit sideways, or transverse, in the vehicle. FWD transmissions are typically called transaxles. A transaxle is a compact design that combines both the transmission and the differential in one housing. This makes them ideal for mid-size and small vehicles. In recent years the other transaxle designs such as CVTs (continuously variable transaxles) and DCTs (dual-clutch transaxles) are popular is smaller vehicles.
A rear-wheel drive (RWD) platform, on the other hand, is found in most light-duty pickup trucks and large SUV’s. Here, the transmission sits longitudinally, meaning its in line with the vehicle’s drive wheels. In this setup, the transmission and differential are often separate units. This layout is common for trucks and larger vehicle SUV’s where towing and durability are a priority.
One important misconception is that transmissions somehow produce power. This isn’t the case. An engine produces power while a transmission provides a way to deliver that power to the drive wheels in a controlled and efficient manner. Gear ratios are what make this possible. Let’s look at ratios from a GM 6L90 transmission.
Low gears such as 1st and 2nd are high in ratio, which means they provide more torque for strong acceleration. A typical 1st gear ratio is about 6:1.
Higher gears such as 4th, 5th, and 6th are lower in ratio (1.15:1, 0.85:1, and 0.667:1). These gears offer less torque but enable vehicle speed and efficiency.
Remember: transmissions do not add power. In fact, they actually consume a small amount of engine power due to friction and how torque flows through the gear sets.
Finally, let’s touch on transfer gears. Some transmissions, like the one pictured in the figure below, use transfer gears to redirect the flow of power. The three gears at the rear of the unit are all transfer gears. Here’s how they work:
The top gear connects directly to the main shaft of the transmission.
The middle gear transfers that power downward.
The bottom gear sends power to the final drive unit, which delivers it to the wheels.
Components of an Automatic Transmission
Let’s begin with the torque converter. This component is located between the transmission and the engine, inside an area called the bell housing. A torque converter is a fluid coupling (not a direct mechanical connection) and it contains four main elements: the impeller, turbine, stator, and clutch (see Figure 11). The torque converter is the device that engages and disengages engine power to the transmission based on engine rpm. It ensures that engine power is delivered to the transmission smoothly and effectively.
A torque converter works much like the demonstration in Figure 12. Imagine two fans facing each other. One is plugged in and powered, while the other spins only from the air moved by the first fan.
The powered fan represents the impeller, which is bolted directly to the engine’s crankshaft.
The unpowered fan represents the turbine, which receives force from the fluid moved by the impeller.
Other major transmission components include:
Transmission case – Houses all the gears, clutches, valve body, and some of the hydraulic passages. Usually made of aluminum, it also includes the bell housing where the torque converter sits.
Transmission pump – Pressurizes transmission fluid, which is essential for both lubrication and operation. It supplies pressurized fluid to the torque converter, valve body, lubrication circuit, and multi-disc clutches.
Gears/gear sets – Automatic transmissions use planetary gears, which are always in constant mesh. This design is why it is impossible to “grind gears” in an automatic — they never disengage.
Clutches and Bands
Clutches and bands are the mechanical devices that control which gear ratios are achieved.
Clutch drums and bands (see Figure 19 and Figure 20) serve two purposes:
They house the clutch plates and piston.
They provide a surface for a band to apply against.
For example, in a 4L60E transmission:
When the band holds the drum stationary while the clutch plates are released → 4th gear is engaged.
When the clutch pack applies and the band releases → reverse is engaged.
Modern transmissions (6-speed and higher) rarely use bands. Instead, they use clutch-to-clutch shifting, which is easier to program and provides greater durability compared to bands.
Bands act as holding or braking devices, keeping a planetary gear member stationary.
Gear Ratio Terms
Gear ratios were explained in a previous chapter, but let’s dive deeper into some gear ratio terminology. The first term is underdrive. Underdrive gear ratios are low gear/high ratio and are used to help get the vehicle moving from a stop or from a slow speed. The next gear term is direct drive. This is simply a ratio of 1:1. Keep in mind that as a transmission shifts from a lower gear to the next higher gear, vehicle speed increases, but as speed increases torque to the wheels decreases. This is a key principle: as vehicle speed increases, less torque is available to the drive wheels, because the drive and the driven gears in the transmission become more similar in size, thus, less leverage is needed to keep the vehicle in motion once it is in motion.
As higher gears are selected, gear ratios are reduced. Referring to Figure 21, first gear has the highest ratio of 4.02:1. Third gear on the other hand is a higher gear but is low ratio – 1.53:1. In the case of direct, the drive and driven gears would be identical in size and the ratio would be 1:1. Quite often the 4th gear in many six speed transmissions is direct drive. Figure 21 doesn’t offer a true direct drive ratio, but 1.15:1 is very close.
Overdrive is the third term to describe. It means output speed is more than input speed. This six-speed transmission offers two overdrive gears, .85:1 and .67:1 respectively.
Planetary Gears Operation
For a planetary gear set to produce a specific outcome, two conditions must always occur:
One gear member must serve as the input (driving member).
Another gear member must be held stationary.
This combination forces the third gear member to act as the output, creating a specific gear ratio.
This process is what allows transmissions to provide multiple gear ratios and smooth shifting.
A primary operating principle with planetary gear sets is that in order to have a power outcome from the gear set, one gear has to be a drive/input member, a second gear has to be held in place (a reactionary member), while the third gearset member is the power output gear.
There are eight possible outcomes relating to planetary gear set operation. The outcomes are:
Neutral – This is when only one of the principles stated above occurs. In other words, one gear is a driving member but there is not a gear that is held stationary. Therefore, there is no power output.
Reverse – This occurs whenever the carrier is held stationary and either the sun or the ring is a power input member. Regardless of car manufacturer, the key to achieving reverse is when the carrier is held and when one of the other gears is the power input gear. This forces the third gearset member to act as the power output. There are two possible reverse ratios but only one is a realistic outcome.
Here’s why:
Let’s suggest that the sun gear has 39 teeth, and the ring gear has 71 teeth. The gears/teeth on the planetary carrier do not have to be counted because they will simply function as power transfer gears. Reverse is calculated as if two gears are in mesh with each other, much like what occurs in a manual transmission. The formula to count R-reverse is:
Driven ÷ Drive = ratio
When the sun is the drive gear and the ring is driven: 71 ÷ 39 = 1.82:1 ratio, which is a realistic speed reduction ratio.
When the ring gear is the drive gear and the sun gear is the driven gear: 39 ÷ 71 = .55:1 ratio.
A ratio of .55:1 is an overdrive ratio and is not useful in a real-world application since moving a vehicle from a stop requires good torque—not high speed.
The point is there are two different reverse possibilities, but only one is realistic.
Underdrive
Underdrive gear ratios produce good torque but not high speed. As stated earlier, low gears like 1st, 2nd, and 3rd are higher in ratio. However, to calculate underdrive ratios with planetary gear sets, a different formula is used:
Underdrive = (# of teeth on the SUN gear + # of teeth on the RING gear) ÷ (# of teeth on the DRIVE gear)
Example:
Sun gear = 39 teeth
Ring gear = 71 teeth
The planetary gear tooth count is handled in a different manner when it comes to calculating underdrive and overdrive ratios. Just like any planetary gear ratio outcome, there is always an input gear and a held gear.
The key to underdrive is that the carrier is the output member. To achieve this, we’ll use the following arrangement in our example:
Sun gear = drive member
Ring gear = held member
Carrier = output
Calculation:
39 + 71 = 110 ÷ 39 = 2.82:1 ratio
This is a realistic underdrive outcome, providing strong torque for moving the vehicle from a stop.
Another Underdrive Outcome
There is another possible underdrive gear ratio. This occurs when the ring gear is the drive member and the sun gear is the held member. Again, the key is that the carrier remains the output member.
Formula:
Underdrive = (# of teeth on SUN gear + # of teeth on RING gear) ÷ (# of teeth on DRIVE gear)
Example:
Sun gear = 39 teeth
Ring gear = 71 teeth
Ring gear = drive member
Sun gear = held member
Carrier = output member
Calculation:
39 + 71 = 110 ÷ 71 = 1.55:1 ratio
This is a valid underdrive ratio, but compared to the earlier 2.82:1, it provides less torque multiplication and more speed.
Try this video:
If this is all a bit difficult to understand, try this video. It explains these principles visually:
Planetary Gear Calculation and Operation
Planetary Gear Calculation and Operation
Both of these calculations (2.82:1 and 1.55:1) are realistic ratios. But remember, by combining other ratios the steps or shifts from one gear to the next can be refined, which enables smoother outcomes than relying on only a single gear set. This is why modern transmissions often use three or four planetary gear sets, so they can work together to create better shifting events.
Direct Drive/ This one is simple. Direct drive occurs when any two of the three gears are locked together. One of those locked members also serves as the input member. With direct drive, no member is held stationary. Instead, the entire gearset rotates as a single unit, which results in a ratio of 1:1.
Overdrive. As with reverse and underdrive, there are two possible overdrive outcomes. The key to overdrive is that the carrier is the input member.
Formula:
Overdrive = (# of teeth on DRIVEN gear) ÷ (# of teeth on SUN gear + # of teeth on RING gear)
Example 1:
Carrier = input
Sun gear = held
Ring gear = driven
Calculation:
71 ÷ 110 = 0.65:1 ratio
This ratio is considered overdrive because the output speed (ring gear) is greater than the input speed (carrier).
The other overdrive possibility is where the ring gear is held, the carrier is the power input, and the sun is the driven member.
Calculation:
39 ÷ 110 = 0.35:1 ratio
In regard to these two overdrive ratios, 0.65:1 is realistic, but 0.35:1 is not. As stated earlier, this is why more than one gear set is necessary in order to work together in order to achieve reasonable gear ratios. What will be shown next is how two or more planetary gear sets interact with each other in order to achieve useful gear-ratio outcomes. If you didn’t watch this video on a previous slide, here is the link once again. It will help you visualize a bit more how gearsets interact with each other in order to achieve final outcomes.
Planetary Gear Calculation and Operation
Recap of the Eight Planetary Outcomes
Neutral is when no gear is held, but there is a power input gear OR vice versa.
Reverse occurs whenever the carrier is held and when either of the other two gears is the power input gear. Also, there are two possible outcome ratios, but only one of these ratios is realistic.
Underdrive also has two possible outcomes. The key to underdrive ratios is to have the carrier as the output member.
Direct drive has one outcome possibility which occurs when any two of the three gears are locked together.
Overdrive is the result whenever the carrier—which is considered the largest of the three gears—is the power input gear. It also has two possible outcomes.
Gearset Configurations
One last thing about gearsets is that there are various configurations. In the automotive world, the following are the most common gearset designs used:
Simple. As figure 27 shows, there is one sun, one set of planets, and one ring gear. This design is commonly used in 8, 9, and 10-speed transmissions.
- Simpson. As shown in Figure 28, there is one common sun gear, two sets of planets, and two ring gears. This type was used in the three-speed transmission era.
- Ravigneaux. As displayed in Figure 29, this design has two planet sets that mesh with each other, two sun gears of different sizes, and one common ring gear. Ravigneaux gearsets are found in some 4-speed but are quite prevalent in 6-speed units.
- Lepelletier. As shown in figure 30, Lepelletier gear designs are a combination of both Simple and Ravigneaux. They are used in many 6-speed transmissions.
Gearset design enables transmission engineers to obtain desired gear ratio results. Simpson gear sets were popular in 3-speed transmissions several years ago. Today 6, 8, 9, and 10-speed transmissions utilize Simple, Ravigneaux, and Lapellitier designs.
Drive vs. Hold Members
So, how is it that a planetary gear member is either a DRIVING or a HOLDING member? This depends on the clutch assembly’s design.
Video: AT Clutches and Bands
To recap the video, clutches can be used to either drive or hold a gear, while a band can only be used to hold a gear set member.
Transmission Fluid
Let’s now focus on transmission fluid. Why the term fluid instead of oil? Transmission oil is considered a hydraulic fluid since is used to propel the vehicle. Transmission fluid serves several purposes. It hydraulically applies clutches and bands, lubricates gears and bearings, removes heat, and, most importantly, propels the vehicle by conveying fluid force from the impeller to the turbine inside the torque converter.
Automotive service professionals need to be keenly aware that there are several different types of automatic transmission fluid. The primary factor in determining which specific fluid is used in a transmission is the clutch lining material. Transmission fluid must be engineered to be compatible with those linings so that the friction material grips the steel plates properly. The friction coefficient of both the smooth steel plates and the linings, along with the fluid’s additives and viscosity, are huge factors in achieving smooth transmission shifting. Because manufacturers frequently introduce new or improved lining materials, fluid specifications also change.
Another consideration for smooth shifting is how the shift solenoids pulse width modulate (PWM) fluid to the clutch assemblies. Automatic transmissions operate according to three principles: mechanical, hydraulic, and electrical. Clutches are applied hydraulically by electronic control, while the gears themselves are mechanical. The fluid is pressurized by the pump and directed through passages to engage various clutches thus, providing gear ratios.
Low Fluid Levels
When fluid levels are too low, hydraulic components such as clutches and bands cannot apply adequately. In addition, planetary gears and bearings are starved of lubrication and cooling. Since the same fluid that engages clutches, lubricates and cools components, low fluid levels will quickly lead to damage.
Low fluid leads to overheating. Excessive temperatures break down the fluid and deteriorate its additives. This prevents smooth clutch engagement. This is the reason why a fluid flush occasionally makes sense. Overheated and low fluid levels allow clutches to slip and burn, which leads to transmission replacement.
Aeration: Too Low or Too High
Transmission fluid levels need to be rather precise, some say within ½ quart (1 pint / 0.48 liter) of the full line when checked. Otherwise, aeration can occur.
Low fluid levels allow air to mix in, creating foam. Air is compressible so clutches cannot apply with full force, causing slippage and overheating.
High fluid levels allow the gears and drums to contact the fluid, whipping air into it and creating the same problem - aeration.
Aerated fluid runs hotter, turns dark, and loses its ability to lubricate, transfer force, and carry heat away. Overfilling is just as harmful as underfilling.
Fluid Service and Debate
The question of when to replace transmission fluid has long been debated. For years, the common practice was to flush or replace fluid every 30,000–50,000 miles. Many service professionals still follow this interval.
However, some manufacturers claim their transmissions are “sealed” or “filled for life.” This is misleading. Transmission fluid can always be checked and serviced on any transmission. Here are some things to consider about exchanging transmission fluid.
It is true that today’s fluids are more advanced and last longer, especially under normal driving conditions.
However, overheated fluid loses key properties and changes color, reducing its ability to lubricate and apply clutches smoothly.
Dirty or overheated fluid causes clutch lining deterioration. This creates debris to contaminate the fluid. Thus dirty circulates through the transmission, accelerating wear to components.
Clutches and Diagnostics
Modern automatic transmissions use both hydraulic multi-disc clutches and mechanical clutches. The focus of this section is the three types of mechanical one-way clutches (OWCs), as well as a fourth type of mechanical clutch called the dog clutch.
Most automatic transmissions include at least one OWC. These clutches are compact and, more importantly, they allow a vehicle to coast during deceleration. The three types of OWCs are roller, sprag, and diode. The dog clutch, which works differently, is only found in one model of transmission, the ZF 9HP. If you’d like to see these clutches in action, check out the One Way Clutches and Dog Clutch Demo videos linked on this page.
When it comes to diagnosing transmission problems, one of the most useful tools is the application chart. Figure 34 shows an example: a clutch application chart for the GM 8L90. Each transmission model has a similar chart. These charts show which clutches are applied in each gear range, making it possible to trace symptoms back to the failing clutch.
For example, if the 2-3-4-6-8 clutch fails, the transmission will only have 1st and Reverse, and the driver will experience engine flare (the engine revs but the car doesn’t move) when trying to shift into 2nd gear. In other words, the transmission “neutrals out” during the shift. Along with this, the transmission control module (TCM) would store a “gear ratio error” diagnostic trouble code (DTC).
| Range | Gear | 1-3-5-6-7 Clutch | 4-5-6-7-8 Reverse Clutch | 2-3-4-6-8 Clutch | 1-2-7-8 Reverse Clutch | 1-2-3-4-5 |
| Park | P | Applied** | Applied** | |||
| Reverse | R | Applied | Applied | Applied | ||
| Neutral | N | Applied** | Applied** | |||
| Drive | 1st | Applied | Applied | Applied | ||
| 2nd | Applied | Applied | Applied | |||
| 3rd | Applied | Applied | Applied | |||
| 4th | Applied | Applied | Applied | |||
| 5th | Applied | Applied | Applied | |||
| 6th | Applied | Applied | Applied | |||
| 7th | Applied | Applied | Applied | |||
| 8th | Applied | Applied | Applied |
Figure 34
Looking at the chart again, notice that both the 1-2-7-8-Reverse and 1-2-3-4-5-Reverse clutches are applied in Park. This is because when the driver selects Drive or Reverse, a third clutch must also apply to complete the gear range. If Drive is chosen, the 1-3-5-6-7 clutch engages. If Reverse is chosen, the 4-5-6-7-8-Reverse clutch engages along with the other two.
The key point is that every gear requires three clutches to be applied for the transmission to deliver power. If any one clutch fails, certain gears will not work. For example, if the 1-2-7-8-Reverse clutch fails, there will be no movement in Drive or Reverse and DTCs will be stored. Another example: for 3rd gear to function, every clutch with a “3” in its name must be working — the 1-3-5-6-7, 2-3-4-6-8, and 1-2-3-4-5-Reverse clutches.
Solenoid Charts
Another diagnostic chart used to diagnose transmission issues is a solenoid application chart (see Figure 35). It works the same way as a clutch chart but focuses on solenoids instead of clutches. If symptoms don’t match the clutch chart, checking the solenoid chart is the next step.
Let’s look at two examples using the 8L90 solenoid chart:
If S9 (solenoid 9) fails, the vehicle will lose 6th, 7th, and 8th gears, but all other ranges, including Reverse, will still work.
If S5 fails, the vehicle will lose Reverse and only 1st, 2nd, and 3rd will operate.
As always, proper diagnosis also includes checking DTCs, looking for technical service bulletins (TSBs), and performing a visual inspection.
| Range | Gear | S1 (NH) 1-2-7-8 Reverse | S2 (NL) 1-2-3-4-5 Reverse | S3 (NL) 1-3-5-6-7 | S4 (NH) 2-3-4-6-8 | S5 (NH) 4-5-6-7-8 Reverse | S7 (NL) TCC | S8 (NC) Default Control | S9 (NC) 1-2-3-4-5 |
| Park | P | On | On | Off | Off | Off | Off | Off | Off |
| Reverse | R | On | On | Off | Off | On | Off | Off | Off |
| Neutral | N | On | On | Off | Off | Off | Off | Off | Off |
| Drive | 1st | On | On | On | Off | Off | On° | Off/On | Off |
| 2nd | On | On | Off | On | Off | On° | Off/On | Off | |
| 3rd | Off | On | On | On | Off | On° | Off* | Off | |
| 4th | Off | On | Off | On | On | On° | Off* | Off | |
| 5th | Off | On | On | Off | On | On° | Off* | Off | |
| 6th | Off | Off | On | On | On | On° | Off* | On | |
| 7th | On | Off | On | Off | On | On° | Off* | On | |
| 8th | On | Off | Off | On | On | On° | Off* | On |
NOTE: Off = Solenoid control port not pressurized.
Off/On = soleinoid control port is not pressurized at low speed in 1st gear, solenoid control port is pressurized at high speed in 1st gear.
On/Off = soleinoid control port is pressurized at low speed in 2nd gear, solenoid control port is not pressurized at high speed in 2nd gear.
* = Default valve is hydraulically latched in the stroked position in this state, default solenoid can be commanded on for lube override.
° = The TCC may apply from 1st through 8th gear depending upon shift condition determined by the TCM
Failsafe (Default) Mode
Most modern transmissions also have a failsafe mode (sometimes called “default mode” or “limp mode”). This mode protects the transmission by limiting operation when the TCM detects a serious fault or loses power. In failsafe, the transmission usually only allows one forward gear plus Reverse.
On the GM 8L90, the failsafe gear is 2nd gear. If the transmission is stuck in one gear and won’t upshift but still has Reverse, it’s in failsafe.
On the Dodge Ram 68RFE, failsafe is called “Limp-in mode.” In Drive, the transmission defaults to 4th gear. If the driver manually selects “2” or “L,” the transmission stays in 2nd gear, allowing the vehicle to limp home at reasonable speed.
On the Ford 10R80 and GM 10L80, there is no failsafe mode at all. If something serious goes wrong, the vehicle has no forward gears and no Reverse — it must be towed.
On the ZF 8HP, the default depends on the vehicle’s state when power is lost. If the failure happens in Drive, the transmission defaults to 6th gear. If the failure happens in Park, Reverse, or Neutral, the transmission defaults to Park and stays there.
Leaks
Just like engines, transmissions are very sensitive to leaks, and even small leaks can cause big problems if ignored. The most common leak points are the pan gasket, axle seals or drive line seals, and the cooler lines (the lines that carry transmission fluid to and from the cooler).
One important rule: avoid using stop-leak additives. These products don’t actually fix leaks. Instead, they introduce debris into the transmission fluid, which can clog small passageways and shorten the life of the transmission. The same is true for radiator stop-leak products — they provide no lasting repair. The only real solution is to fix the leak properly and as soon as possible. Putting off a leak is risky; in fact, neglecting leaks is one of the top reasons of both transmission and engine failure. Preventative maintenance is always cheaper than waiting until something overheats/fails.
One common cause of leaks is improper torque on the transmission pan bolts. If the bolts are over-tightened, the pan metal bends around the bolt holes and prevents the gasket from sealing. Always use the proper torque specs when installing pan bolts. Another mistake is trying to stack two gaskets (“double gasketing”) to stop a leak. This does not work. The REAL fix is to make sure the pan rail (the flat surface around the bolt holes) flat. If the pan can’t be flattened replace it.
The type of gasket used also matters. Cork gaskets are low-quality and should generally be avoided. Most modern transmissions now use reusable gaskets (see Figure 36). These are durable, reliable, and can be reused multiple times. They usually feature raised rubber ribs and steel collars around the bolt holes, which help maintain a consistent seal without over-compressing the gasket.
| Figure 36 | Figure 37 |
Conclusion
Not everyone in the automotive industry fully understands automatic transmissions or their operating principles. Those who do have a clear advantage because this knowledge helps them see how the entire vehicle and how all of its systems work together.
A solid foundation in planetary gear set outcomes is essential when learning about transmissions. Just as important is recognizing that automatic shifting depends on three main sensor inputs:
Engine load (MAP or MAF sensor)
Vehicle speed (VSS)
Throttle angle (TPS)
Another key diagnostic principle is DTC prioritization. Transmission-related codes are considered third in line, behind U-codes (network issues) and engine performance codes. In nearly every diagnostic situation, U-codes must be addressed first, then engine codes, before focusing on transmission codes.
When diagnosing transmission issues, clutch application charts and solenoid charts are powerful tools. They allow technicians to narrow down problems and connect symptoms to specific components. Automatic transmissions are also designed with a failsafe (default) mode, which activates when the TCM loses power or detects a serious issue. This mode ensures some level of drivability, depending on the transmission model.
Proper fluid service is another major factor in transmission health. Using the correct type of fluid and maintaining the correct fluid level are both critical for reliable operation and long service life. Technicians need to become comfortable with fluid inspection procedures, which will be emphasized further in a later chapter and during the lab portion of this class.
Finally, torque converters were briefly introduced. While not covered in depth here, they are a crucial component, since they transfer engine torque into the transmission. Many people don’t fully understand how torque converters operate. Their structure and function will be explored in another chapter. For now, review the basics of torque converter operation by watching 1:00–5:00 of the video in Figure 39.Chapter Review
Use the following questions to review and check your understanding of the concepts introduced in this chapter:
What kind of gears form the foundation of a traditional automatic transmission?
What does TCM stand for, what role does it play, and where is it typically located?
When addressing diagnostic trouble codes (DTCs), at what point should transmission codes be considered? Which codes should always be addressed first?
What are the three primary sensor inputs that enable automatic transmission shifting?
What are three secondary sensors that also influence the shift schedule?
Can you define the transmission model designations 8F35 and 10R80?
What are the four main elements found inside a torque converter?
Where is most of the heat in an automatic transmission generated?
Define these terms: longitudinal transmission, transaxle, and transfer gears.
Which transmission component functions as the control center of the transmission?
In order for a planetary gearset to create an output ratio, what two conditions must occur?
What components are used to either hold or drive a planetary gearset member?
When are clutch application charts and solenoid application charts most valuable?
Why do so many different types of transmission fluid exist?