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 handle both basic maintenance and advanced repairs.

The engine and transmission work together as the core of the powertrain, but in modern vehicles they no longer operate in isolation. 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, making transportation safer and more efficient for the driver. 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 on the network.

With that in mind, technicians must learn the 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 resolved.

Often, once the communication fault is repaired, the other codes will disappear on their own.

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 shifting. If the engine itself is not running properly, the TCM cannot make correct decisions about shift timing or line pressure. 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 detects these irregular fluctuations, but the TCM interprets the unstable readings as a transmission issue, which may cause 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 may respond 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 line—until the network and engine systems are delivering reliable information, the transmission 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 in some cases reverse. Fail-safe is triggered by two main conditions:

1. 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—it could be 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 this occurs.

2. Protective default triggered by the TCM.
   Sometimes the TCM detects a problem that could damage 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 several 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 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 shorthand for “intake manifold pressure.”

• VSS (vehicle speed sensor).
  This sensor, usually located on the transmission case near an axle shaft, reports actual vehicle speed to the TCM. It is not to be confused with wheel speed sensors, which measure rotation at each wheel for systems like ABS or traction control.

Shift Inputs

The MAP/MAF, VSS, and TPS sensors are the primary inputs the TCM relies on to make shifting happen. Without them, the transmission can’t calculate when to shift.

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. Same deal for the TCM — it watches vehicle speed and engine load to decide the right moment.

Secondary 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 is warmed up, following factory programming.

• TFT (transmission fluid temp): Critical for both TCC application and shift timing. Best performance happens once the fluid is warm.
  
  

Transmission Inputs from 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 manufacturers require these 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 this group of 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 automatic 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 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 the same component:

• TRS – Transmission Range Switch (Ford)

• MPLS – Multi Lever Position Switch (GM)

• Manual Shift Shaft Position Switch (GM)

• Inhibitor Switch (Subaru)

• Park/Neutral Switch and Transmission Control Switch (Toyota – these two work together to perform the same functions as a TRS)

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 more often. This improves fuel economy, performance, and emissions.

It’s very similar to how a multi-speed bicycle works. 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 even more effectively because they offer infinite ratios, they do not match the towing strength and durability of traditional automatics. That’s why manufacturers continue to invest heavily in multi-speed planetary 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 drivetrain sits sideways, or transverse, in the vehicle. These transmissions are actually 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, new types of transaxles have become common, such as CVTs (continuously variable transaxles) and DCTs (dual-clutch transaxles). Even though the terms transmission and transaxle are often used interchangeably, technically a transaxle is a single unit that contains both a transmission and a differential and sits transverse in the vehicle.

A rear-wheel drive (RWD) platform, on the other hand, is found in most light-duty pickup trucks. Here, the transmission sits longitudinally, meaning in line with the vehicle’s drive wheels. In this setup, the transmission and differential are separate units. This layout is common for trucks and larger vehicles where towing and durability are a priority.

One important misconception is that transmissions somehow produce power. That isn’t true. The engine produces power, while the 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. For example, 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 trade torque for 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.
  
  

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.

Valve body – A complex hydraulic and electronic control unit. It contains passages, valves, springs, the pressure control solenoid, and the shift/clutch control solenoids. The valve body takes pressurized fluid from the pump and routes it to the clutches, bands, gear sets, and torque converter. It acts as the control center of the transmission and is itself controlled by the Transmission Control Module (TCM).

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.

• A single planetary gear set can produce several different outcomes.

• Transmissions with three speeds or more use at least two planetary gear sets.

• Modern units (6-, 8-, 9-, and 10-speed) often use three or four planetary gear sets.

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:
  

1. They house the clutch plates and piston.

2. 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.

• Clutches can either drive a gear member or hold it 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 4^th 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.

To rephrase, low gears have high ratios. They offer good leverage and produce good torque, which helps the vehicle move effectively at low speeds. High gears are of low ratio, but they enable speed increase, which results in less torque, as vehicle speeds increase. 

Planetary Gears Operation

For a planetary gear set to produce a specific outcome, two conditions must always occur:

1. One gear member must serve as the input (driving member).

2. 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 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

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