The Automatic Transmission is complex, to say the least. You’ll need years of practice to master. In this article, we’ll learn the components that make up an automatic transmission. We’ll learn the basic principles of operations of these components. The torque converter, the fluid pump, planetary gear sets and their variations, clutch drums, bands, clutch apply charts, and finally, valve bodies.
TORQUE CONVERTER
The torque converter is the primary component for transmitting power from the engine to the automatic transmission. It typically consists of the Front Cover, Lock-up Clutch, Stator, and the Impeller (Figure 1). 
The torque converter is bolted to the engine flywheel, also known as the flexplate, so it turns at the same speed as the engine. The torque converter provides four major functions.
• It provides a fluid coupling for a smooth transfer of torque from the engine to the transmission.
• It multiplies torque from the engine, which enables the vehicle to achieve additional performance when required.
• It provides a mechanical link between the engine and transmission when necessary, eliminating fluid coupling and increasing fuel economy.
• It mechanically drives the transmission oil pump.
PUMPS
The automatic transmission runs on hydraulic pressure. This pressure is supplied by the pump, which is driven by the torque converter. There are many different styles of pumps. Some pumps are georotor style (Figure 2), some have two gears and a crescent (Figure 3), some are vane and slide, and some are driven by chain and sprocket arrangements (Figure 4).
Planetary Components: The planetary gear set ultimately determines the output of a transmission. A simple gear set consists of a sun gear, ring gear, pinion or “planet” gears, and a pinion carrier (Figure 5).
- The sun gear, ring gear, or pinion carrier can all be input members of the gearset, which means they can all be coupled to driving clutches in the transmission.
- These three members can also be held stationary by friction clutches, one-way (mechanical) clutches, or bands.
- The three members can also be driven by other gearset members to serve as the output of the gearset.
- Individual pinion gears can only rotate on their pins, transferring power between the other three elements. Pinion gears cannot be input or output members.
All planetary gearsets follow basic rules. It is important to understand these rules in order to understand automatic transmission operation.
- Two external gears in mesh (sun gear and pinion gears) will rotate in opposite directions.
- An internal (ring) and external (pinion) gear in mesh will rotate in the same direction.
- There must be an input member and a held member to get output from a planetary gearset (except in direct drive).
- When two planetary gearset components are driven in the same direction at the same speed, direct drive (1:1) ratio will result.
- A carrier will always follow its input.
- If a carrier is the driving member, overdrive will result.
- If a carrier is the output member, reduction will result.
- If a carrier is held, reverse will result.
The Simpson gearset is a compound planetary gearset with the following features:
- Two ring gears
- Two pinion carriers (each with its own set of pinion gears)
- A common sun gear.
The example shown is from a 5R55E. This transmission also has an overdrive gearset (simple planetary) before the input shaft to provide overdrive input. A design characteristic of the Simpson gearset is that both the front carrier and the rear ring gear are connected to the output shaft, providing two possible output components.
The Ravigneaux gearset is also a compound planetary unit. The Ravigneaux system features a small sun gear, a large sun gear, a single pinion carrier, long pinion gears, short pinion gears, and the common ring gear (Figure 6). In systems with overdrive, the planet is being driven, and the large sun gear is being held stationary. The rule is that when the carrier is the driving member, overdrive will result.
Key elements of Ravigneaux gearset operation include the following:
- Although they appear connected, the forward and reverse sun gears operate independently.
- The small sun gear engages the short pinions only.
- The large sun gear engages the long pinions only.
- The short pinions engage the long pinions.
- The long pinions engage the short pinions and ring gear and are the only members which drive the output.
- The ring gear is used as the output member.
The Lapelletier geartrain (Figure 7), as used in this transmission, consists of:
- A front planet with the sun gear fixed to the front pump.
- A planetary gear set fixed to an input drum.
- A ring gear attached to the input shaft.
This assembly is used to drive a Ravigneaux gearset in the back of the transmission.
Mr. Lapelletier figured out that if the planetary gearset in the back of the transmission is turned at one speed and the sun gear is turned at another speed — usually slower — the combination creates an overdrive ratio. This overdriven ratio is slightly lower than the one created by holding the sun gear and driving the planet. The important thing to understand is that the combination of the single planet being driven faster than the sun gears, which are being driven in reduction, creates the different ratios.
CLUTCHES AND BANDS
The key to understanding power flow is to know how “friction elements” (clutches and bands) apply and hold members of the planetary gear set. Three basic types of components can hold or drive planetary members: friction clutches, one-way clutches, and bands. The rules of operation for these elements are as follows:
- Friction clutches are fiber-covered plates that are hydraulically applied and can “drive” planetary members (Figures 8 and 9). They do this by “coupling” a planetary member to a driving element, usually the input shaft and its extensions.
- Friction clutches can also hold planetary members. A friction clutch must have one set of friction plates anchored to the transmission case to do this.
- One-way clutches are used to drive or hold planetary members mechanically. They do this by their design, which allows free movement in one direction but no movement in the other (Figure 10).
- Bands are covered with friction material (Figure 11) and are applied by hydraulic pistons, or servos (Figure 12). They are used to hold planetary members. They are similar to drum brakes in that they have a surface of friction material that hold elements when forced against them. Bands can wrap directly around planetary members or around the drums and housings that couple these members.
Keep these rules in mind as you study the power flow descriptions. They help you understand the action of clutches and bands in relation to planetary power flow. Bands are used to stop a drum from turning. A band can be a rigid or flexible metal strip with friction material bonded to it. Bands are applied hydraulically by servos. The higher the pressure, the more clamping force there is.
A one-way clutch allows rotation in only one direction. In the opposite direction, the one-way clutch freewheels. There are three different types of one-way clutches: Spring and roller, sprag type, and ratchet sprag, or diode. They all perform the same function in different manners. The sprag is used in the Simpson gear set to hold the rear planet from turning. Then, when the shift to second gear takes place, it will freewheel. This eliminates the need for an apply component on the 2-1 shift. Doing so removes the chance of any down-shift shift shock.
Each roller has its own ramp. When the outside race is turned clockwise, the motion forces the roller against the spring and down the ramp. When it rotates counterclockwise, the motion and spring force the roller up the ramp, stopping the rotation.
Your ability to accurately read and interpret the Clutch and Band Apply Chart (Figure 13) is essential for accurately diagnosing automatic transmissions. Knowing what the clutches and band are doing while driving and during shifts will enable you to narrow broad symptoms to a few possibilities.
Knowing how to interpret apply charts will help you understand the operation of the planetary gear sets as they are described in this article. Use the apply chart to determine which apply components are malfunctioning on a customer’s transmission.
All of today’s transmissions are controlled electronically (Figure 14). The vehicle computer measures road speed using a vehicle speed sensor (VSS), and engine load by measuring engine vacuum flow through the intake manifold or throttle position. The computer then determines the transmission shift points and controls the shifts by energizing or de-energizing a series of solenoids. These solenoids usually supply pressure to the shift valves, stroking the valves and shifting the transmission.
Some computer systems also control mainline pressure through a solenoid by creating a pulsed, duty-cycled signal to adjust the position of the solenoid pintle. This creates a metered leak in the pressure from the pump, controlling mainline pressure. For a more detailed view of transmission hydraulic operation, pick up a copy of Hydraulic FunDamentals, available from the ATRA Book Store.
There are many different types of valve bodies. Some are very simple solenoid bodies (Figure 15). Others are very complex, with many valves. All of them have their own unique characteristics. The apply components are fed through the solenoids which are controlled by the TCM. The pressure regulator and TCC valves are in the pump body.
This valve body uses solenoids to control valves. These valves regulate the oil pressure to the apply components. Many valve bodies have several small parts (Figure 16). Always mark the locations of these small parts for proper installation. Take pictures!
An automatic transmission is a complex hydraulic circuit. Learning how to read and interpret oil schematics is one of the keys to being able to understand hydraulic functions of an automatic transmission and project the outcome of a targeted modifications. In the next issue of GEARS, we’ll cover the definitions of key components.
