Table of Contents
Overview
Most automatic transmissions used for passenger vehicles have a torque converter as a coupling device. The main roles of the torque converter are: to decouple the engine from the transmission, when the vehicle is at standstill, and to transmit torque to the transmission when the engine is increasing speed. The torque converter allows the engine to idle when the vehicles is stopped even though the transmission has a gear engaged. As the name suggests, the torque converter converts (amplifies) the input torque of the engine into a higher output torque. This particular feature of the torque converter is not possible with a clutch, which can transmit maximum the engine torque and not more than that.
Image: Automatic transmission
Credit: ZF
The torque converter is mounted between the internal combustion engine and the automatic transmission, in the same place where a clutch would have been in the case of a manual transmission. The main components of the torque converter are:
- the impeller (also known as pump)
- the turbine
- the stator (mounted on a one-way mechanism)
- the lock-up clutch
Image: Torque converter – exploded view
where:
- torque converter front cover
- clutch friction discs
- clutch pressure disc with torsional damper
- turbine
- stator mounted on an one way mechanism
- impeller housing
The torque converter is filled with automatic transmission fluid (ATF), which is a type of transmission oil. The impeller is connected to the crankshaft and the turbine is connected (splined) to the input shaft of the transmission. In the image below you can see a detailed section of a torque converter with its main components. The impeller, stator and turbine have curved vanes which causes the fluid to flow inside the torque converter.
Image: Main components of a torque converter
Credit: ZF Sachs
where:
- turbine
- impeller
- stator
- torsional damper
- brake lining
- lock-up clutch
The impeller (1) is rotated by the crankshaft and it converts the mechanical energy of the crankshaft into kinetic energy by putting the fluid in motion. The fluid then hits the turbine (1) and the inverse process happens, the kinetic energy is converted back into mechanical energy. The increase in torque happens due to the stator (3), which deflects the flow of fluid when entering the turbine.
As you can see, there in no direct (mechanical) connection between the impeller and the turbine. The power between the engine and transmission is transferred through the fluid in motion. Due to this, the efficiency of the torque converter is relatively low, especially at low temperatures and high speed difference (slip) between the impeller and turbine.
The fluid is put in motion by the impeller blades, which directs it to the stator blades, which further redirects the fluid into the turbine blades. When there is high speed difference between the impeller and the turbine, the stator doesn’t rotate, achieving the torque amplification. This phase is called the converter phase. The torque converter can multiply the engine torque up to 2.5 times. When the turbine speed gets close to the impeller speed, the stator begins to rotate and the torque converter enters the clutch phase. In this phase there is no engine torque multiplication happening.
The lock-up clutch (6), also known as the torque converter clutch (TCC), has the role of mechanically connecting the impeller with the turbine in order to limit the power losses. When the speed difference between the impeller and turbine is not too large, the torque converter clutch is closed and the connection engine to transmission is direct, without any torque converter losses in place.
For more information about the torque converter, read also the article How a torque converter works.
Efficiency
The torque converter functions like a hydraulic coupling device. The mechanical power transmitted from the internal combustion engine is converted into hydraulic power by the impeller and back into mechanical power by the turbine. All these power conversions come with some losses. These loses are mainly due to friction within the fluid layers. The lost power is dissipated as heat.
The efficiency of the torque converter depends on the speed ratio ν [-] between the impeller and the turbine. The torque converter speed ratio is defined as the ratio between the output speed (turbine) and the input speed (impeller):
where:
ωT – turbine angular speed [rad/s]
ωP – impeller (pump) angular speed [rad/s]
Since the turbine speed lags all the time behind the impeller (pump) speed, the speed ratio is less than 1. This means that there is friction in the transmission fluid which leads to power losses. The lower the speed ratio, the higher the friction, the higher the power losses, the lower the overall torque converter efficiency.
Image: Torque converter efficiency function of speed ratio
Credit: [4]
- S (stall point): in this point the turbine speed is zero and the impeller is rotating; the efficiency in the stall point is minimum, around 50%, which means that half of the power coming from the engine is lost through friction and dissipated as heat; at this point the torque conversion is at its highest value, which is beneficial for vehicle pull-off capabilities
- M (maximum efficiency point): in this point the torque converter hits its maximum efficiency as torque converter, the fluid flows without impact losses from the impeller wheel to the next.
- C (lock-up point): in this point the stator starts to rotate with the turbine and there is no torque conversion possible; from this point on the torque converter behaves like a hydraulic clutch, only transfering power from the engine to the transmission without any torque amplification
- F (free-flow point): at this point there is no load on the turbine, the speed ratio is very close to 1, which means that the turbine speed matches the impeller speed; in this point
Lock-up clutch
During most of the operation time of the torque converter, the speed match between the impeller and turbine is never achieved. At cruising speed, the torque converter can only pass around 85 % of the engine power to the transmission. This means that there is a lot of power lost in the torque converter, dissipated as heat. In order to improve its efficiency, manufacturers added a lock-up clutch to the torque converter.
The Torque Converter Clutch (TCC) mechanically locks the engine to the transmission by connecting the impeller with the turbine through a wet clutch. This way the torque converter slip is eliminated and the efficiency increased. Another advantage is that the heat dissipated into the automatic transmission fluid is substantially reduced.
There are several ways of locking the torque converter clutch. These differences are function of the hydraulic circuit controlling the clutch actuation.
Depending on the number of passages (ports) for controlling the oil flow through the torque converter clutch, there are several types of torque converters:
- two-passage (2-pass) torque converters
- three-passage (3-pass) torque converters
- four-passage (4-pass) torque converters
The most common type of torque converter is the two-passage torque converter. In this type, the torque converter clutch is activated by reversing the flow of the automatic transmission fluid (ATF) through the converter.
 Image: Torque converter clutch (Toyota) – open |  Image: Torque converter clutch (Toyota) – closed |
In a two-passage torque converter clutch actuation system (as in the images above), the lock-up clutch is installed on the turbine hub, in front of the turbine. The dampening spring absorbs the torsional vibrations during clutch engagement in order to prevent shock transfer. The friction material applied on the lock−up piston is the same type of material as the one used on multiplate clutch disks in the automatic transmission.
The engagement and disengagement of the lock-up clutch depends on the direction the fluid enters the torque converter. The automatic transmission fluid can enter either through the front of the lock-up clutch or between the impeller and turbine, behind the clutch. By controlling the pressure behind and in front of the clutch, we control the engagement and disengagement of the lock-up clutch.
In some applications, the transmission fluid used to control the torque converter lock−up clutch is also used to remove heat from the torque converter and transfer it to the main engine cooling system through the heat exchanged in the radiator.
The control of the oil pressure in the lock-up clutch is done through two valves: the relay valve and the signal valve. In this type of arrangement the signal valve controls the pressure on one side of the relay valve, which controls the pressure in the lock-up clutch. By default, both valves are kept into position by springs, leaving the clutch in the disengaged position and the torque converter unlocked. When higher line pressure is applied to the lower part of the signal valve, it moves up and connects the line pressure to the relay valve lower end. This causes the relay valve to move up and to reconfigure the oil flow circuit in such a way that pressure is applied to the back of the clutch and engages it. To disengage the clutch, pressure is removed from the lower end of the signal valve and the oil circuit changes to the initial layout, which applies the pressure in front of the clutch disengaging it.
 Image: Torque converter clutch (solenoid) – open |  Image: Torque converter clutch (solenoid) – closed |
Modern torque converters have electronic control of the clutch operation. The pressure in the torque converter clutch is regulated via a main regulator valve which is piloted by a solenoid (see pictures above).
When the solenoid is energised, the line pressure acting on the right side of the regulator valve is low since the fluid escaped toward drain. In this state, the regulator valve will be positioned to the right and the oil will flow through the front of the clutch, keeping it open. Switching off the solenoid causes the pressure to increase on the right side of the regulator valve, which will move to the left. This operation will reconfigure the oil circuit in such a way that the oil will flow through the rear of the clutch, closing it.
When the torque converter clutch is closed, the impeller is mechanically linked to the turbine and the engine power is transferred to the transmission without losses in the torque converter.
With the three-passage torque converter, two passages are used for transmission oil (ATF) flow through the torque converter and for clutch cooling, while the third passage is used independently to control the lock and unlock of the clutch.
Image: Comparison of a two-passage and a three-passage torque converter
Credit: Schaeffler
The torque capacity of the converter clutch depends on several properties:
- the area of the piston where the transmission oil pressure is applied
- the effective radius of the friction material
- the number of friction surfaces
- the friction coefficient of the friction material and steel
- the actual transmission oil pressure applied to the clutch piston
While the geometric and material properties of the clutch are fixed, the oil pressure can be adjusted in order to control the position, thus the state of the clutch. The state of the torque converter clutch can be either:
- open
- closed (locked)
- slipping
The state of the clutch depends on throttle valve position (engine load) and engine speed. In general terms, at low engine speed the torque converter clutch is open, while at high engine speeds, the clutch is closed. The clutch is maintained in a slip state usually at medium-low engine speed and load.
Image: Operating ranges of a torque converter clutch
In an ideal torque converter, the torque capacity of the clutch and its slip could be regulated exclusively by controlling the applied oil pressure. This is not possible in a real torque converter due to the fact that there are several interference factors, which complicate the process of controlling the clutch slip. These interference factors are [1]:
- pressure drop across friction material: in two-passage torque converters, the friction material is used to transmit torque and also as a sealing component on the outside diameter of the piston; in order to cool down the clutch, a groove pattern is often pressed into the friction material; when transmission oil flows through the grooves from the high pressure side of the piston to the low pressure side, it experiences a pressure drop; the magnitude of this pressure drop depends on the groove geometry, consistency of the friction surfaces, temperature, and slip speed.
- absolute system speed: after the transmission oil has flowed through the friction material grooves in a two-passage converter, it must be transported radially from the outside diameter of the converter to the inside towards the transmission input shaft; since the entire system is spinning, the fluid particles are subjected to Coriolis forces on their way to the inside, leading to the formation of a spiral flow in front of the transmission input shaft; this results in back pressure that reduces the effective pressure on the piston.
- system pressure variation: fluctuations in converter charging pressure affect the high-pressure side of the piston in a two-pass system and the low-pressure side of the piston in a three-pass system.
- differential speed (slip): during open or slipping conditions, two and three-pass systems have components such as the damper, turbine or cover on either side of the piston which are rotating at different speeds; these components dominate the mean rotational speed of the ATF on either side of the piston, which results in a different centrifugal force, creating a relative pressure across the piston.
Interference factors 1 and 2 can be largely neutralised by a three-passage system. The remaining interference factors can also be improved significantly in a three-passage system or compensated by the calibration software in the transmission. However, in order to be able to compensate all factors entirely without additional software requirements, a different principle is needed: the four-passage torque converter. As the name suggests, this is a converter system with four hydraulic passages.
Image: Torque converter with four hydraulic passages
Credit: Schaeffler
Like the three-passage system, two of the passages are used for the flow through the converter, and the third passage serves to control the clutch. The unique feature of the four-passage torque converter is the additional fourth passage, which feeds a pressure compensation chamber. This results in identical fluid speed conditions on both sides of the piston. The dynamic centrifugal force of the ATF is identical on both sides of the piston because the outside diameters of the activation and compensation chamber seals are the same. This means that the piston pressure is now independent of slip speed, and furthermore, the pressure chambers of the clutch are shielded from system pressure variations, i.e. from charge pressure fluctuations.
With the four-pass converter, the clutch can be controlled very precisely, independent of operating conditions. Schaeffler started volume production of the system presented in 2014 and is currently working on
its implementation with other customers. A study was completed of production two-passage, three-passage and four-passage torque converters to compare the slip speed during operation.
Image: Torque converter clutch slip speed comparison
Credit: Schaeffler
The comparison shows that in this specific four-pass application the lock-up clutch can be engaged even in first gear. Besides fuel consumption savings, this also means that the lock-up clutch can be
used as a launch device in line with the torus of the converter. This allows a smaller and more lightweight design of the torus. In higher gears, the four-pass converter can be operated at a very low slip speed
due to its precise controllability. As a result, the damper can be designed on a smaller scale, allowing a more space-saving design of the converter as a whole.
Clutch solenoid
Modern torque converters use electro hydraulic control for the lock-up clutch. The hydraulic circuit which locks/unlocks the torque converter clutch is managed using hydraulic valves. The valves are actuated directly or indirectly by solenoids.
A solenoid is a linear electrical actuator. When is being energised (supplied with electric energy) it pushes or pulls a rod which is connected to the hydraulic valve. There are different types of solenoid used for torque converter clutch control, but the principle of working is basically the same.
 Image: Torque converter clutch pulse width modulation valve – GM |  Image: Torque converter clutch solenoid with connector view |
 Image: Torque converter clutch solenoid and valve with oil filter |  Image: Torque converter clutch solenoid |
The solenoid has two electrical connectors, a plus (+) voltage and a ground (-). It is usually power by the 12 V electrical system of the vehicle and it’s controlled by the transmission control module (TCM).
Clutch diagnostics
On electronically controlled transmissions, the operation of the torque converter clutch solenoid is monitored by the powertrain control module (PCM) or transmission control module (TCM) and can, but will not always, set a diagnostic trouble code (DTC) if a fault is present.
The PCM/TCM will not set a DTC unless there is a problem with the electrical circuit controlling the torque converter clutch solenoid.
Image: Torque converter clutch solenoid circuit schematic example
Using a scan tool we can read the DTC related to the torque converter clutch solenoid. The most common DTCs are:
- P0740
- P0741
- P0742
- P0743
- P0744
The tables below summarises the definition of each DTC, their meaning, possible causes of failure and which are the symptoms at the vehicle level.
| OBD Definition | Meaning | Possible causes | Symptoms |
| P0740 | |||
| Torque converter clutch (TCC) solenoid – circuit malfunction | There is a problem with the electrical circuit of the torque converter clutch solenoid. This means that the control module (PCM/TCM) can not properly control the clutch, which means that the torque converter clutch is either in a permanent/intermittent locked state, or open state or slipping. |
|
|
| P0741 | |||
| Torque converter clutch (TCC) solenoid – performance/stuck off | There is a slip (speed difference) between the engine (impeller) and the transmission input shaft (turbine) when the torque converter clutch is locked. This means that the torque converter clutch doesn’t lock properly or it’s in a permanent open state. |
|
|
| P0742 | |||
| Torque converter clutch (TCC) solenoid – stuck on | The torque converter clutch solenoid is always energised (stuck on) which translates into the torque converter being always locked. |
|
|
| P0743 | |||
| Torque converter clutch (TCC) solenoid – electrical | There is a permanent problem with the electrical circuit of the torque converter clutch solenoid. |
|
|
| P0744 | |||
| Torque converter clutch (TCC) solenoid – circuit intermittent | There is an intermittent problem with the electrical circuit of the torque converter clutch solenoid. Intermittent problem means that the malfunction comes and goes, it’s sporadic. |
|
|
Before beginning any diagnostics, check to make sure the system has power. Next, inspect and test the solenoid’s ground circuit. If the power and ground are ok, test the solenoid with an ohmmeter. If the solenoid’s resistance is within specifications, remove the solenoid from the transmission. Apply power and ground to the solenoid while attempting to blow air through the solenoid. If the solenoid operates properly, the problem is probably in the torque converter itself. If the solenoid does not allow air to flow, place it in a clean container of automatic transmission fluid and electrically cycle it to see if you can remove any blockage from the solenoid. If this does not work, you will need to replace the solenoid [8].
References
[1] Thomas Heck et al., Efficient Solutions for Automatic Transmissions – Torque Converters and Clutch Packs, Schaeffler Symposium 2018.
[2] VAG, Automatisches Getriebe Modelljahr ‘95 – Konstruktion und Funktion, Selbsstudienprogramm 172, 1995.
[3] Toyota, Automatic Transmissions – Course 262.
[4] Ayman Moawad, Aymeric Rousseau, Impact of Transmission Technologies on Fuel Efficiency, Technical Report, Argonne National Laboratory, January 2012.
[5] Yi Zhang, Chris Mi, Automotive Power Transmission Systems, Wiley, 2018.
[6] Harald Naunheimer, Bernd Bertsche, Joachim Ryborz, Wolfgang Novak, Automotive Transmissions – Fundamentals, Selection, Design and Application, 2nd Edition, Springer, 2011.
[7] Robert Fischer, Ferit Küçükay, Gunter Jürgens, Rolf Najork, Burkhard Pollak, The Automotive Transmission Book,
Springer, 2015.
[8] Keith Santini, Kirk VanGelder, Automotive Automatic Transmission and Transaxles, Jones & Bartlett Learning, 2018.