Transmission Control Unit
Encyclopedia
A transmission control unit or TCU is a device that controls modern electronic automatic transmission
s. A TCU generally uses sensors from the vehicle as well as data provided by the Engine Control Unit
to calculate how and when to change gears in the vehicle for optimum performance, fuel economy
and shift quality.
Electronic automatic transmissions have been shifting from purely hydromechanical
controls to electronic controls since the late 1980’s
. Since then, development has been iterative and today designs exist from several stages of electronic automatic transmission control development. Transmission solenoid
s are a key component to these control units.
The evolution of the modern automatic transmission and the integration of electronic controls have allowed great progress in recent years. The modern automatic transmission
is now able to achieve better fuel economy, reduced engine
emissions
, greater shift system reliability, improved shift feel, improved shift speed and improved vehicle handling
. The immense range of programmability offered by a TCU allows the modern automatic transmission to be used with appropriate transmission characteristics for each application.
On some applications, the TCU and the ECU are combined into one unit, a Powertrain Control Module
or PCM.
technology, this is often a shared input between the ECU and TCU. The input is used to determine the optimum time and characteristics for a gear change according to load on the engine. The rate of change is used to determine whether a downshift is appropriate for overtaking, for example, the value of the TPS is also continually monitored during the journey and shift programmes are changed accordingly (economy, sport mode, etc.). The TCU can also reference this information with the vehicle speed sensor to determine vehicle acceleration and compare this with a nominal value; if the actual value is much higher or lower (such as driving uphill or towing a trailer) the transmission will change its gearshift patterns to suit the situation.
. The TCU uses the input shaft speed to determine slippage across the torque converter and potentially to determine the rate of slippage across the bands and clutch
es. This information is vital to regulate the application of the torque converter lock-up clutch smoothly and effectively.
pressures according to the changing viscosity of the fluid based on temperature, and also to determine regulation of the torque converter lock-up clutch.
if the transmission detects that the vehicle is going downhill.
module in order to change gearchange behaviour to take into account the throttle is not being operated by the driver, this eliminates unexpected gearchanges when the cruise control is engaged.
communications or similar protocols (such as Chrysler's CCD bus, an early EIA-485-based vehicle local area network
). In older vehicle designs, as well as in aftermarket TCUs sold into the racing and hobbyist markets, the TCU receives only the signals needed to control the transmission (engine speed, vehicle speed, throttle position or manifold vacuum, shift lever position).
Ultradrive
and its follow-ons) use the solenoids to control the clutches directly, by way of a greatly simplified valve body.
if a serious problem is found. An output to the cruise control module is also often present to deactivate the cruise control if a neutral gear is selected, just like on a Manual transmission
.
Automatic transmission
An automatic transmission is one type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually...
s. A TCU generally uses sensors from the vehicle as well as data provided by the Engine Control Unit
Engine Control Unit
An engine control unit is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters an internal combustion engine needs to keep running...
to calculate how and when to change gears in the vehicle for optimum performance, fuel economy
Fuel efficiency
Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. Overall fuel efficiency may vary per device, which in turn may vary per application, and this spectrum of variance is...
and shift quality.
Electronic automatic transmissions have been shifting from purely hydromechanical
Hydraulic machinery
Hydraulic machines are machinery and tools that use liquid fluid power to do simple work. Heavy equipment is a common example.In this type of machine, hydraulic fluid is transmitted throughout the machine to various hydraulic motors and hydraulic cylinders and which becomes pressurised according to...
controls to electronic controls since the late 1980’s
1980s
File:1980s decade montage.png|thumb|400px|From left, clockwise: The first Space Shuttle, Columbia, lifted off in 1981; American President Ronald Reagan and Soviet leader Mikhail Gorbachev eased tensions between the two superpowers, leading to the end of the Cold War; The Fall of the Berlin Wall in...
. Since then, development has been iterative and today designs exist from several stages of electronic automatic transmission control development. Transmission solenoid
Transmission solenoid
A transmission solenoid is an electro-hydraulic valve that controls fluid flow into and throughout an automatic transmission. Solenoids can be normally open or normally closed. They operate via a voltage or current supplied via the transmission computer or controller...
s are a key component to these control units.
The evolution of the modern automatic transmission and the integration of electronic controls have allowed great progress in recent years. The modern automatic transmission
Automatic transmission
An automatic transmission is one type of motor vehicle transmission that can automatically change gear ratios as the vehicle moves, freeing the driver from having to shift gears manually...
is now able to achieve better fuel economy, reduced engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
emissions
Emission standard
Emission standards are requirements that set specific limits to the amount of pollutants that can be released into the environment. Many emissions standards focus on regulating pollutants released by automobiles and other powered vehicles but they can also regulate emissions from industry, power...
, greater shift system reliability, improved shift feel, improved shift speed and improved vehicle handling
Car handling
Automobile handling and vehicle handling are descriptions of the way wheeled vehicles perform transverse to their direction of motion, particularly during cornering and swerving. It also includes their stability when moving at rest. Handling and braking are the major components of a vehicle's...
. The immense range of programmability offered by a TCU allows the modern automatic transmission to be used with appropriate transmission characteristics for each application.
On some applications, the TCU and the ECU are combined into one unit, a Powertrain Control Module
Powertrain Control Module
A Powertrain Control Module, abbreviated PCM, is an automotive component, an electronic control unit , used on motor vehicles. It is generally a combined control unit, consisting of the engine control unit and the transmission control unit. It commonly controls more than 5 factors in the car or...
or PCM.
Input parameters
The typical modern TCU uses signals from engine sensors, automatic transmission sensors and from other electronic controllers to determine when and how to shift. More modern designs share inputs or obtain information from an input to the ECU, whereas older designs often have their own dedicated inputs and sensors on the engine components. Modern TCUs are so complex in their design and make calculations based on so many parameters that there are an indefinite amount of possible shift behaviours.Vehicle speed sensor (VSS)
This may also be known as an output speed sensor (OSS). This sensor sends a varying frequency signal to the TCU to determine the current speed of the vehicle. The TCU uses this information to determine when a gear change should take place based in the various operating parameters.Wheel speed sensor (WSS)
Modern automatic transmissions also have a wheel speed sensor input to determine the true speed of the vehicle to determine whether the vehicle is going downhill or uphill and also adapt gear changes according to road speeds, and also whether to decouple the torque converter at a standstill to improve fuel consumption and reduce load on running gear.Throttle position sensor (TPS)
The TPS sensor along with the vehicle speed sensor are the two main inputs for most TCUs. Older transmissions use this to determine engine load, with the introduction of drive-by-wireElectronic throttle control
Electronic throttle control is an automobile technology which severs the mechanical link between the accelerator pedal and the throttle. Most automobiles already use a throttle position sensor to provide input to traction control, antilock brakes, fuel injection, and other systems, but use a...
technology, this is often a shared input between the ECU and TCU. The input is used to determine the optimum time and characteristics for a gear change according to load on the engine. The rate of change is used to determine whether a downshift is appropriate for overtaking, for example, the value of the TPS is also continually monitored during the journey and shift programmes are changed accordingly (economy, sport mode, etc.). The TCU can also reference this information with the vehicle speed sensor to determine vehicle acceleration and compare this with a nominal value; if the actual value is much higher or lower (such as driving uphill or towing a trailer) the transmission will change its gearshift patterns to suit the situation.
Turbine speed sensor (TSS)
This may also be known as an input speed sensor (ISS). This sensor sends a varying frequency signal to the TCU to determine the current rotational speed of the input shaft or torque converterTorque converter
In modern usage, a torque converter is generally a type of hydrodynamic fluid coupling that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load...
. The TCU uses the input shaft speed to determine slippage across the torque converter and potentially to determine the rate of slippage across the bands and clutch
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
es. This information is vital to regulate the application of the torque converter lock-up clutch smoothly and effectively.
Transmission fluid Temperature sensor (TFT)
This may also be known as Transmission Oil Temperature. This sensor determines the fluid temperature inside the transmission. This is often used for diagnostic purposes to check ATF (Automatic Transmission Fluid) at the correct temperature. The main use of this has been as a failsafe feature to downshift the transmission if the ATF becomes extremely hot. On more modern TCUs this input allows the TCU to modify the line pressure and solenoidSolenoid
A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...
pressures according to the changing viscosity of the fluid based on temperature, and also to determine regulation of the torque converter lock-up clutch.
Kick down switch
One of the most common inputs into a TCU is the kick down switch which is used to determine if the accelerator pedal has been depressed past full throttle. When activated the transmission downshifts into the lowest permissible gear based on current road speed to use the full power reserves of the engine. This is still present in most transmissions though is no longer as necessary to use as the TCU uses the throttle position sensor and rapid rate of change to determine whether a downshift may be necessary, thus there is no need to use the kickdown feature in most circumstances.Brake light switch
This input is used to determine whether to activate the shift lock solenoid to prevent the driver selecting a driving range with no foot on the brake. In more modern TCUs this input is also used to determine whether to downshift the transmission to increase engine braking effectEngine braking
Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes....
if the transmission detects that the vehicle is going downhill.
Traction Control System (TCS)
Many TCUs now have an input from the vehicles traction control system, if the TCS detects unfavourable road conditions, a signal is sent to the TCU. The TCU can modify shift programmes by upshifting early, eliminating the torque converter lock-up clutch application, and also eliminating the first gear totally and pulling off in 2nd.Switches
These simple on/off electric switches detect the presence or absence of fluid pressure in a particular hydraulic line. They are used for diagnostic purposes and in some cases for controlling the application or release of hydraulic control elements.Cruise control module
An overlooked factor is that many TCUs also have a signal from the Cruise controlCruise control
Cruise control is a system that automatically controls the speed of a motor vehicle. The system takes over the throttle of the car to maintain a steady speed as set by the driver.-History:...
module in order to change gearchange behaviour to take into account the throttle is not being operated by the driver, this eliminates unexpected gearchanges when the cruise control is engaged.
Inputs from other controllers
A wide variety of information is delivered to the TCU via Controller Area NetworkController Area Network
Controller–area network is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer....
communications or similar protocols (such as Chrysler's CCD bus, an early EIA-485-based vehicle local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
). In older vehicle designs, as well as in aftermarket TCUs sold into the racing and hobbyist markets, the TCU receives only the signals needed to control the transmission (engine speed, vehicle speed, throttle position or manifold vacuum, shift lever position).
Output parameters
The typical modern TCU sends out signals to shift solenoids, pressure control solenoids, torque converter lockup solenoids and to other electronic controllers.Shift lock
Many automatic transmissions lock the selector lever via a shiftlock solenoid to stop a driving range being selected if the brake pedal is not depressed.Shift solenoids
Modern electronic automatic transmissions have electrical solenoids which are activated to change gears. Simple electronic-control designs (such as Ford's AOD-E, AXOD-E and E4OD) use the solenoids to modify the shift points in an existing valve body, while more advanced designs (such as the ChryslerChrysler
Chrysler Group LLC is a multinational automaker headquartered in Auburn Hills, Michigan, USA. Chrysler was first organized as the Chrysler Corporation in 1925....
Ultradrive
Ultradrive
The Ultradrive is a 4-speed automatic transmission from Chrysler Corporation. It was produced starting in 1989. It was originally paired primarily with the Chrysler 3.3 engine in vehicles with transverse engines...
and its follow-ons) use the solenoids to control the clutches directly, by way of a greatly simplified valve body.
Pressure control solenoids
Modern electronic automatic transmissions are still fundamentally hydraulic. This requires precise pressure control. Older automatic transmission designs only use a single line pressure control solenoid which modifies pressure across the entire transmission. Newer automatic transmission designs often use many pressure control solenoids, and sometimes allow the shift solenoids themselves to provide precise pressure control during shifts by ramping the solenoid on and off. The shift pressure affects the shift quality (too high a pressure will result in rough shifting; too low a pressure will cause the clutches to overheat) and shift speed.Torque converter clutch solenoid (TCC)
Most electronic automatic transmissions utilize a TCC solenoid to regulate the torque converter electronically. Once fully locked, the torque converter no longer applies torque multiplication and will spin at the same speed as the engine. This provides a major increase in fuel economy. Modern designs provide partial lockup in lower gears to improve fuel economy further, but this can increase wear on the clutch components.Output to ECU
Many TCUs provide an output to the ECU to retard the ignition timing, or reduce the fuel quantity, for a few milliseconds to reduce load on the transmission during heavy throttle. This allows automatic transmissions to shift smoothly even on engines with large amounts of torque which would otherwise result in a harder shift.Outputs to other controllers
The TCU provides information about the health of the transmission, such as clutch wear indicators and shift pressures, and can raise trouble codes and set the Malfunction Indicator Lamp on the Instrument ClusterDashboard
A dashboard is a control panel placed in front of the driver of an automobile, housing instrumentation and controls for operation of the vehicle....
if a serious problem is found. An output to the cruise control module is also often present to deactivate the cruise control if a neutral gear is selected, just like on a Manual transmission
Manual transmission
A manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications...
.