Transmission control unit

Transmission control unit

A transmission control unit or TCU is a device that controls modern electronic automatic transmissions. 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 solenoids 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.

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.

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 rotational speed of the output shaft or differential in a typical vehicle. The TCU uses tire size, gear ratios and final drive ratio to calculate the current vehicle speed.

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 converter. 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 clutches.

Transmission fluid Temperature sensor (TFT)

This may also be known as Transmission Oil Temperature (TOT). This sensor determines the fluid temperature inside the transmission. This allows the TCU to modify the line pressure and solenoid pressures according to the changing viscosity of the fluid based on temperature. This provides for more appropriate shifting in extreme temperatures, and allows for temperature failsafe systems to take control in case the transmission overheats.


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.

Inputs from other controllers

A wide variety of information is delivered to the TCU via Controller Area Network 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).

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 solenoids

Modern electronic automatic transmission 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 Chrysler Ultradrive 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 lock the torque converter electronically. Once locked, the torque converter no longer applies torque multiplication and will spin at the same speed as the engine (assuming the converter is fully locked). This provides a major increase in fuel economy, and modern designs provide partial lockup in lower gears to improve fuel economy further.

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 if a serious problem is found.


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