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 (ECU) to calculate how and when to change gears in the vehicle for optimum performance, fuel economy and shift quality.

Electronic automatic transmissions have been changing in design from purely hydromechanical controls to electronic controls since the late 1980s. 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 a single unit as a powertrain control module (PCM).

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

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. The TCU also uses a ratio between the TSS and WSS which is used to determine when to change gears. If either the TSS or WSS fails or malfunctions/becomes faulty, the ratio will be wrong which in return can cause problems like false speedometer readings and transmission slipping. To test these parts, check the resistance to make sure it's within manufacturer specs.

...
Wikipedia