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Understanding Transmission Control Units (TCU): Functions, Types, and Common Issues

The transmission control unit, also known as the TCU, is the control unit that controls the gearbox. Almost all automatic gearboxes are controlled by a TCU. Read on quickly! 

What does a TCU do in a car?

The TCU or gearbox control unit is the brain behind the automatic transmission and ensures that it works optimally by responding to every movement of the driver. When the accelerator pedal is pressed, the car shifts up, and when braking it shifts down again. The TCU must continuously calculate what the driver is likely to do, because there are no fixed formulas for the correct shifting moments. The system takes many factors into account, such as the current speed, the position of the accelerator pedal, the angle of the road, the brake pressure, the steering angle, any wheel slip and even data from the navigation system. In addition, the transmission control unit takes into account the activation of the manual gear selector and cruise control. The TCU ensures that the correct gear is selected at all times. The transmission control unit controls the shift valve, the control valve and the shift motor, among other things. It does this by means of CAN messages. 

Most common defects of the TCU

There are many different possible defects with the TCU. These are the most common faults with the transmission control unit:

  • Illogical shifting sequence
  • Gearbox does not shift
  • Automatic transmission shifts poorly
  • Car does not start
  • Complete electronic failure of gearbox

TCU car

What automatic gearboxes are there?

There are various automatic gearboxes on the market, all of which are controlled by a TCU. Car brands often refine their own technology. These four variants are the most common:

  • Planetary gear set with torque converter
    The 7G-tronic from Mercedes-Benz is somewhat reminiscent of the Hydramatic from General Motors due to the torque converter, but underneath this gearbox is much more advanced. It uses a combination of a planetary and Ravigneaux gear system, which results in a compact structure. A Transmission Control Unit (TCU) continuously monitors parameters such as engine speed and accelerator pedal position to select the optimal gear at all times. This ensures a smooth and efficient driving experience. 
  • Robotized manual transmission
    In cars with a robotized manual transmission, the Transmission Control Unit (TCU) plays a crucial role. It controls the shift motors and the clutch actuator. This system is a cost-effective way to achieve an automatic transmission, where a conventional manual transmission is converted with automatic functions. Examples of this technology include Opel's Easytronic and Ford's Durashift, but brands such as Alfa Romeo, BMW, Fiat, Honda and Toyota have also developed their own variants of this system. 
  • Continuously variable transmission
    A car with a continuously variable transmission (CVT) does not use fixed gears, but adjusts the gear ratio steplessly by means of an innovative system. It consists of two conical pulleys, which are connected by a strong, flexible push belt. The pulleys can change their distance from each other, which changes the effective diameter of the pulley and thus the transmission ratio. The engine drives the input pulley, while the output pulley drives the wheels via a differential. The TCU monitors this process and ensures optimum performance. The Mercedes-Benz 722.8 and Audi Multitronic are examples of modern CVT systems that are known for their efficiency and smoothness in driving. 
  • DSG: Double Clutch Transmission
    The Volkswagen Group's DSG is a transmission system that is becoming increasingly popular. It uses two clutches: one for the even and one for the odd gears. This ensures that the next gear is always ready, allowing the gear shift to take place extremely quickly. In addition, two primary shafts are used, each responsible for a set of gears. This configuration significantly reduces shifting time and provides a much smoother driving experience. Although many car brands have adopted this technology, DSG remains one of the most recognizable and trusted names in this field.