Transmission Control Units (TCU): How they work, and how they fail

The Transmission Control Unit, sometimes referred to as the TCU or TCM, is the control unit that is responsible for your automatic gearbox functionality. Today, almost all automatic gearboxes are controlled by a TCU. Read on to learn more about how they work and how they fail! 

TCU: What does it do? 

The TCU/TCM or Gearbox control unit is the brain behind your automatic transmission. It ensures that the transmission functions optimally by responding quickly and efficiently to a driver's inputs. 

When the throttle pedal is pressed, the car shifts up, and when braking, the car shifts down. The TCU must continuously calculate what the driver is likely to do. There are no fixed formulas for the correct shifting pattern, so the system must take into account multiple factors – speed, throttle position, steering angle, brake pressure, wheel slip and even data from the navigation system. 

On top of this, the TCU takes into account the activation of special functions like manual gear selection and cruise control. The TCU controls the shift valve, the control valve, the shift actuator and many other components, all via CAN communication lines. 

The most common TCU faults 

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

• Implausible shifting sequence
• Failure to change gear
• Poor gear changes 
• The car does not start
• Complete electronic failure of the gearbox

Not all automatic gearboxes are created equal! 

There are numerous types of automatic gearboxes on the market – the majority of modern automatics are controlled by TCUs with manufacturers refining their unique take on the same problem. 

• Torque Convertor Automatic
  
  The oldest-school type transmission in our list is the 7G-tronic fitted to many Mercedes-Benz models. Whilst in principle, a torque converter automatic has been around for decades, the combination of a planetary and Ravigneaux gear system results in a compact structure. 
  
  A transmission control unit (TCU) continually 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, suitable for a premium Mercedes. 
  
  
• Automated Manual Transmissions
  
  The TCU of an automatic manual transmission plays a crucial role, controlling 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 fitted with automated functionality. 
  
  It's been seen fitted to numerous vehicles, like the Vauxhall Easytronic, and Ford Durashift, but also by brands like Alfa Romeo, Fiat, Honda and Toyota have developed their variants. 
  
  
• Continuous Variable Transmission
  
  Sometimes referred to as a rubber-band gearbox, the CVT gearbox doesn't use a fixed gearbox, but instead adjusts the gear ratio without a step change. It consists of two conical pulleys, which are connected by a strong yet flexible push belt. The distance between pulleys can change, effectively creating a variable diameter and thus the ratio of the transmission. 
  
  The engine drives the input pulley whilst 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 some examples of modern CVT systems that are known for their smooth and efficient driving experience. 
  
  
• DSG: Double Clutch Transmission
  
  VAGs DSG is a transmission system that has grown increasingly popular. Utilising an innovative dual-clutch design, one for the even and one for the odd gears. The result is a transmission that always has the next gear ready, allowing the shift to take place extremely quickly. 
  
  In addition, two primary shafts are used, each responsible for a set of gears. This configuration significantly reduces the time a gear change takes and provides a much smoother driving experience. Although many manufacturers now offer dual-clutch gearboxes, the DSG remains one of the most recognisable names.