What Is a TCU? Transmission Control Unit (TCU) Explained

A part of the Electronic Control Unit (ECU), the Transmission Control Unit covers everything related to the transmission of a vehicle. It ensures smooth gear shifts and optimal fuel economy and performance. Below we will discuss how does it work and major input and output sensors it uses.

How Does TCU work?

It has the same principle as an ECU. Using input sensors it gathers data then compares it to its onboard memory and then sends out a signal accordingly to manage the output to what its memory recommends. All this is done to keep the performance optimal.

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The TCU similar to an ECU uses multiple sensors to gather data and then sends signals to necessary parts of a vehicle. Below are the input and output ports of a TCU.

Inputs 

These are the sensors giving the information to the TCU ranging from engine sensors to gear sensors. Some of these are mentioned below.

1. Vehicle Speed Sensor (VSS)
   The VSS allows the TCU to figure out when a gear shift would be necessary by determining the current speed of the car. The VSS alone however isn’t enough and the TCU needs more sensors.
2. Wheel speed sensor (WSS)

   

   The WSS helps determine the true speed of the vehicle and with the ratio of VSS to WSS the TCU can provide gear shifts that are both economical and smooth. The WSS also helps determine if the car is going uphill or downhill which in turn lets the TCU know if a gear shift is necessary.

3. Throttle Position Sensor (TPS)
   One of the major inputs for the TCU, the TPS provides information about the possible gear change according to the load on the engine. The change in load on the engine allows it to determine if a downshift or upshift of gear is required in the current condition.
4. Turbine Speed Sensor (TSS)
   The TSS sends data about the rotational speed of the torque converter. This helps TCU figure out if there is the slippage or not. It will then lock up the torque converter to prevent anymore slippage. This helps with the fuel economy.

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Outputs

With the data from input sensors the TCU outputs signals to the following sensors.

1. Shift Solenoids
   TCU controls the shift solenoids that change gears. It quite simply shifts gears when necessary according to the signal provided by the TCU. Perfect gear shift helps boost fuel economy.
2. Torque Converter Clutch Solenoid (TCC)
   

   

   After a lockup because of the TCC, the torque converter will rotate at the same speed as the engine after getting locked up with through the use of solenoids. This provides a big boost to fuel economy as there is no more slippage possible.

3. Engine Control Module (ECM)
   TCU provides signals to the ECM to slow down the engine a bit which allows the gear shifts to keep up with the heavy throttle. The ECM responds with slowing down the ignition timing thus providing enough time for the TCU to shift gears.

These are some of the sensors that TCU uses to ensure optimal vehicle transmission. The TCU has a lot more sensors and duties to perform but those are outside the scope of this blog. This concludes the brief explanation of TCU.

FAQs

1. What is the purpose of the transmission control unit?

A transmission control unit (TCU) or a transmission control module (TCM) is a device that receives the signals of sensors such as engine speed, vehicle speed, throttle position and based on the same information, decides when to switch gears in an automatic transmission to achieve smooth gear changes and improved fuel consumption.

2. What happens if a TCU fails?

In case of failure of the TUC, the car could experience gearing that is sluggish, erratic, or mixed gearing, become unable to change gear, shift erratically, achieve low fuel mileage, or not shift at all, making the car unsafe and a hassle to drive.

3. What is the cost of a replacement of a TCM?

Replacement of a TCM is determined by what the car is as well as the model and the labour. It can cost several hundred dollars (or other materials) to replace certain parts of cars, but handling the labour and reaching the transmission can increase the cost considerably.

4. How to check if TCM is faulty?

A faulty TCM can be indicated by the presence of gear-shifting issues, low fuel economy, check-engine light, slipping or jerking in transmission, or abnormal delays/ failures in shifting gears. Transmission-related error codes can be identified with the help of a diagnostic scan, which proves that TCM problems exist.