What is a mechatronic unit?

A mechatronic unit is the control centre of the gearbox. It is found in the gearbox where we have the direct-shift gear oil (DSG). Its task is to manage the torque converter and the clutch pad of the gearbox via the transmission fluid under specific pressure. It is capable of performing shift operations in a second. The gearbox’s control unit is in constant communication with the engine control unit, and this helps to determine the right time when changing gear.

Simply put, the mechatronic unit is a computerised control part of the dual-clutch system. This unique advanced dual-clutch automatic transmission is found in many cars, but mostly Audis and Volkswagens. The dual-clutch transmission consists of two sub gearboxes, each with its clutch, that work together to shift the gear. 

The electronic control unit is made up of sensors and valves (actuators) in one single component with the main function of opening one clutch and closing the other at the right moment. This results in smooth shifting without any interruption of the tractive force. The unit processes the sensor signal, operates the valves, measures the RPMs, and slides of the hydraulic gear mechanism.

The mechatronic unit evaluates the selection of data. From the data, it recognises whether the gearbox needs to change from first to second gear, for example.

Components of a mechatronic unit

The components of a mechatronic unit are:

• Control unit
• Valve body
• Solenoids

The control units collect and analyse data about the current condition of the mechatronics and fluid via speed sensors, temperature sensors, and pressure sensors. From the gathered information, the control unit is able to optimise and equally execute the program for gear changing and at the same time, monitor for errors in the electronics and mechanics.

The solenoids are electrohydraulic valves, and they are the executive elements of the control unit in the mechatronic. Their main function is to manage the hydraulic valves in the valve body by allowing the transmission fluid to flow under specific pressure. The solenoids can work on a higher frequency, and this can help in faster gear changing.

The valve body is responsible for hydraulic control in the automatic transmission. Its role is to manage the fluid flow to the various section of the gearbox and the torque converter. The valve body consists of the following components:

• Plates with fluid pad and valve seat
• Gasket with gaps in specific places
• Hydraulic valves
• Springs
• Plastic and metal balls
• Manual valves (on the upper plate) – it determines whether the car is in parking, drive or neutral or reverse position.

Common Problems 

The most common defects occurring in the control units are: incorrect solenoid operation in the computer and periodic errors during diagnostic. More than one sensor can get faulty, and getting the data from them will be impossible. Electronic components on the mainboard may get damaged, and errors can appear on the software program of the gear change control due to incorrect data.  

Many times, the failure begins with sensor temperature failing - either failing outright or failing out of specification. If data sent to the sensor is incorrect, the computer will the do what it thinks is right based on the incorrect data; hence, failure results. When this happens, certain gears stop working, either at a time or all at once. You may notice grinding or bad fuel economy. 

The most common problem on the valve body is usually the appearance of gaps between the valve seat and the valve under normal wear. The presence of elements that are not part of the composition of the transmission fluid can lead to rapid wear. Failures in the mechatronic unit can also result due to a problem in the automatic transmission in general. 

Until the problem is fixed, the mechatronic unit will continue to break down. Proper diagnostic and maintenance are very crucial.

Faults associated with mechatronic units

• Clonking in gears
• Selector warning lights
• Inability to select gears
• “PRNDS” lights flashing
• Gearbox keeps selecting the neutral position

Troubleshooting a mechatronic unit 

The mechatronic unit is complex and challenging to troubleshoot when things start to go wrong. Reading or getting the trouble codes is important because it makes troubleshooting so much easier and reduces the cost during repairs. 

All mechatronics look the same, but they have several different HW (hardware) versions and multiple SW (software) versions, analogy or digital inputs. Getting an exact similar unit can be difficult when one starts to become faulty. This is the reason why you may sometimes end up with an incorrect version, and this causes driving difficulties, wrong shifting points, rapid clutch wear, trouble codes or inability of the mechatronic unit to actually work at all. That is why a mechatronic unit taken from another car MUST have the same HW version and MUST be re-flashed with the car's specific software.

The problems will typically be intermittent, with only the “PRNDS” light flashing, the gearbox going into a neutral position or losing reverse. The fault becomes more permanent with time, so replacement units must be arranged right away since damage might become more lasting if not dealt with promptly. 

First things first: Visit a qualified workshop. Next, get a proper diagnosis, then have the necessary repairs. 

If there is a diagnosed problem in the electronics, the mechatronic unit must be dismantled from the gearbox and the control unit separated from the valve body. After that, it proceeds to a laboratory, where it is fully inspected to determine the status of the unit (fault on solenoid control, speed sensor, range sensor or mainboard electronics). 

After inspection, necessary repairs can be done on the areas affected. Sometimes, repairs are not possible, when this is the case, there is a need for a reconditioned control unit. In case of a defect, they are replaced with new solenoids.

If a mechanical defect arises after diagnosis, the mechatronic unit is dismantled, and all mechanical components are inspected, including the valve body, gearbox and torque converter. 

The valve body will be checked, and the necessary repairs can be carried out. It is then disassembled into its composite elements. After that, a test is carried out for the presence of any freeze in the transmission fluid. 

The oil passages in the plates are visually inspected for residues or any foreign particles. After this is done, the valve body is cleaned up. If there are still some foreign particles left, then it means there is a serious problem in some of the mechanical parts of the transmission, which need to be located and removed.

To determine wear in the valve seat, two different tests are carried out. The first is performed manually by a valve machine for each valve. The other test is carried out on a specialised high-tech stand, which stimulates the pressure and temperature of the valve body in real conditions. The correct or incorrect operation of the valve is known with the help of chats. If the test conducted is passed successfully, the valve body is then assembled to the control unit, and the mechatronic is mounted in the gearbox. 

(Blog from carpart.com.au)