A good closer look into Mechatronics Units testing

The Go4Trans Team is happy to present to you part 1 of the 2-part research over the testing methods of TCMs / mechatronics units. For part 2 - click here.

Every automatic transmission specialist has their own method of dealing with Transmission Control Modules and mechatronics units issues. Some rebuild them, some replace them, but most curse this complex part of the gearbox. However, do we all know them well? To begin, let us review on what we know about valve bodies.

Valve body as part of the automatic gearbox

The main purpose of the valve body (VB) in the automatic transmission structure is an accurate shifting of gears. The valve body design includes a labyrinth of channels and passages within it and they transfer automatic transmission fluid (ATF) to the numerous valves. These valves actuate the corresponding clutch package or band servo to engage a specific gear suitable for each driving situation. All valves in the valve body have a particular purpose; their names frequently refer to their functions (for example, the 2-3 shift valve – is responsible for the 2nd gear to 3rd gear upshifting). The manual valve, which is directly attached to the gear shifting lever, is the most crucial valve. The driver directly controls the position of this valve, it opens and closes various passages depending on the gear stick position. For example, when the driver shifts the lever to the D gear, the manual valve directs ATF to the clutch pack(s) that engages 1st gear. It also intended to keep track of car speed and control throttle position so that it can specify the optimal time and the force for the 1-2 shift.

Image of valve body assembly and it being disassembled

On computer-controlled transmissions (or, in other words, those coupled with Transmission Control Units), there also are electrical solenoids mounted in the valve body to direct fluid to the appropriate clutch packs or bands under computer control. This allows for more precise control of shift points.

Clarification of the terminology: TCU / TCM / mechatronics / TEHCM / other names and brands

A transmission control unit, or TCU, is a special mechanism that controls operation of advanced automatic transmissions. Thanks to the information received from numerous sensors and the engine control unit (ECU), TCU calculates how and when to shift gears in the car for best operational characteristics, fuel efficiency, and gear change quality. It should be kept in mind that ECU and ECM are two completely different units. ECM is short for ‘electronic control module’; while TCU is also known as TCM (short for ‘transmission control unit / module’, accordingly). In some cases, the TCU and the ECU are combined into a single unit as a powertrain control module (PCM).

Image of a TCM box

The Transmission Control Module takes in signals from numerous sensors and then employs this data to actuate the torque converter clutch (TCC), monitor line pressure and ensure well-timed gear shifting. The correctness of the TCM operation is ensured by the fact that the unit compares the preset (master) data with the inputs before giving any commands to subordinate elements. The brake controller transmits a signal to the computer when the brakes are activated. Thanks to this signal, the TCM controls the TCC. The list of sensors implemented in automatic transmissions includes the following: engine coolant temperature sensor (ECT), mass air flow sensor (MAF), throttle position sensor (TPS), transmission fluid temperature sensor (TFT), turbine shaft speed (TSS), and more. Thanks to these numerous components, the process of gear shifting is performed timely and efficiently. The quality of transmission operation has a direct impact on other car components.

ECMs, TCMs, and PCMs are built with microchips that can be reprogrammed electronically. This software update procedure is called reflashing, or reprogramming process. Another names you definitely have come across: mechatronics units and TEHCM. TEHCM acronym: TRANSMISSION – ELECTRO – HYDRAULIC – CONTROL – MODULE. TEHCMs are units combining the functions of the transmission control module, valve body solenoids, and pressure switches/sensors and temperature sensors into a single assembly. TEHCM is the name for the assemblies employed in various 6T and 6L makes by GM.

Mechatronics units are a combination of both electrical and mechanical systems. In fact, mechatronics as a branch of engineering includes even a wider combination: that of robotics, electronics, computer, telecommunications, control, and product engineering. Mechatronics units are employed in German and American transmission makes by ZF, Getrag, Borg Warner, LUK.

Clarification of differences between TCU / TEHCM / Mechatronics unit

While there is no established explanation on the difference between a TCU and a mechatronics, key difference lies in location of the systems in the transmission design (the mechatronic is located inside the transmission, while the TCU, is usually installed elsewhere on the vehicle). Moreover, a mechatronics unit has numerous built-in sensors, while a TCU receives required data from its sensors remotely.

Non-professionals and inexperienced repair specialists frequently get confused and mix up these definitions. So let us make things clear, and briefly specify the main differences between TCM and TEHCM. As stated above, these units have a lot in common. Both mechanisms collect and analyze vehicle characteristics and issue adequate commands to ensure accurate gear shifting. They differ in the following aspects: location, technical complexity and associated type of transmission. The TCM sits on the outside of the transmission (for example, the GM 8-Speed). The GM’s TEHCM combines the function of a TCM with valve body solenoids, integrated electronic and hydraulic elements, pressure and temperature switches, filter plate and a modified lead frame – all in a single unit (for instance, the GM 6-Speed).

Image of tcm vs mechatronics

One may wonder: why they invented the TEHCM? To avoid speculation, we stick to treating it as a brand name for these TCMs employed in the specific GM makes. It’s worth adding, that there is a certain growth in the demand for replacement remanufactured TEHCM units, which shall be primarily attributed to high cost of new OEM units.

Modern applications of TCMs / TEHCMs

We believe that the industry is in for a trend of moving away from the troublesome TEHCM and mechatronics back to external TCMs (as with ZF9HP). Key reasons for that be high cost of replacement of the former and easier repair procedure for the latter. Obviously, TCMs will remain an intrinsic part of the modern non-manual gearbox – by making automatic transmissions ‘smarter’, manufacturers have been able to increase fuel efficiency and driving comfort considerably. Smoother shifting and improved vehicle handling for the cars with TCMs is complemented by reduced engine emissions, which has become a truly serious concern in the 21st century.

How TEHCM and mechatronics units are coupled with valve bodies

It would be a serious dig into a sophisticated topic if we were to describe how TEHCM and mechatronics units are coupled with valve bodies. To put things simply, it shall be stated that TEHCM / mechatronics units are designed to comprise a truly single unit including the valve body in it. It shall be noted that there are many cars with various -tronic units available on the present-day automotive market: Multitronic / S Tronic in Audi, 7G-Tronic / 9G-Tronic in Mercedes, Easytronic in Opel / Vauxhall, Tiptronic in Porsche, etc. Some of these names are highly popular nowadays but the likes of Tiptronic or Easytronic are familiar to a smaller audience only.

Just to give you an impression on how seriously valve bodies are integrated into the -tronic products, we can tell of such a fact: according to the info we had back in 2018, Mercedes-Benz declined to sell replacement valve bodies to independent repair shops claiming that these components were related to the security of the vehicle. Yet they would sell complete transmissions. Mercedes-Benz claims that they will not sell the valve body because it could aid in automobile theft. The nature of such a threat lies in the integrity of the assembly with the immobilizer device, which is servicing for electronic security and is fitted to an automobile preventing the engine from running unless the specific token (digital key) is present. Cars that have an immobilizer (usually dated 2000 and up) will most certainly need to be reprogrammed by a locksmith, dealer, or licensed mechanic. Their key purpose is prevent the car from being "hot wired", when a motor vehicle's ignition interlock could be bypassed and thus started without the physical key.

Immobilizer sign picture

Some transmission specialists claim that such a restriction from Mercedes for selling valve bodies was due to the policy of providing dealers with the unfair advantage of being the only party able to repair these cars, while independent repair shops were limited at being able to fix these cars.

Mechatronics are single units being an integrity of the valve body, as hydraulic pressure distributor, control unit, as sensor data analyzer, and solenoids. Some repair shops provide the service of mechatronics units repair by repairing control units or replacing them and by doing a thorough job on valve bodies with valve replacement and reaming if required. More generally, mechatronics units are simply replaced.

TCM flashing and virginizing - why and when required?

The Internet has certain articles of information on TCU coding and updating. TCU coding is fundamentally related to reduced fuel consumption and subsequent emissions control. Therefore, as a rule, one should know the EHS (Environment, Health, and Safety) number to do correct software update and do SCN (Software Calibration Number) coding. The Volkswagen emissions scandal has shown how serious this matter is treated these days.

‘Virginizing’ means complete erasing of the vital file from the chip; ‘flashing’ means programming the blank chip with the appropriate new file. In the industry forums one may easily come across the term ‘resetting’ which denotes the same process of wiping out the key file off the chip. Shops that allow for TCM virginizing / reflashing do not disclose this precious information regarding the implemented technology but basically in large industrial cities there certainly are available solutions. The situation in smaller towns or in less industrial regions is different and such services as virginizing / reflashing are scarce.

Some shops claim to have insights that allow for facilitation of the repair procedure for certain modern makes. For instance, a renowned USA company has a solution to eliminate the need for any programming on vehicles that utilize the 7G Tronic - however, such insights remain a company’s know-how and any ideas to disclose them are to be sought separately.

On approaches to testing and repair problems with TCMs

We would like to imply that there’s definite scarcity of publicly-available information on proper testing for TCMs and remanufacturing of its varieties, which leaves this subject as ‘terra incognita’ for the common drivers and even the majority of general automatic transmission repair specialists. For instance, an official guide by ZF on BMW 6-speed mechatronic replacement says nothing about where one should be getting a new or remanufactured mechatronics unit. The part concerning reprogramming is quite laconic too: ‘Operating vehicle in failsafe mode may cause transmission damage. If on-site programming tools are not available then the vehicle must be towed to your vehicle specific dealer for programming’.

As practice shows, mechatronics units are not highly reliable and these devices are expensive and complex in terms of design. For a number of reasons, until recently, these units were considered to be non-repairable. In other words, both official and unofficial services in case of problems with mechatronics were often limited only to flashing and / or replacement of the unit.

Today, the situation has changed somewhat, as the widespread use of DSG boxes and the faults associated with this transmission first generated demand and then supply. Repair of mechatronics and DSG boxes in general has become one of the available options that allow to avoid complete replacement of expensive components.

Picture of a DSG box mechatronics unit

Multiple repair shops claim on their websites that they allow for the following services with mechatronics units: ensuring that all electrical currents are measured and adjusted precisely. Shift pressure testing for mechatronics units, calibration of the control modules with the individual pressure actuators. Provision of corresponding algorithms for control of the module’s permanent memory. Complete reprogramming for the mechatronics units, etc.

Professional editions, which are scarce, provide better and clearer pictures on what is the right approach with TEHCM / mechatronics units failures. Always look for any factory Technical Service Bulletins (TSB’s) or re-flashes before removing the transmission from the vehicle. In many cases, some specialists have removed a transmission to find out that it is fine internally. Later and after they mount it back on the car they find out the problem merely needed a software update.

Picture of Technical Service Bulletins

Some transmission shops provide precise guides on how to remove of a mechatronics unit and send it to their facilities for repairs. Such approach seems fairly valid too. Generally, there is a tip that transmission shops should definitely use a tester for the basic settings, oil changes and for trouble codes. These testers are not always universal and each variations may require a particular tool.

Regarding the scanning equipment: it is clear that the OE main dealers supply the best scanning tools for the job however that means owning many different tools to cover all the the different transmission types. Aftermarket tools are available that can be used as a more universal tool but they won't be as reliable across all the types as a OEM tool.

Fortunately, in some cases the problem is caused not by the module itself, despite the fact that the signs of malfunction may be quite similar. The issue can be triggered by a blocked passage or a defective valve solenoid in the transmission. The module cannot operate correctly because it does not receive the required amount of voltage supply due to certain problems with the mentioned elements. In that case, the solution is quite simple – you just have to replace the faulty component and avoid unnecessary expenses.

TCM learning as part of transmission work cycle

The TCM unit calculates individual clutch parameters by means of a special procedure for 6-speed automatic transmissions (the transmission adaptive values learn). When the TCM gets the clutch info, the procedure converts it into the adaptive data units, which the TCM uses for clutch control at speed changing.

The scanning tool allows the transmission experts initiating the transmission adaptive values learn procedure, which should be executed after any of the following repair services: transmission internal service or rebuild, valve body repair or replacement, control solenoid valve assembly replacement, TCM software or calibration update, any service in response to shift quality. Keep in mind that during the process of a TEHCM programming (re-programming), the software and calibration adjustments are eliminated as part of the process. If the programming process can’t be finished because of a conflict, the TEHCM won’t operate correctly. The system will then switch to limp mode.

If is essential that a repair shop performs diagnostic checks and test drive using a scan tool. A quote for a repair without checking for bulletins and updates could prove costly in the long term. Today a repair shop specialist needs to look at the whole vehicle and all control modules associated with powertrain management. This is the end of part 1 from our 2-part insight into the TCM / mechatronics units testing.

This is the end of part 1 from our 2-part insight into the TCM / mechatronics units testing. For part 2 - click here.