What Does the Transmission Control Module Do?
As its name suggests, the transmission control module controls and manages gearshifts on automatic transmissions, as well as control and monitor the operation of the torque converter lock-up clutch to improve the quality and feel of gearshifts.
Why is the Transmission Control Module Needed?
There was a time when automatic transmissions did not need electronic control modules to function. These iterations of transmissions were controlled by the engine vacuum, and while gearshifts were generally harsh, this control mechanism did not only work reasonably well; it was also robust and reliable for as long as the vacuum line that connected the transmission to the engine’s intake manifold was free of leaks or restrictions.
However, this control mechanism did not work well with the first fuel injection systems, and as fuel injection technology developed, car manufacturers began devising ways to improve the quality of gearshifts by adopting progressively more complex electronic systems that used engine speed and load data as the primary input for the first rudimentary electronic transmission control systems.
We need not delve into the fine details of the history of the development of transmission control systems here, but suffice it to say that modern transmissions cannot work without the highly developed control modules that form integral parts of modern engine management systems. Put differently, this means that the transmission control module on any vehicle with an automatic transmission is a critical component, without which a modern vehicle cannot operate.
How Does the Transmission Control Module Work?
It is perhaps worth mentioning the fact that modern transmission control modules can, and do perform several tasks. Although their primary function is to control and manage gearshifts, a modern transmission control module also controls the engagement of the torque converter lock-up clutch to improve the quality of gearshifts, while at the same time, working with the engine management system to retard the ignition timing to reduce engine power during gearshifts.
In terms of operating principles, though, a transmission control module is not much different from any other modern control module, except of course, for its programming. In practice, however, transmission control modules receive inputs from the engine control module regarding the engine speed and load via a complex interface between the engine and transmission control modules.
In addition to this data, the transmission control module also uses data it generates itself. This includes information about-
- which gear is currently selected
- the rotational speed of the torque converter turbine
- the rotational speed of the transmission output shaft
- the pressure of the transmission fluid in the transmission’s principal hydraulic circuits
- the temperature of the transmission fluid
- the ON/OFF status of the shift solenoids
- the integrity of communications between itself and other control modules, which usually include the engine control module, throttle control module on drive-by-wire systems, and some ADAS modules like the stability control, traction control, and adaptive cruise control modules
Provided all input data are valid, correct, and/or plausible, the transmission control module will calculate appropriate shift points and execute gear shits by de-energizing some shift solenoids, and energizing others to activate different hydraulic circuits. Switching pressurized transmission fluid between hydraulic circuits is the mechanism that accomplishes actual gearshifts. Therefore, the transmission control module will monitor all operational parameters very closely to ensure that a) gear shifts occur at calculated shift points, and b) that there are no faults, such as insufficient line pressure or excessively high fluid temperatures, present in the transmission that could affect the operation and/efficiency of the transmission.
If such faults are detected, the transmission control module may lock the transmission into one gear as a safety measure to protect mechanical components in the transmission. In addition, the transmission control module may also assume control over the electronic throttle control system to reduce throttle inputs. In some cases, depending on the vehicle and/or the nature of the failure or problem, a transmission control module may also initiate a no-start condition that will persist until the fault is corrected.
Where is the Transmission Control Module Located on the Engine?
In some cases, the transmission control module may be located close to the transmission, such as in the example (circled) shown above, where the module is attached to the casing of the transmission.
In most cases, though, the transmission control module is incorporated into the engine control module, where it forms a part of the main circuitry of a combined engine/transmission control module. In such designs, the transmission control circuits of the combined module can only be identified by referring to dealer-level service and repair information.
What Does the Transmission Control Module Look Like?
This image shows a combined PCM/TCM (Transmission Control Module) from a 2006 Dodge Charger. Note, though, that since the outward appearance of transmission control modules varies greatly between vehicle makes and models, and as a result, there is no single example of a transmission control module that is representative of all transmission control modules.
Note that in designs like this, where both the engine management system and the transmission control system plug into the same unit, unplugging the wrong connector could cause the engine management system to lose critical programming. Therefore, we highly recommend that you consult reliable service information that applies to your specific vehicle if you need to locate and/or identify the transmission control module to prevent creating serious programming faults.
What are the Symptoms that the Transmission Control Module is Bad?
The most common symptoms of failed or defective transmission control modules are largely similar across all vehicle makes and models and could include one or more of the following-
- Stored trouble code(s) and possibly an illuminated warning light, depending on both the vehicle and the nature of the problem
- Depending on the nature of the problem, multiple trouble codes that relate to serial communications problems, transmission fluid temperature and/or pressure issues, turbine or output shaft speed issues, and/or torque converter lock-up clutch control solenoid issues could be present
- The transmission may fail to select or accept any gear
- The transmission may skip some gears
- The transmission may be locked into one gear
- Gearshifts may be harsh, erratic, or unpredictable
- The transmission may hesitate, or slip severely in one or more (or all) gears upon acceleration
- A no-start condition may be present
NOTE: While all or most of the symptoms listed here typically result from mechanical failures within the transmission, these kinds of symptoms can also be produced by certain types of programming failures or defects in the transmission control module. Additionally, most of the symptoms listed here can also be produced by some categories of serial communication failures between the transmission control module and other modules, including the engine control module and some ADAS-related modules.
How do you test the Transmission Control Module?
It should be noted that a) transmission control modules contain no user-serviceable parts, and b) transmission control modules cannot be tested on a DIY basis.
Having said that though, many specialist repairers can test at least some aspects of a transmission control module’s operation by using highly advanced diagnostic equipment to obtain critical readings and then comparing test readings with OEM specifications- which does not guarantee that the module will work as expected even if all test values agree with OEM specifications.
In the real world, the only objective test of whether or not, as the case may be, a transmission control module works as expected is to install it in a vehicle to see if a) it reacts to all the inputs it receives, and b) to see if the module generates appropriate outputs based on the inputs it receives.
The problem with bench-testing control modules and especially transmission control modules is that it is simply not possible to simulate all of the hundreds of inputs any transmission control module receives every second with any degree of accuracy or consistency- regardless of what many specialist repairers claim
How do you replace the Transmission Control Module?
It should be borne in mind that while it is a relatively straightforward affair to unplug a control module and plug in a replacement if the module is accessible, all transmission control modules have to be either programmed or integrated into the serial communications systems of a vehicle if the module comes pre-programmed.
However, module programming is a somewhat tricky procedure that requires access to not only advanced diagnostic equipment but also access to vendors of OEM-level service and repair information to obtain the correct software files. Module programming also requires specific knowledge of programming procedures, as well as the skill to recognize and prevent programming failures, which is a common feature of control module programming.
If the transmission control module is located inside the transmission (as is common) all of the above still applies, but with the added complication that replacing the module requires some disassembly of the transmission- which comes with its own complications, issues, and potential problems.
Based on the above, we do not recommend that non-professional mechanics attempt a transmission control module replacement on any vehicle under any circumstances. The likelihood of causing severe, and even fatal damage to the vehicle and/or its electoral system is extremely high if mistakes are made, so play it safe, and seek professional assistance with diagnosing and repairing suspected transmission control module issues or problems.