What Does the Powertrain Control Module Do?
SPECIAL NOTES: PCMs, aka Powertrain Control Modules go by several different names, including, but not limited to the following-
- PCM – Ford / General Motors
- DME/DDE – Digital Motor Electronics/ Digital Diesel Electronics (BMW / MINI)
- INJECTION CONTROL UNIT- (Peugeot, Citroën, Fiat, Alfa Romeo, Lancia)
- ECM – Engine Control Module
- ECU – Electronic Control Module
The above names/abbreviations are often used interchangeably by repair shops, mechanics, and even dealerships in the US automotive market, which might imply that all of the above control modules perform the same range of functions, but the fact is that PCMs are somewhat different from the other modules.
From an engine management perspective, the most notable difference is that PCMs, usually, (but not always) control both the greater engine management system and the transmission on Ford and General Motors products, which is generally not the case with some otherwise-named [engine] control modules.
Note also that the term “ECU” is often used by many mechanics and repair shops to refer to engine [management] control modules. This usage can be confusing because “ECU” is a generic term that is best used to refer to control modules that are not involved in engine management and transmission control systems. Some examples of this would include the ABS Control Module, Body Control Module, and Power Steering Control Module.
Based on the above, this guide will use the term PCM (Powertrain Control Module) throughout to avoid confusion and misunderstandings. END OF SPECIAL NOTES.
Regardless of whether a PCM is called a PCM or some other name, all PCMs are the primary processing hubs for the vast majority of data a vehicle generates during use. These processing functions include, but are not limited to, controlling the ignition, fuel, and emission control systems, as well as acting as a sort of “clearing house” that manages communications between groups, or classes of control modules on the vehicle.
Put differently, no vehicle can function if the PCM does not process data generated by systems like fuel delivery, ignition, and variable valve timing that are not only closely integrated; these systems are also interdependent to operate as expected. Therefore, the PCM is arguably the most critical component in the larger data processing, data transmission, and fault detection processes that, collectively, form the basis of all vehicle control strategies. So, put simply, if the PCM does not work as expected, the vehicle cannot function as designed.
Why is the Powertrain Control Module Needed?
Today there is hardly a function on modern vehicles that is not monitored, controlled, or managed by one or more electronic control modules, which in their simplest form, are small computers, aka microprocessors. Moreover, modern vehicles can have up to 50 or more control modules/microprocessors that are interconnected with each other by several hundred feet of wiring.
We need not delve into the complexities of how the dozens of control modules on an average modern vehicle work or communicate with each other but suffice it to say that even a compact vehicle has several serial communications systems that transmit data between control modules in a strictly hierarchal system. Put differently, this means that critical data, such as information from the wheel speed sensors, or input data from the ignition system will be transmitted before data that concerns the operation of say, the power windows will be transmitted.
In practice, though, the above is a gross oversimplification of an incredibly complex conglomeration of communications systems that all have different functions and data transmission speeds. Nonetheless, as a practical matter, no rational communication would be possible between control modules unless there was a central “clearing house” in the form of a PCM to manage and maintain the rationality, integrity, and precedence of individual data transmissions between both control modules and serial communications networks.
How Does the Powertrain Control Module Work?
In its simplest form, a PCM is just a computer that is programmed to receive inputs from sensors and subordinate control modules, and then compare the input data with expected values. However, it is essential to note that PCMs are not “smart”, in the sense that they can take autonomous actions or decisions on how best to run or manage the vast number of systems and functions that make a modern vehicle work as designed.
As a practical matter, though, modern vehicles generate several GB of data every second it is in operation, with most of this data deriving from a multitude of sensors that monitor many critical operations continuously. In fact, a modern vehicle generates so much data that a PCM requires up to 100 million* lines of code to process all of it efficiently.
* By way of comparison, a modern heavy passenger airliner requires only about 15 million lines of code to operate safely.
We need not delve into the complexities of actual PCM programming protocols here, but suffice it to say that the bulk of a PCMs programming consists of extensive lookup tables and other types of pre-programmed data, including various forms of memory that serve as reference points against which sensor inputs are evaluated or assessed for accuracy, plausibility, and/or rationality.
We can put the above into context by using a practical, albeit a simplified example of how a PCM works-
All modern vehicles require a specified minimum pressure to be present in the fuel rail before the engine will start. The PCM “knows” the value of the minimum allowable fuel pressure because it is programmed with this value, but modern fuel delivery systems are also equipped with fuel rail pressure sensors that monitor the actual fuel pressure continuously.
In practice, the PCM will activate the fuel pump briefly to build pressure in the fuel system when the ignition is switched on. The fuel rail pressure sensor then relays the actual fuel pressure to the PCM, and if the actual fuel pressure (as reported by the fuel rail pressure sensor) agrees with the pressure values that are programmed into the PCM, the PCM will activate the starting circuits to allow the engine to start.
If, however, the actual fuel pressure is lower than the minimum pressure the PCM expects to see, the PCM will not activate or enable the starting circuits, thereby preventing the engine from starting to prevent damage to the engine from occurring as a result of the fuel pressure falling outside of an acceptable range of pressures.
Based on the above, it should be clear that the PCM does not get to “decide” what the fuel pressure should be for the engine to start. In this example, the PCM merely compares an input (from the fuel rail pressure sensor) with a preprogrammed value, and if the two values agree the PCM creates an output, which in this example, is the activation of the starting circuits.
Conversely, if the input does not agree with a preprogrammed value (the minimum allowable fuel rail pressure) the PCM will not create an output, which in this example, is the activation of the staring circuits. Note that in the context of the role PCMs play in the overall control and management of critical systems on modern vehicles, an “output” is best defined as a preprogrammed response to an input that does not meet expected criteria or parameters.
So while the operating principles of PCMs revolve around the comparison of inputs with programmed values, and then creating outputs based on the results of said comparisons, recent and ongoing advances in computing technologies have greatly increased the number of operations an average PCM can perform per second. Put differently, this means that modern PCMs can process vastly more data in much less time than their predecessors could, but at their cores, PCMs are just computers that use lots of code to produce outputs based on the inputs they receive.
Where is the Powertrain Control Module Located on the Engine?
This image shows the location (circled) of the PCM on a 2009 Ford F150 truck. On most vehicles, the PCM is located under the dashboard, but other common locations include placements in center consoles, under front seats, inside the engine compartment (as in this example), or in some cases, in wheel wells behind plastic splash guards, as on some Ford and Chrysler products.
Note, though, that since these are also common locations of other stand-alone control modules, we strongly recommend that reliable service information be consulted to locate and identify the PCM correctly.
Note also, that even if the PCM is easily accessible, it a) contains no user-serviceable parts and b) that merely disconnecting a PCMs electrical connector(s) could not only cause the loss of some control functions but potentially also the corruption of critical driver circuits and erasable memory capabilities. For these reasons, we do NOT recommend that non-professional mechanics disconnect, remove, install, or reprogram any control module(s), and particularly not PCMs.
What Does the Powertrain Control Module Look Like?
This image shows the appearance of a typical PCM, but note that the dimensions and construction of PCMs vary somewhat between vehicle makes and models.
For instance, some PCM have simple plastic or sheet metal cases, while others are contained in aluminum cases that incorporate cooling fins (as shown in this example) and other features such as complex latching mechanisms that ensure positive latching of the electrical connector(s) that link the PCM to the vehicle’s wiring.
What are the Symptoms that the Powertrain Control Module is Bad?
Symptoms of a defective or failing PCM vary greatly not only between vehicle makes and models, but also as a result of the nature of such defects and/or failures. In fact, in many instances of PCM failure, it may be difficult, if not always impossible to link some symptoms to a defective PCM without the use of a suitable diagnostic scanner because some symptoms that often accompany PCM failures such as-
- cylinder misfires
- non-starting conditions
- harsh or erratic gearshifts, or
- instrument cluster failures
– (among others) can be caused by a wide variety of issues that do not involve defective PCMs at all. Nonetheless, some common symptoms of defective or failing PCMs could include one or more of the following, but note that not all of the possible symptoms listed here will always be present on all vehicles-
- One or more stored trouble code(s)-especially multiple codes relating to a loss of communication between the PCM and several other control modules
- One or more warning lights may be illuminated, depending on the vehicle and the nature of the failure
- The engine may crank normally, but won’t start
- The engine may not crank
- One or more electrical systems may not work, with lighting and instrumentation being common examples of this category of symptoms
- The transmission may be locked in one gear
- It may not be possible to lock or unlock the vehicle’s doors
- On some vehicles, it may not be possible to release the electronic parking brake
How do you test the Powertrain Control Module?
Be aware that confirming a defective or failing PCM as the root cause of one or more of the above symptoms can sometimes be challenging even for professional mechanics, and even more so if the affected vehicle is fitted with a secure gateway module that prevents communication between the vehicle’s PCM and external diagnostic equipment such as unsecured scan tools. Therefore, we do NOT recommend that non-professional mechanics attempt any diagnostic procedure that may involve a PCM and/or other control modules due to the high risk of inadvertently damaging previously undamaged modules during the diagnostic attempt.
Moreover, it should also be noted that even in cases where a PCM is known to be defective, the actual testing of a PCM should only be undertaken by suitably qualified and experienced personnel who have access to both the required specialized test equipment and software, and the relevant OEM service/repair information.
As a practical matter, PCM testing and repair is a highly specialized field of car repair, and as such, these procedures should NOT be performed by persons who are not specialist repairers because PCM repairs performed by non-specialists are guaranteed to be neither effective nor reliable.
How do you replace the Powertrain Control Module?
While the actual physical replacement of a PCM is no more complicated than unplugging the old unit and plugging in the replacement, getting the replacement to work as expected is a whole lot more complicated.
For instance, the new PCM has to be programmed with a software calibration version that is compatible with all the other control modules in the vehicle. This is usually the software version that was in use when the PCM failed, and while this is relatively easy to determine, obtaining the correct programming software might not be as easy for non-professional mechanics, which typically, do not have access to OEM resources and/or third party vendors that supply OEM service information and software.
From a practical standpoint, the best course of action to follow when you experience issues with a PCM is to seek professional assistance with diagnosis and repair. It is perhaps worth mentioning that module programming can fail for a variety of reasons even in a professional environment, meaning that the procedure is even more likely to fail or abort repeatedly during a DIY programming event.
At the risk of overstating the case, PCM replacements and subsequent programming procedures are best left to professional mechanics who have the knowledge, experience, equipment, and technical information to ensure that the replacement is performed to OEM standards and specifications.