A Failing Powertrain Control Module Has a Range of Potential Symptoms

One of the most critical and reliable parts on the majority of vehicles is called a powertrain control module (PCM). It’s often referred to as the heart of the vehicle, but more precisely should be called the brain of the engine. Every active component of the powertrain relies on the PCM to tell it what to do and when to do it. When there’s a problem with the vehicle, it might be the PCM. The symptoms can mimic other issues, but there are some signs that often point to a faulty unit.

The Days of Yore

Prior to the introduction of computers into vehicles, the powertrain was largely controlled by the changing state of vacuum and pre-set timing. A distributor would send the ignition spark to different cylinders based on its position in the rotation, modified by a vacuum hose.

Originally, this was done using points that would open and close based on springs, weights and vacuum changes. Eventually, the points changed to electronic ignition – a major improvement. Some systems, such as GM’s high energy ignition (HEI), were so revered, they were aftermarket installed on non-GM vehicles. However, they still didn’t offer control over the entire system, and as emissions and gasoline mileage became more important, even electronic ignition wasn’t enough.

The PCM Fixed the Inefficiencies Native to Mechanical Analog Control

The PCM was the solution to these issues. The powertrain control module didn’t magically appear overnight; it was gradually developed from the first attempts to computerize cars in the 1960s and 1970s. While the early computers were generally disappointing, the experience showed engineers that digital electronics were the future.

Once the kinks were worked out, it turned out that making vehicles with a powertrain control module was not only amazingly effective at saving gas and improved emission controls, but that it was actually cheaper to manufacture them with a PCM than without. Computer prices dropped extremely rapidly in the late 1970s, and by the 1990s, it was effectively inconceivable to build a vehicle without a central computer.

Instead of relying on rotation and vacuum to determine the perfect timing for an ignition spark, the PCM uses at least 100 factors to create ideal conditions inside each cylinder for the most efficient use of gasoline. The system uses sensors throughout the powertrain to update the PCM thousands of times per second. By ensuring all the parts are working in synchrony, a significant reduction in wear and tear occurs, friction and losses are reduced, and more power is produced using less gasoline.

The PCM Is a Bit of a Genius

Today’s powertrain control modules are much like the technology in your smartphone. While they don’t require as fast a computer, they use very advanced algorithms to make the most of the chips inside the PCM to provide extra power when it’s needed and save gas when it’s not.

Each manufacturer uses a different set-up for the PCM. Some have the PCM separate from the other drivetrain control units, such as the engine control unit (also called the engine control module or ECM) and the transmission control module (TCM). Chrysler is known for using that configuration, but most automobile manufacturers combine all major control modules into one housing. Technically, each unit is still separate from each other, but when they are all inside one module, they must all be replaced together when any of the control units aren’t working properly.

An Engine With a Bad Powertrain Control Module Can Do Funny Things

A working PCM combines a great deal of data, such as the air-to-gas ratio, fuel mixture, fuel economy and precise timing to create a very smooth operation regardless of the weather or power demands. It uses this data for overall performance monitoring based on a matrix of data stored in its memory banks. If there’s a problem, it can manifest as several symptoms, since all the components in the powertrain depend on each other for the best performance. For instance, if the ignition spark is off as little as 2/10ths of a second, the engine can misfire and even start shaving off metal inside the cylinder.

Many other symptoms may – or may not – indicate a bad PCM, including

  • The engine running rough
  • The vehicle is hard to start
  • The exhaust is smoky or smells odd
  • The check engine light is on
  • The gas mileage unexpectedly worsens
  • The engine stalls

The most infamous of these is the check engine light. While this – like all the other symptoms listed above – can be caused by other issues, it is frequently a sign that the powertrain control module needs replacement.

A Faulty PCM Also Means You Should Locate and Fix the Cause

Since all the symptoms could have several causes, many PCMs are replaced unnecessarily. In fact, the most common problem isn’t usually the unit itself but a bad sensor or sensor wiring. Each sensor is much closer to the vibrating powertrain as well as inclement weather conditions. The PCM itself is normally either inside the passenger cabin (often under a seat) or under the hood, and is contained in a rugged plastic housing, so it doesn’t usually get much direct contact.

However, if the PCM is bad, the cause needs to be identified, or the new unit could suffer the same fate. For instance, a voltage overload is a common cause of a bad PCM. If wiring develops a short circuit, it can instantly zap the module. Check the starter solenoid or actuator to make sure they aren’t the problem.

Water intrusion is also a cause of failing powertrain control units. If the housing gets even a hairline crack, water can be forced inside – almost a guarantee of a dead computer. If the unit is located under the seat, spilled drinks have been known to fry the computer.

Excessive vibrations are a somewhat less likely cause of failure. If an engine mount breaks, the stress can build up in the housing, causing tiny cracks. However, if caught in time, the housing can be repaired.

When you suspect a failing PCM, you’ll need to take it to a shop with a diagnostic computer. Most PCMs will provide special error codes that can quickly identify the problem. Some issues can be repaired, but in most cases if the unit is failing, it needs to be replaced before engine damage happens.