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Powertrain Control Module (PCM): Brains of the Car

The PCM controls ignition timing, fuel delivery, valve timing in engines with Variable Valve Timing), emissions functions, turbo boost pressure in turbocharged engines, idle speed, throttle position and cruise control.

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The Powertrain Control Module (PCM) is the electronic brain that controls almost everything the engine (and transmission in many cases) does. It controls ignition timing, fuel delivery, valve timing in engines with Variable Valve Timing), emissions functions, turbo boost pressure in turbocharged engines, idle speed, throttle position and cruise control.

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The PCM also controls the automatic transmission in vehicles that do not use a separate Transmission Control Module (TCM). It also communicates with the Body Control Module, ABS/traction control/stability control module, electronic steering, climate control system, various lighting modules, keyless entry system and other subsystems to share information, data and interactive functions. In short, the PCM is involved in a lot of things that reach far beyond the engine itself.

Take the anti-theft system as an example. The PCM looks for a coded signal from a key fob or the key itself when a motorist enters their vehicle. With late-model push button start applications, the PCM won’t activate the starter if it doesn’t see the correct code from the key fob. With older key ignitions, the PCM won’t energize the fuel pump circuit if the key code isn’t correct. Consequently, no-start problems can be caused by communication issues between the PCM, ignition switch and key or key fob, by a fault in the PCM’s anti-theft circuitry, or by a problem in the starter, ignition or fuel delivery system. That’s why accurate diagnosis of any suspected PCM-related fault is so important. There are often many different possibilities that may be causing a problem, and the PCM is only one of the possible causes.

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The Onboard Diagnostic system can detect many kinds of internal PCM faults such as memory issues, math errors and driver failures. But some PCM problems won’t set a fault code and can only be diagnosed by a process of elimination. Unfortunately, some do-it-yourselfers as well as professional technicians jump to conclusions prematurely and condemn the PCM before they have ruled out all of the other possibilities. Consequently, they replace the PCM only to discover the newly installed PCM doesn’t fix their problem. They often blame the replacement PCM and return it because they think the unit is defective. But the real fault is not in the PCM but their failure to diagnose the vehicle correctly.

The PCM is an expensive piece of hardware, so it should not be replaced unless there is good evidence that a problem is within the PCM itself and not something else. Many drivability and emissions problems can be cured by simply reflashing the PCM with updated software from the vehicle manufacturer. This requires a J-2534 pass-thru tool or a professional grade scan tool that is capable of reflashing a PCM, and access to the OEM service information website via the Internet to obtain the download.

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If the fault turns out to be the PCM itself, it’s essential to get all of the application information so the correct replacement PCM and calibration can be obtained for the vehicle. This includes the obvious information such as year, make and model of vehicle, engine, transmission, emissions calibration and/or VIN number. You may also need the part number on the original PCM. On older PCMs with removable calibration chips, the chip needs to be swapped to the replacement computer.

Some aftermarket suppliers of PCMs provide their “reman” PCMs preprogrammed and ready to run, while others supply their PCMs unprogrammed, which requires the installer to load the calibration software. Many late-model PCMs also require special “learning” procedures to be performed after they have been installed. This may require a scan tool and/or special manual procedures so the PCM can learn the basic settings it needs to operate various devices. Some accessories or subsystems may not function normally until the required learning procedure(s) have been performed.

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