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The engine management system controls all of the engine’s vital functions including spark timing, fuel delivery, camshaft timing (on engines with variable valve timing), and various emission functions. It also controls the cooling fan, fuel pump, starter motor, charging system and interacts with the transmission control module (if it is a separate component), the Antilock Brake System, traction control system, stability control system, climate control system, body control module, anti-theft system and most of the other electronic systems that are found in vehicles today.
The Powertrain Control Module (PCM) is the brain of the engine management system. It makes all the decisions that control the fuel mixture, throttle opening, ignition and overall engine operation based on information it receives from its many sensors as well as other modules in the vehicle.
Since 2008, all new cars and light trucks have been equipped with “Controller Area Network” (CAN) electrical systems that allow the PCM and other modules to share data and interact. CAN electrical systems started phasing in as far back as 1992 on some European imports, so many late-model vehicles have them. What this means is that some types of engine performance and emissions problems can now be caused by other systems in the vehicle. That makes diagnosis more complicated than ever, and also requires up-to-date professional grade scan tools that can do more than just read codes or basic OBD II data.
The OBD II system keeps a watchful eye on how well the engine management system is doing with respect to keeping emissions within limits. But it won’t always set a code or turn on the Check Engine light when a problem occurs. OBD II can detect most engine-related sensor failures, engine misfires, fuel vapor leaks (anything from a tiny pinhole to a loose or missing gas cap) and communication faults within the CAN system or PCM itself. But it can also miss some problems, or fail to set a code when there is an obvious problem such as a no-crank, no-start or intermittent stalling.
All too often, people rely on fault codes to tell them what to replace. If there’s no code, they don’t know what to do. And even when there is a code (or multiple codes), the code doesn’t tell you what to replace. It only tells you a fault has occurred in a particular system or circuit. A misfire code, for example, tells you the engine is misfiring but not why it is misfiring. It could be fuel-related, ignition-related or compression-related. Finding the cause requires more diagnostic tests and a thorough understanding of how the engine management system works.
Many problems that are blamed on the PCM are not the computer’s fault. A high percentage of replacement PCMs that are returned under warranty (either because the PCM failed to fix the customer’s problem or because the engine did not run properly after it was installed) have “no fault found.” The problem is misdiagnosis and jumping to conclusions.
PCMs that are defective may have been damaged by “installer error,” a catch-all term that means somebody screwed up somehow when they installed the part. They may have crossed wires, zapped the electronics with a jolt of static electricity, disconnected or reconnected wires while the key was on, or failed to follow the proper reset, relearn or reinitialization procedures that may be required to get everything working again. PCMs typically fail for one of two reasons: voltage overloads (often due to a short in a solenoid or actuator circuit) or environmental factors (corrosion, thermal stress or vibration). If the shorted solenoid or actuator isn’t found and repaired, the replacement PCM may suffer the same fate.
As for environmental factors, water is the main thing to avoid. When water gets inside a PCM, it can short circuits and set up irreversible corrosion that ruins electronic connections. That’s why most insurance companies consider flood-damaged vehicles a total loss. The electronics will never be the same again. Even if they still work, corrosion will likely cause all kinds of problems later.
If a PCM has failed and a replacement is needed, it must be the correct one for the application. PCMs are wired differently and calibrated differently for various makes and models. To accurately identify the application, you will probably need the vehicle year, make, model, engine size, VIN number and the OEM part number on the old PCM.