Powertrain control module
 | This article contains instructions, advice, or how-to content. (May 2012) |
A powertrain control module, abbreviated PCM, is an automotive component, an electronic control unit (ECU), used on motor vehicles. It is generally a combined control unit, consisting of the engine control unit (ECU) and the transmission control unit. On some cars, such as many Chryslers, there are multple computers: the PCM, the Transmission Control Unit, and the Body Control Module, for a total of three separate computers as an example. These automotive computer's are generally very reliable. The PCM commonly controls more than 100 factors in a car or truck. There are many hundreds of error codes that can occur, which indicates that some subsection of the car is experiencing a problem. When one of these errors occurs, usually it will turn on the "check engine" light on the dashboard. The PCM is one of potentially several onboard computers, or essentially the "brain" of the engine control system. When the "brain" does not function correctly, neither will the engine or anything else that the microprocessor controls, which may include the charging system, transmission, various emission controls and communications with other onboard control modules. The PCM should be replaced only when it is diagnosed to be defective.
On board Diagnostic (OBD II) diagnostic trouble codes that typically indicate a fault with the powertrain control module include:
- P0600....Serial Communication Link
- P0601....Internal Control Module Memory Check Sum Error
- P0602....Control Module Programming Error
- P0603....Internal Control Module Keep Alive Memory (KAM) Error
- P0604....Internal Control Module Random Access Memory (RAM) Error
- P0605....Internal Control Module Read Only Memory (ROM) Error
- P0606....ECM/PCM Processor
- P0607....Control Module Performance
- P0608....Control Module VSS Output
If you see any of these codes when diagnosing the vehicle with a code reader or scan tool, it may mean the PCM has failed and must be replaced. There can be other reasons for these codes as well, so the presence of any of these codes does not definitively mean the PCM needs to be replaced. Additional diagnostic tests will usually be necessary to confirm the problem is really the powertrain control module and NOT something else. Refer to the OEM diagnostic charts for what these tests are. Usually it involves checking certain inputs to the PCM to see if it outputs the correct response. No response or an incorrect response usually means the PCM is defective and needs to be replaced. Professional Automotive Technicians use extremely expensive equipment ($4,000 to $8,000) to do the extensive and complicated diagnostics that are required before eliminating all other possibilities, to be able to arrive at a firm conclusion that the PCM has failed.
The primary inputs to the PCM come from many sensors, of different types, that are spread around the car. Most of them are oriented toward engine management and performance. These sensors fail at a much higher rate than any of the computers do.
Accurate PCM Identification
Because there are so many different PCMs, accurate identification of the PCM and its correct replacement is absolutely essential to prevent unnecessary returns. Accurately identifying the PCM requires not only the vehicle year, make, model and engine size, but also the OEM part number on the PCM itself. Most supplier catalogs list replacement PCMs both ways. Many PCMs appear to be exactly the same on the outside (same sized box and connectors) but may be wired or calibrated differently inside.
If the wrong PCM is installed in a vehicle, it may run but probably will not run well. Close enough is not good enough when it comes to replacing PCMs. It must be the correct replacement to work right. Always refer to the OEM number on the PCM and look it up in the suppliers cross reference index to find the right part. On some occasions, after the new PCM is installed, it requires a "flash" operation, to "adapt it" specifically to the vehicle it is installed in. This flash can also set the computer into a more advanced condition that takes advantage of knowing the history of the vehicle, such as how many miles are on the car, how efficiently the fuel injection to the engine has been working, and other factors. This flash is usually performed at a dealer.
Bibliography
Duffy, James E. (2003). Modern Automotive Technology.
Ribbens, William B. (2002). Understanding Automotive Electronics.