Powertrain control module PCM

Powertrain control module

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A powertrain control module, abbreviated PCM, is an automotive component, an electronic control unit (ECU), used on motor vehicles. It is generally a combinedcontrol unit, consisting of the engine control unit (also known as an ECU) and the transmission control unit. It commonly controls more than 100 factors in a car or truck. The onboard computer, or Powertrain Control Module (PCM),is the brains of the engine control system, so when the brain is not functioning correctly neither is 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. Once a diagnosis has been made (and we emphasize the word diagnosis), then and only then should the PCM be replaced.

Onboard 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 'A' P0609....Control Module VSS Output 'B' P0610....Control Module Vehicle Options Error 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. 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.

All too often, technicians tend to blame that which they understand least. If an engine is not running right and the cause is not obvious, they may blame the computer. Throwing parts at a problem in an attempt to solve it may be good for the parts business, but when a customer brings a PCM back because it failed to fix their problem, nobody wins. Warranty returns on complicated and expensive components like powertrain control modules are an ongoing problem that costs everyone money.

UNNECESSARY POWERTRAIN CONTROL MODULE RETURNS. Over 50 percent of PCMs that are returned under warranty (either because the PCM failed to fix a performance problem or because the engine did not run properly after it was installed) have nothing wrong with them! So it is obvious a lot of people are swapping computers to see if a different PCM will fix their problem.

The trouble with returns is if the PCM has been on the car, you have no way of knowing if it is still "good" or not. Somebody may have crossed up some wires, zapped the PCM with too much voltage or who knows what? The computer needs to be tested and verified before it can go back on the shelf and be sold to somebody else.

Unfortunately, there is no easy way of doing that in a parts store. The PCM has to be hooked up to a sophisticated simulator that exercises all of the computer's input and output circuits to make sure it works correctly - which means the PCM has to go back to the supplier, be retested, and if no fault is found, repackaged and put back into stock.

Be warned, though, that many parts stores have a policy of "no returns or refunds on electronic components."

WHY DID THE POWERTRAIN CONTROL MODULE DIE?

One way to reduce the risk of PCM warranty problems is to find out why the old PCM died. Determining the cause of death may not always be possible, but it may be essential to prevent the same thing from damaging the replacement PCM in some cases.

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 is not found and repaired, the voltage overload it creates may damage the replacement PCM, too.

As for environmental factors, water is the main thing to avoid. If water gets inside a PCM, it can short circuits and set up irreversible corrosion that ruins electronic connections. Most remanufacturers will not even attempt to repair a PCM if the vehicle it came out of was submerged in a flood. Replacement is the only option. Thermal stress and vibration can form microcracks in circuit boards (which are repairable). This often has more to due with the ruggedness of the circuit design than operation factors in the vehicle itself.

ACCURATE POWERTRAIN CONTROL MODULE IDENTIFICATION

Because there are so many different PCMs, accurate identification of the PCM and its correct replacement is absolutely essential to prevent unnecessary returns. 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 for the application.

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. So if in doubt, always refer to the OEM number on the PCM and look it up in the suppliers cross reference index to find their

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