The (PCM)onboard computer, or Powertrain Control Module (PCM),is the life brains of the engine control system, so when the brain is not operating correctly neither is the engine or anything else that the microprocessor controls – which could include the charging system, transmission, different emission controls and communications with other onboard control modules. Once a diagnosis has been established, then and only then should the PCM be changed.
PCM Trouble Codes
Onboard Diagnostic (OBD II) diagnostic trouble codes that typically shows 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
P0607….Control Module Performance
P0608….Control Module VSS Output ‘A’
P0609….Control Module VSS Output ‘B’
P0610….Control Module Vehicle Options Error
If you observe any of these codes when diagnosing the car with a code reader or scan tool, it may mean the PCM has failed and must be changed. Additional diagnostic tests will often be necessary to confirm the issue is really the powertrain control module and NOT something else. Refer to the OEM diagnostic charts for what these tests are. Often 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 has to be changed.
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 an issue 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 issue, nobody wins. Warranty returns on complicated and expensive components like powertrain control modules are an ongoing issue that costs everyone money.
POWERTRAIN CONTROL MODULE WARRANTY ISSUES
Over 50 percent of PCMs that are returned under warranty (either since the PCM failed to fix a performance issue or because the engine did not run properly after it was mounted) 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 issue.
The trouble with returns is if the PCM has been on the vehicle, 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 has to be tested and verified before it can go back on the shelf and be sold to somebody else.
Sad to say, 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 implies that the PCM has to go back to the supplier, be retested, and if no fault is detected, repackaged and put back into stock.
Be warned, though, that many parts stores have a policy of “no returns or refunds on electronic part.”
WHY DID THE POWERTRAIN CONTROL MODULE DIE?
One way to lower the risk of PCM warranty problems is to find out why the old PCM died. Knowing the cause of death may not always be possible, but it may be essential to stop the same thing from spoiling the replacement PCM in some cases.
PCMs typically fail for one of two reasons: voltage overloads (often because of 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 stay off. If water gets inside a PCM, it can short circuits and set up irreversible corrosion that spoils electronic connections. Most remanufacturers will not even attempt to repair a PCM if the car 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 because of the ruggedness of the circuit design than operation factors in the car itself.
Identification label on back of General Motors PCM.
POWERTRAIN CONTROL MODULE IDENTIFICATION
Because there are so many different types PCMs, accurate identification of the PCM and its correct replacement is absolutely essential to stop unnecessary returns. Many PCMs seem 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 mounted in a vehicle, it may run but probably will not run well. Close enough is not good enough when it comes to changing PCMs. It must be the correct replacement for the application.
Accurately identifying the PCM needs 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 replacement part number.
The calibration chip and PROM contains the programming instructions for the car application. That is why it often does not come with the replacement PCM. There are too many different possibilities. On many newer vehicles, flash memory or “EEPROMs” (Electronically Erasable Program Read Only Memory) are used. If the replacement PROM is not properly programmed for the application, it must be reprogrammed after it has been mounted.
Unfortunately, the ability to do this type of reprogramming is not readily offered to the aftermarket. The car makers do not want aftermarket technicians messing around with the calibration of their onboard computers since they are afraid doing so may alter emissions or performance. But that is another issue. One such example is Chrysler transmission modules. They must be reset with the factory DRB scan tool and dealer codes to set the “pinion factor,” which regulates the operation of the speedometer.
REMAN POWERTRAIN CONTROL MODULES
Because a powertrain control module can be very expensive to change, almost all aftermarket replacement PCMs are “remanufactured” units. A PCM is not rebuilt in the same way that an alternator or water pump is rebuilt since there are no mechanical parts that wear out. Remanufacturing in this case often means testing the powertrain control module, isolating and repairing any faults that may be found, then retesting the powert5ain control module to ensure everything works correctly.
A remanufactured PCM is typically sold one of two ways: on an exchange basis from stock, or on a custom rebuild basis. If a particular PCM is not in stock or is unavailable, you may be able to send an old PCM to a remanufacturer for repair. Turn around time is typically a few days and the cost is about the same as an exchange unit except there is no core charge). The hard part is finding a remanufacturer who can test and repair your powertrain control module.
Some PCMs, though, may actually not be repairable. As we said earlier, most remanufacturers will not even touch a PCM if it came out of a flooded car.
PCM REPLACEMENT TIPS
Changing a PCM is essentially a matter of swapping boxes. Accessibility can be a problem on some cars because the PCM is often buried under or behind other components in the instrument panel, climate control system or console. Some are found under a seat and need removing the seat.
Irrespective of the PCM’s location, though, one thing you should do prior to removing the old PCM and mounting the replacement PCM is disconnect the battery.
Once the PCM has been installed and reconnected, the battery can be reconnected, too. But the job is not done yet. Many PCMs have passed through a “relearning” procedure after they have been mounted or if they have been disconnected from the battery. On some newer cars, a scan tool may be needed to reprogram the PCM and to reset the anti-theft system.
On some applications, there may be a specific relearn procedure for establishing the base idle speed and other operating parameters. On others, it may be necessary to take the car for a short test drive so the computer can adjust itself. The exact requirements will be spelled out in the car service manual. The best advice here is to test drive the car after the powertrain control module has been mounted. A short drive cycle that includes going over 35 mph will usually reset most PCMs so they will function properly.
The powertrain control module will also continue to learn and make small adjustments to the fuel mixture and other functions over time as the car accumulates miles. If the PCM also controls the transmission, it may take awhile to relearn the driver’s habits so the transmission may not shift exactly the same as before until this happens.
Finally, if the Malfunction Indicator Lamp (MIL) or Check Engine light comes back on after the PCM has been changed, it means there is still a problem with the vehicle. The fault is probably NOT the PCM, unless the fault code is for an internal PCM fault. The presence of fault codes means additional diagnosis is needed to identify and repair the fault. And until the real problem is found and fixed, the PCM may not operate normally.
If the engine control system is not going into closed loop, chances are the coolant sensor or oxygen sensor may not be working properly. If spark timing seems to be over advanced or retarded, the problem may be a faulty MAP sensor, misadjusted throttle position sensor or overly sensitive knock sensor. And if nothing seems to work right, low charging voltage due to a weak alternator or poor battery connections may be the fault.
Always bear in mind, a powertrain control module requires all its sensor inputs, proper battery voltage, a good ground and the ability to send out control signals to operate normally.