Automotive Engine Performance

Powertrain Control Module

19-02 Explore electronic control module components and operational features.

The PCM is used to operate the engine and all of its required systems. From ignition control to fuel control, the PCM must coordinate the operation of all of engine systems to maintain engine operation (FIGURE 19-13). In addition to these two systems, the PCM must interact with the vehicle theft deterrent (VTD) system, entertainment control, stability control, and HVAC to operate a modern vehicle. This is done with a series of inputs and outputs. The vehicle requires the PCM to be the brains of its operation for everything from operating simple switches to making complex calculations.

FIGURE 19-13 The PCM must seamlessly connect all of the different modules necessary for vehicle operation and adapt to the needs of the driver, the engine, and the environment.

Analog and Digital Signals

The input for the various sensors to the PCM depend on what type of sensor it is. The PCM must be able to understand all inputs: from a sine wave created by a permanent magnet (PM) sensor in the engine to the digital square wave produced by a Hall-effect sensor (FIGURE 19-14). This requires the PCM have a converter that electronically converts the analog signal to a digital one that the PCM can understand. The converter is integrated into the circuitry within the PCM. It takes the analog signal and generates the digital signal based on the position of the sine wave signal. This can be misinterpreted at times if the sine wave has a lot of interference in it that causes the converter to detect a false signal. When diagnosing an analog signal to the PCM, make sure to verify that the signal is clean and correct.

FIGURE 19-14 Digital signals have an exact on and off, whereas a sine wave signal has gradual sloping on and off. Some sensors can only be sine wave based on the application, which may require a converter to generate the appropriate signal for the PCM to understand what is happening with the sensor.

Memory

The electronically erasable programmable read-only memory (EEPROM) holds all of the software needed to operate the vehicle. This type of memory is located in the PCM and holds the calibrations necessary to conduct all of the functions that the PCM is required to accomplish. This is similar to a hard drive on a computer in that it retains all of the “learned” features. As the vehicle operates, the PCM learns different aspects that can change based on the operational situations that the vehicle is put in. This includes the idle rpm, transmission shift points, fuel trim strategies, and other features that are adjustable based on the way the driver operates the vehicle. This memory cannot be replaced and has no moving parts. These modules may fail due to a voltage spike or vibration; to fix a failure in this system, the PCM must be replaced with a new unit. The PCM will have to relearn all of the adaptive parameters when it is installed. This situation may cause the driver to think that there are other issues with the vehicle until the PCM has had time to relearn the way the driver operates the vehicle.

Output Drivers

The term “drivers” will be used a lot in the description of how the PCM operates the various components on the engine. An output driver is a switch that controls the operation of an output component within the engine. This could be a fuel injector, ignition coil, VVT solenoid, or one of many others (FIGURE 19-15). Anything that is controlled by the PCM has an output driver associated with it. These output drivers are usually transistors, which are low-voltage switches with no moving components, a design that allows for quick activation and a long life. The downside with these types of switches is that they are not very robust when they try to control a lot of amperage flowing through them. This is why these types of output drivers usually use a relay to control the high-amperage side of the circuit.

FIGURE 19-15 Drivers are needed in the operation of the engine components to control their operation. These drivers are used to switch the component on and off, depending on what the PCM is commanding.

The PCM also controls the use of PWM. PWM is used to operate the component with a metered on time to increase the component’s life and decrease the electrical consumption of the component. Quickly pulsing it on and off allows the component to maintain the needed output while powering it on for only a percentage of time. The on time is usually measured in duty cycle. The duty cycle is the amount of time during which the component is on and operating. For example, a 50% duty cycle would indicate the component was switched on for 50% of the time. The other 50% of the time, it was in the off state. A transistor usually controls duty cycle because the quickness of the switch is needed to maintain the operation of the component.

Calibration/Recalibrating

The calibration programmed in to the module is the software that operates the module and controls its outputs (FIGURE 19-16). These calibrations are designed to operate the outputs that the module is supposed to control. They consist of controlled outcomes for the inputs that are expected from the various sensors on the engine. For instance, by using the electronic coolant temperature (ECT) sensor as an input, the calibration can determine what fuel injector output and ignition timing advance are necessary to operate the engine correctly. Calibrations are generated by the original equipment manufacturers (OEMs) when the vehicle is built, and then they are tested in the research and development process. Calibrating a sensor to the module is slightly different because the module is put into a learn mode where it recognizes the new component, which allows the module to adjust to the “new normal.” Failing to calibrate new components may cause premature failure or driver complaints. The service information should inform the technician of when this requires that the component be replaced. At times, some components may need to be recalibrated if they are not operating properly. The reason for this is either the module “forgets” the parameters of the component or there has been an event that pushed the component out of its parameters and this calibration process will put it back into specification within the module.

FIGURE 19-16 Using the correct calibration for the module is crucial to the proper operation of the component. If the wrong calibration is used, the module will not operate correctly.

Unfortunately, the OEMs cannot anticipate every situation the vehicle will be operating in, so the vehicle may need to be updated from time to time. Such updates are used to fix any bugs that have been identified (FIGURE 19-17). This is where a reintialization or reflash comes into play as the technician is changing the calibration in the module to the new updated one available from the OEM. Using the wrong calibration or corrupted calibrations can render the control module useless, in which case it may need to be replaced.

FIGURE 19-17 Following the technical service bulletin (TSB) in the required process will guide the technician to the proper repair for the complaint.

Programming/Reprogramming

There are situations when the control module will need an updated or calibrated to maintain module operation. In this case, the module will need to be flashed or programmed. Because all of the vehicles sold in the US are On-Board Diagnostics second generation (OBDII) compliant, they all operate under the same computer protocol. This enables one tool to reprogram every type of PCM available on the market. This standard is the SAE J2534 standard (SAE: Society of Automotive Engineers). The J2534 standard is a governmental regulation that requires a specific structure of communication networks on all vehicles sold. This allows the service industry to have one tool standard to communicate with the vehicle to repair the PCM (FIGURE 19-18). This standard applies only to the powertrain, but in the interest of simplicity, a lot of manufactures have applied it to other modules on the vehicle.

FIGURE 19-18 The J2534 reprogramming tool allows the service technician to program or reprogram PCMs on any OBDII vehicle. This standardization of operational protocol of these computers allows the technician to have just one tool for all makes and models sold with OBDII.

As mentioned in the Calibration/Recalibrating section of this chapter, at certain times during vehicle repair, the PCM or some other module will need to be updated. The manufacture of the vehicle is constantly updating the operational software of the various modules to correct software glitches or to enhance vehicle operation (FIGURE 19-19). When these new calibrations are published on the OEM’s website, they can be downloaded to the vehicle through the J2534 device (FIGURE 19-20). Another time that programming a module may be necessary is when the technician is replacing the module for an internal failure. Most modules come unprogrammed from the parts store because they are used on various versions of that vehicle that may require different programming based on vehicle options. By using the J2534 device, the technician can upload the correct software program.