The Powertrain Control Module (PCM) is given responsibility for the operation of the emission control devices, cooling fans, ignition and advance and in some cases, automatic transmission functions. Because the PCM oversees both the ignition timing and the fuel injector operation, a precise air/fuel ratio will be maintained under all operating conditions. The PCM is a microprocessor or small computer, which receives electrical inputs from several sensors, switches, and relays on and around the engine.
Based on combinations of these inputs, the PCM controls outputs to various devices concerned with engine operation and emissions. The PCM relies on the signals to form a correct picture of current vehicle operation. If any of the input signals is incorrect, the PCM reacts to what ever picture is painted for it. For example, if the coolant temperature sensor is inaccurate and reads too low, the PCM may see a picture of the engine never warming up. Consequently, the engine settings will be maintained as if the engine were cold. Because so many inputs can affect one output, correct diagnostic procedures are essential on these systems.
One part of the PCM is devoted to monitoring both input and output functions within the system. This ability forms the core of the self-diagnostic system. If a problem is detected within a circuit, the control module will recognize the fault, assign it a Diagnostic Trouble Code (DTC), and store the code in memory. The stored code(s) may be retrieved during diagnosis.
While the PCM is capable of recognizing many internal faults, certain faults will not be recognized. Because the PCM sees only electrical signals, it cannot sense or react to mechanical or vacuum faults affecting engine operation. Some of these faults may affect another component, which will set a code. For example, the PCM monitors the output signal to the fuel injectors, but cannot detect a partially clogged injector. As long as the output driver responds correctly, the computer will read the system as functioning correctly. However, the improper flow of fuel may result in a lean mixture. This would, in turn, be detected by the oxygen sensor and noticed as a constantly lean signal by the PCM. Once the signal falls outside the pre-programmed limits, the PCM would notice the fault and set a trouble code.
Additionally, the PCM employs adaptive fuel logic. This process is used to compensate for normal wear and variability within the fuel system. Once the engine enters steady-state operation, the PCM watches the oxygen sensor signal for a bias or tendency to run slightly rich or lean. If such a bias is detected, the adaptive logic corrects the fuel delivery to bring the air/fuel mixture towards a centered or 14.7:1 ratio. This compensating shift is stored in a non-volatile memory which is retained by battery power even with the ignition switched OFF . The correction factor is then available the next time the vehicle is operated.
The electronic engine control format is called OBD-II. OBD-II vehicles are able to monitor certain systems of the vehicle through various input sensors and output hardware. These monitors include the Catalyst Efficiency, Engine Misfire Detection, Comprehensive Component, EGR System Flow (where fitted), EVAP System Integrity, Secondary Air (if equipped), Fuel System, and Heated O 2 Sensor Monitors.
Some of the hardware included with these monitors are an O 2 sensor behind the catalyst, an LDP on the EVAP system, and a High Data Rate CKP sensor to help monitor engine misfires. All monitors in the OBD-II system have separate trouble codes and are diagnosed by standard diagnostic methods.
MALFUNCTION INDICATOR LAMP (MIL)
The Malfunction Indicator Lamp (MIL) is located on the instrument panel. The lamp is connected to the control unit and will alert the driver to certain malfunctions detected by the PCM. When the lamp is illuminated, the PCM has detected a fault and stored a DTC in memory.
The light will stay illuminated as long as the fault is present. Should the fault self-correct, the MIL will extinguish but the stored code will remain in memory.
Under normal operating conditions, the MIL should illuminate briefly when the ignition key is turned ON . This is commonly known as a bulb check. As soon as the PCM receives a signal that the engine is cranking, the lamp should extinguish. The lamp should remain extinguished during the normal operating cycle.