Engine Control Module: Description and Operation
POWERTRAIN CONTROL MODULE (PCM) DESCRIPTIONPowertrain Control Module (PCM) Description:
The Powertrain Control Module (PCM) is a precision unit consisting of a one chip microprocessor, an Analog/Digital (A/D) converter, and an Input/Output (I/O) unit. The PCM is an essential part of the electronic control system. The PCM is responsible for such major functions as control of the fuel injectors, the idle air control (IAC) valve, the fuel pump relay, etc. The PCM performs the OBD 2 diagnostic tests of the emission related systems. The PCM supplies a buffered voltage to the various information sensors and switches. The PCM controls most components with an electronic switch that completes a ground circuit when turned on. The PCM is also responsible for a self-diagnosis function and a fail-safe function.
The PCM (1) is located below the glove box, underneath the instrument panel, on the right side of the passenger compartment.
PCM EPROM
The PCM has an Erasable Programmable Read-only Memory (EPROM) computer chip that contains the calibration information used by the PCM to control fueling, idle speed, ignition timing, transaxle shifts, and vehicle emissions. The calibration information is based on various aspects of the vehicle, such as engine size, vehicle weight, transaxle type, final drive ratio, etc. The EPROM is programmed (flashed) with the calibration information that is critical to the proper operation of the PCM. The EPROM is soldered into the PCM and cannot be serviced separately. Replacement PCMs used for service require programming after installation.
Self-Diagnosis Function
The PCM diagnoses any troubles which may occur in the engine control system when the ignition switch is in the On position with the engine running. The PCM indicates a malfunction by illuminating the Malfunction Indicator Lamp (MIL) when a fault occurs in any of the following systems:
- The Heated Oxygen Sensor 1 (HO2S 1)
- The Heated Oxygen Sensor 2 (HO2S 2)
- The Engine Coolant Temperature (ECT) sensor
- The Throttle Position (TP) sensor
- The Mass Air Flow (MAF) sensor
- The Vehicle Speed Sensor (VSS)
- The Intake Air Temperature (IAT) sensor
- The Manifold Absolute Pressure (MAP) sensor
- The Camshaft Position (CMP) sensor
- The Crankshaft Position (CKP) sensor
- The Knock Sensor (KS)
- The Evaporative Emission (EVAP) control system
- The Idle Air Control (IAC) system
- The misfire detection
- The fuel trim
- The catalyst monitor
- The cooling fan control
- The Central Processing Unit (CPU) of the PCM
When the PCM detects a malfunction in one of the above areas, the PCM will illuminate or flash the MIL in order to notify the driver of the occurrence of a fault. The PCM will store a Diagnostic Trouble Code (DTC) when the PCM illuminates the MIL.
The PCM will turn off the MIL after 3 consecutive ignition cycles, in which the diagnostic runs, without the malfunction occurring. The DTC will remain stored in the PCM memory after the MIL is OFF.
Fail-Safe Function
When a malfunction occurs within the engine control system, the PCM maintains control over the fuel injection system, the idle speed control system, etc. The PCM controls these systems by using calculated values and backup programs stored within the PCM.
This function is called the fail-safe function. With the fail-safe function, a certain level of engine performance is available even when a malfunction occurs. The fail-safe function prevents a complete loss of engine performance.
The systems covered by the fail-safe function are as follows:
- The engine coolant temperature (ECT) sensor
- The throttle position (TP) sensor
- The mass air flow (MAF) sensor
- The intake air temperature (IAT) sensor
- The vehicle speed sensor (VSS)
- The barometric (BARO) pressure sensor
- The fuel level sensor
- The central processing unit (CPU) in the PCM
Control Module Learning Ability
The control module has a "learning" ability which enables the control module to make corrections for minor variations in the fuel system. This learning ability can improve driveability. Disconnecting the battery resets the learning process. A change in the vehicle's performance may be noticed when a reset occurs. The vehicle operator can teach the control module in order to regain some of the lost vehicle performance.
In order to teach the control module, ensure that the engine is at operating temperature and drive the vehicle at part throttle with moderate acceleration. The vehicle may also be operated at idle conditions until normal performance returns.
PCM Output Controls
IMPORTANT: Do not use the scan tool's Output Control feature until the vehicle is at normal operating temperatures. The PCM will not respond to a scan tool request for Output Controls until the engine coolant temperature is above 80°C (175°F). Failure to wait for normal operating temperatures can result in the incorrect diagnosis of a driveability problem.
Output Controls are disabled when any DTCs are stored by the PCM. Clear all DTCs before using Output Controls on the scan tool.
The powertrain control module (PCM) can be directed by the Tech 2 scan tool to operate certain solenoids, valves, motors, and switches. This scan tool function is generally referred to as Output Controls. Output controls can be found under Special Functions on the Tech 2 scan tool. Operating a PCM controlled device with the scan tool should be limited to a maximum of ten seconds per test period.