Computers and Control Systems: Description and Operation
DESCRIPTIONThe Powertrain Control Module (PCM), located at the right hand side of the engine compartment, is the control center of the vehicle. It controls the following:
- Fuel metering system.
- Transaxle shifting.
- Ignition timing.
It constantly looks at the information from various sensors, and controls the systems that affect vehicle performance. The PCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the Malfunction Indicator Lamp (MIL) "Service Engine Soon," and store a DTC which identifies the problem areas to aid the technician in making repairs.
MALFUNCTION INDICATOR LAMP
The MIL (Service Engine Soon / Check Engine) lamp is on the instrument panel and has the following functions:
- It informs the driver that a problem has occurred and that the vehicle should be taken for service as soon as reasonably possible.
- Some applications will flash diagnostic trouble codes stored by the PCM which help the technician diagnose system problems.
- Some applications will flash "Open Loop" or "Closed Loop" operation.
Refer to Computers and Controls / Testing and Inspection / Vehicle On-Board Diagnostics / Without Scan Tool for information on identifying exactly what information the MIL can provide. Without Scan Tool
As a bulb and system check, the MIL (Service Engine Soon / Check Engine) will come "ON" with the key "ON" and the engine not running. When the engine is started, the lamp will turn "OFF." If the lamp remains "ON," the self-diagnostic system has detected a problem. If the problem goes away, the lamp will go out in most cases but a diagnostic trouble code will remain stored.
When the light remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, an On-Board Diagnostic System Check must be performed. The procedures for these checks is located at Computers and Controls / System Diagnosis / Flow of Diagnosis. Testing and Inspection
These checks will expose malfunctions which may not be detected if other diagnostics are performed prematurely.
DIAGNOSTIC INFORMATION
There is a continual self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The language for communicating the source of a malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the PCM, a diagnostic trouble code is set and the Malfunction Indicator Lamp (MIL) "Service Engine Soon / Check Engine" is illuminated on some applications.
The diagnostic charts in the A - Charts and functional checks in the C - Charts are designed to locate a faulty circuit or component through logic based on the process of elimination. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are no multiple failures. All diagnostic charts are located at Computers and Controls / System Diagnosis / Procedures. Testing and Inspection
LEARNING ABILITY
The system has a "learning" ability which allows it to make corrections for minor variations in the fuel system to improve driveability. If the battery is disconnected, to clear diagnostic trouble codes or for other repair the "learning" process resets and begins again. A change may be noted in the vehicle's performance. To "teach" the vehicle, ensure that the engine is at operating temperature. The vehicle should be driven at part throttle, with moderate acceleration and idle conditions until normal performance returns.