Computers and Control Systems: Description and Operation

DESCRIPTION
The Powertrain Control Module (PCM) is the control center of the fuel injection system. It constantly looks at the information from various sensors (inputs) and controls the systems (outputs) that affect vehicle performance. The PCM also performs the diagnostic function of the system. It can recognize operational problems, alert the driver through the "Service Engine Soon" light, and store a code or codes which identify the problem areas to aid the technician in performing repairs.

DIAGNOSTIC INFORMATION
The System Diagnostic Tabless and Diagnostic Trouble Codes are designed to locate a faulty circuit or component through a process of logical decisions. The diagnostic charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are not multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complemented by the diagnostic procedures contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code is set and the Malfunction Indicator Lamp (MIL) "Service Engine Soon" is illuminated on some applications.

MALFUNCTION INDICATOR LAMP (MIL)
The MIL (Service Engine Soon) is on the instrument panel and has the following functions:

- It informs the driver that a fault that affects vehicle emission levels has occurred and that the vehicle should be taken for service as soon as possible.
- As a bulb and system check, the MIL (Service Engine Soon) will come "ON" with the key "ON" and the engine not running. When the engine is started, the MIL will turn "OFF."

When the MIL remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, an On-Board Diagnostic (OBD) System Check must be performed. These checks will expose faults which may not be detected if other diagnostics are performed first.

INTERMITTENT MALFUNCTION INDICATOR LAMP
In the case of an "intermittent" fault, the MIL (Check Engine) may illuminate and then (after three trips) go "OFF". However, the corresponding diagnostic trouble code will be stored in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

A diagnostic trouble code may reset. Consult the "Diagnostic Aids" associated with the diagnostic trouble code. A physical inspection of the applicable sub-system most often will resolve the problem.

CONTROL MODULE LEARNING ABILITY
The control module has a "learning" ability which allows it to make corrections for minor variations in the fuel system to improve driveability. If the battery cable is disconnected, the "learning" process resets. A change may be noted in the vehicles performance. To "Teach" the vehicle, ensure that the engine is at operating temperature, then turn the ignition key "ON".

Another method to "Teach" the vehicle to idle is to drive the vehicle at part throttle, with moderate acceleration. The vehicle may also be operated at idle conditions until normal performance returns.