4.3L Vortec PCM Operation

The Vehicle Control Module (VCM) is the control center for the engine component systems which effect engine operation. The VCM constantly monitors and processes information from the various information sensors and switches. The VCM then sends the necessary electrical responses to control the various control system components.

Review the wiring diagrams in order to determine which systems are controlled by the VCM. The VCM has the ability to perform on-board diagnostic tests on itself, the information system components, and certain control system components.

The VCM has the ability to alert the driver of a malfunction by illuminating a malfunction indicator lamp (MIL). The VCM has the ability to store certain information pertaining to a malfunction, including diagnostic trouble codes (DTCs). Using serial data communication, a scan tool can retrieve this information in order to be viewed by the technician. The technician can use this information in order to identify, diagnose, and verify the repair of the malfunction. The VCM is sometimes referred to as the control module.

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The VCM control calibrations for a particular vehicle are based on the vehicle identification number (VIN). The calibrations are stored in the electrically erasable programmable read-only memory (EEPROM). The EEPROM is part of the VCM and can not be replaced. The EEPROM can be reprogrammed using the Techline equipment.

The electronic ignition control module is located in the VCM. The knock sensor (KS) module contains the knock sensor system calibrations. The knock sensor module is not part of the VCM and is serviced separately from the VCM. The VCM has the ability to make corrections in the fuel control system in order to allow for minor vehicle variations. This information is stored in the random access memory (RAM). RAM needs a constant voltage supply to be retained. The short term fuel trim and the long term fuel trim are stored in RAM.

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