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.

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.
4.3L Vortec Engine Computer Description

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