Ascender 4WD L6-4.2L (2003)
Data Link Connector: Description and Operation
Data Link Communications
Data Link Communications Description and Operation
Circuit Description
The data link connector (DLC) allows a scan tool to communicate with the class 2 serial data line. The serial data line is the means by which the
microprocessor-controlled modules in the vehicle communicate with each other.
Once the scan tool is connected to the class 2 serial data line through the DLC, the scan tool can be used to monitor each module for diagnostic purposes
and to check for diagnostic trouble codes (DTCs).
Class 2 serial data is transmitted on a single wire at an average of 10.4 kbps. This value is an average, class 2 uses a variable pulse width modulation to
carry data and depending on the message it may operate faster or slower. The bus will float at a nominal 7.0 volts during normal operation.
Each module can pull this lower during the transmission. The bus is not at battery positive voltage or ground potential during normal operation. When
the ignition switch is in RUN, each module communicating on the class 2 serial data line sends a state of health (SOH) message every 2 seconds to
ensure that the module is operating properly. When a module stops communicating on the class 2 serial data line, for example if the module loses power
or ground, the SOH message it normally sends on the data line every 2 seconds disappears.
Other modules on the class 2 serial data line, which expect to receive that SOH message, detect its absence; those modules in turn set an internal DTC
associated with the loss of SOH of the non-communicating module. The DTC is unique to the module which is not communicating, for example, when
the inflatable restraint sensing and diagnostic module (SDM) SOH message disappears, several modules set DTC U1088. Note that a loss of serial data
DTC does not normally represent a failure of the module that set it.
On some vehicles, if the PCM is unable to communicate with the VTD system after the vehicle has started, the PCM will consider the VTD system to be
malfunctioning. The PCM will enter a fail enable state and will command the security indicator to illuminate. When the PCM is in a fail enable state the
vehicle will NOT stall or stop running. If the PCM is in a fail enable state when the ignition is switched OFF, the PCM will remain fail enable until
communications with the VTD system has been restored. When the PCM is in a fail enable state the VTD system is NOT active and the vehicle will start.
This feature is NOT available on all Isuzu vehicle lines.
Data Link Connector (DLC)
The data link connector (DLC) is a standardized 16 cavity connector. Connector design and location is dictated by an industry wide standard, and is
required to provide the following:
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Scan tool power battery positive voltage at terminal 16.
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Scan tool power ground at terminal 4.
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Class 2 serial data at terminal 2.
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Common signal ground at terminal 5.
Class 2 Serial Data Line
The class 2 serial data line on this vehicle is a star configuration. The BCM and PCM have an additional class 2 serial data circuit between them. If one
of the class 2 serial data circuits to the BCM or PCM opens, communication will not be interrupted. The following modules communicate on the class 2
serial data line:
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The body control module (BCM)
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The driver door module (DDM)
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The electronic brake control module (EBCM)
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The front passenger door control module (FPDM)
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The HVAC control module
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The inflatable restraint sensing and diagnostic module (SDM)
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The instrument panel cluster (IPC)
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The liftgate control module (LGM)
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The memory seat module-driver w/power seat
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The memory seat module-passenger w/power seat
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The powertrain control module (PCM)
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The radio
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The radio amplifier
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The RSA/HVAC control module-rear auxiliary
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The transfer case shift control module w/4WD
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The vehicle communication interface module (VCIM) w/UE1
The class 2 serial data line allows a scan tool to communicate with these modules for testing purposes, checking for DTCs, and to activate/enable/disable
functions.