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 GM 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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Common signal ground at terminal 5.
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Class 2 signal at terminal 2.
Class 2 Serial Data Line
The class 2 serial data communications circuit on this vehicle is in a star configuration. This configuration has 3
splice packs, located as follows:
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The splice pack - SP205 located near the data link connector
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The splice pack - SP206 located under the steering column
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The splice pack - SP207 located under the LH cowl panel
There is also a direct redundant class 2 serial data circuit between BCM and PCM.
The following modules communicate on the class 2 serial data circuit:
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Splice pack - SP205 attaches to:
{
BCM
2004 Hummer H2
2004 ACCESSORIES & EQUIPMENT Data Link Communications - Hummer H2