Data Link Communications

CIRCUIT DESCRIPTION
Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation when operating information and commands are exchanged among the modules. When a module receives a message for a critical operating parameter, the module records the identification number of the module which sent the message. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. When a module does not associate an identification number with at least 1 critical parameter within about 5 seconds of beginning serial data communication, DTC U1000 or U1255 is set. The DTC will only be reported once if more than 1 critical parameter does not have a sending module identification number associated with it.

CONDITIONS FOR RUNNING THE DTC
^ Voltage supplied to the module is in the normal operating voltage range.
^ Diagnostic trouble codes U1300, U1301 or U1305 are not set.
^ The module setting the DTC requires serial data communication to occur.

CONDITIONS FOR SETTING THE DTC
At least 1 critical operating parameter has not been associated with an identification number within about 5 seconds after beginning serial data communication.

ACTION TAKEN WHEN THE DTC SETS
The module uses a default value for the missing parameter.

CONDITIONS FOR CLEARING THE DTC
^ A current DTC U1000 or U1255 will clear when all critical operating parameter for the module have been associated with an identification number or at the end of the current ignition cycle.
^ A history DTC resets after 50 ignition switch cycles with no repeated failure.

DIAGNOSTIC AIDS
When a malfunction such as an open fuse to a module occurs while modules are communicating, a current DTC is set indicating lost communication with a specific module DTC. When the modules stop communicating, ignition is turned OFF, the current Lost Communication DTC is cleared but the history DTC remains. When the modules begin to communicate again, the module with the open fuse will not be learned by the other modules so DTC U1000 or U1255 is set as current by the other modules. If the malfunction occurs when the modules are not communicating, only DTC U1000 or U1255 is set.

TEST DESCRIPTION

Steps 1-9:

Steps 10-15:

The numbers below refer to the step numbers on the diagnostic table.
1. A Lost Communication with XXX DTC with a history status may indicate the cause of U1000 or U1255.
2. The modules which is not communicating is the likely cause of U1000 or U1255 being set. The following systems communicate on the class 2 serial data circuit:
^ Body Control Module (BCM)
^ Electronic Brake Control Module (EBCM)
^ Electronic Suspension Control (ESC) Module
^ HVAC (with CJ2)
^ Sensing and Diagnostic Module (SDM)
^ Instrument Panel Cluster (IPC)
^ Driver Door Module (DDM)
^ Passenger Door Module (PDM)
^ Seat Control Module (SCM) (with AAB)
^ Powertrain Control Module (PCM)
^ Radio
^ Remote Control Door Lock Receiver (RCDLR)
5. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.
6. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.
7. The module which was not communicating may have set Lost Communication with XXX DTCs for those modules that it was monitoring.
11. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
13. If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
14. The module which set U1000 or U1255 is the likely cause of the malfunction.

"For a description of the RPO Code(s) shown in this article or any of the images therein, refer to the RPO Code List found at Vehicle/Application ID". RPO Codes