Data Link Communications

CIRCUIT DESCRIPTION
Modules connected to the class 2 serial data circuit monitor for serial data communications during normal vehicle operation. 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 for State of Health monitoring. 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 one critical parameter within about 5 seconds of beginning serial data communication, DTC U1000 or U1255 is set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

CONDITIONS FOR RUNNING THE DTC
^ Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.
^ Diagnostic trouble codes U1300, U1301 and U1304 do not have a current status.
^ The module operation 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 parameters for the module have been associated with an identification number or at the end of the current ignition cycle.
^ A history DTC will clear after completing 100 ignition cycles with no repeated malfunction.

DIAGNOSTIC AIDS
When a malfunction, such as an open fuse to a module, occurs while modules are communicating, a Lost Communication DTC is set as a current 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 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 1-11:

Steps 12-14:

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 are not communicating are the likely cause of U1000 or U1255 being set. The following modules communicate on the class 2 serial data circuit:
^ Powertrain control module (PCM)
^ Theft deterrent control module
^ Inflatable restraint sensing and diagnostic module (SDM)
^ Vehicle interface unit (VIU)
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.
10. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
12. If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
13. The module which set U1000 or U1255 is the likely cause of the malfunction.