Data Link Communications

CIRCUIT DESCRIPTION
During normal vehicle operation modules connected to the class 2 serial data circuit monitor the class 2 serial data circuit for serial data communications. Operating information and commands are exchanged among the modules. When a module receives a critical operating parameter, the module records the identification number of the transmitting module for the purpose State of Health monitoring Node Alive messages. A critical operating parameter is one which, when not received, requires that the module use a default value for that parameter. Once an identification number is learned by a module, it will monitor the class 2 serial data circuit for that module's Node Alive message. Each module on the class 2 serial data circuit is required to send a Node Alive message every 2 seconds. When no message is detected from a learned identification number for 5 seconds, a DTC U1XXX where XXX is equal to the 3 digit identification number is set.

DTC U1001-U1254:

The control module ID number list below provides a method for determining which module is not communicating. A module with a class 2 serial data circuit malfunction or which loses power during the current ignition cycle will have a Loss of Communication DTC set by other modules that depend on information from that failed module. The modules that can communicate will set a DTC indicating the module that can not communicate. When no message is detected from a learned identification number for 5 seconds, a DTC U1XXX where XXX is equal to the 3 digit identification number is set.

When more than one Loss of Communication DTC is set in either one module or in several modules, diagnose the DTCs in the following order:
1. Current DTCs before history DTCs unless told otherwise in the diagnostic table.
2. The DTC which is reported the most times.
3. From the lowest number DTC to the highest number DTC.

CONDITIONS FOR RUNNING THE DTC
^ The following diagnostic trouble codes do not have a current status:
^ B1982
^ B1983
^ U1300
^ U1301
^ U1305
^ The vehicle power mode requires serial data communication to occur.

CONDITIONS FOR SETTING THE DTC
A message from a learned identification number has not been detected for the past five seconds.

CONDITIONS FOR CLEARING THE DTC
^ A current DTC clears when the malfunction is no longer present.
^ A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

DIAGNOSTIC AIDS
When multiple Loss of Communication DTCs are set concurrently, the cause is likely to be two opens in the class 2 serial data circuit. Use the Control Module ID Number list in order to determine which modules are not communicating. Use the class 2 serial data circuit schematic in order to determine the location of the opens.

TEST DESCRIPTION

Steps 1-5:

Steps 6-9:

The numbers below refer to the step numbers on the diagnostic table.
6. The module which was not communicating due to an open in the class 2 serial data circuit may have set Loss of Communication DTCs for those modules that it was monitoring.
8. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.