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 ID number of the module which sent the message for 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. When a module does not associate an ID number with at least one critical parameter within about 5.5 seconds of beginning serial data communication, DTC U1000 or U1255 is set. When more than one critical parameter does not have an ID number associated with it, the DTC will only be reported once.

CONDITIONS FOR RUNNING THE DTC
The ignition switch run power mode is selected.
The modules voltage supply is within normal operating value, approximately 9-16 volts.

CONDITIONS FOR SETTING THE DTC
At least one critical operating parameter has not been associated with a modules assigned ID number within about 5.5 seconds after beginning serial data communication.

ACTION TAKEN WHEN THE DTC SETS
The SERVICE VEHICLE SOON indicator illuminates.
The module will use default values for the missing parameters.

CONDITIONS FOR CLEARING THE DTC
^ A current DTC will clear when all critical operating parameters for the module have been associated with an ID number, or at the end of the current ignition cycle.
^ A history DTC clears when the module ignition cycle counter reaches the reset threshold, without a repeat of the malfunction.

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

TEST DESCRIPTION

Steps 1-2:

Steps 3-7:

Steps 8-14:

The numbers below refer to the step numbers on the diagnostic table.
1. A DTC U1001 - U1254 "Lost Communications with XXX" with a history status may indicate the cause of U1000 or U1255.
2. The modules not communicating are the likely cause of U1000 or U1255. The modules that are available on the class 2 serial data circuit are listed in the Circuit Description.
7. The module which was not communicating due to a poor connection to the class 2 serial data circuit may have set DTC U1001 - U1254 "Lost Communications with XXX" for those modules that it was monitoring.
9. The modules which can communicate indicate the module which cannot communicate. You must clear the serial data communication DTCs from these modules to avoid future misdiagnosis.
11. If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
12. The module which set U1000 or U1255 is the likely cause of the malfunction.