Data Link Communications

DTC U1000

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 1 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 to 16 volts.
- Diagnostic trouble codes U1300, U1301 and U1305 do not have a current status.
- The vehicle power mode requires serial data communication to occur.

CONDITIONS FOR SETTING THE DTC
At least one 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 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 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 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 - 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 module which is not communicating is the likely cause of U1000 or U1255 being set. The following modules communicate on the class 2 serial data circuit:
- Auxiliary Power Drop Connector
- BCM
- DIC
- DRR (U2K or U2L)
- EBCM
- IPC
- PCM
- Radio
- SDM
- OnStar(UE1)
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.