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 one critical parameter within 5 seconds of beginning serial data communication, DTC U1000 or U1255 are set. When more than one critical parameter does not have an identification number associated with it, the DTC will only be reported once.

The class 2 serial data line on this vehicle is a star configuration. The following modules communicate on the class 2 serial data line:
- The body control module (BCM)
- The chime module
- The electronic brake control module (EBCM)
- The inflatable restraint sensing and diagnostic module (SDM)
- The instrument panel cluster (IPC)
- The powertrain control module (PCM)
- The remote control door lock receiver (RCDLR)
- The radio
- The vehicle communication interface module (VCIM), w/UE1

DTC DESCRIPTORS
This diagnostic procedure supports the following DTCs:
- DTC U1000 Class 2 Data Link
- DTC U1255 Class 2 Data Link

CONDITIONS FOR RUNNING THE DTC
- Voltage supplied to the module is in the normal operating voltage range of 9-16 volts.
- DTCs U1300, U1301 or U1305 do not have a current status.
- The vehicle power mode requires serial data communication to occur.

CONDITIONS FOR SETTING THE DTC
A node alive message has not been received from an unidentified module within the last 5 seconds after establishing class 2 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 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 U1000 or U1255 are set current by the other modules. If the malfunction occurs when the modules are not communicating, only U1000 or U1255 are set.

TEST DESCRIPTION

Steps 1-7:

Steps 8-15:

The numbers below refer to the step numbers on the diagnostic table.
2. A DTC U1001 - U1254 Lost Communications with XXX with a history status may indicate the cause of U1000 or U1255.
3. 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.
10. 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.
13. If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
15. The module which set U1000 or U1255 is the likely cause of the malfunction.