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 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, U1304 and U1305 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.

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 DTC U1001-1254 Loss of XXX Communications is set current. When the modules stop communicating the current DTC U1001-1254 Loss of XXX Communications 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 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 3-5:

Steps 6-9:

Steps 10-14:

The numbers below refer to the step numbers on the diagnostic table.
1. A DTC U1001 - U1254 Loss of XXX Communication with a history status may indicate the cause of U1000 or U1255.
2. The modules not communicating is the likely cause of U1000 or U1255. The following modules are available on the class 2 serial data circuit.
- Inflatable restraint sensing and diagnostic module (SDM)
- Powertrain control module (PCM)
- Theft deterrent control module
- Vehicle interface unit (VIU)
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 Loss of XXX Communication for those modules that it was monitoring.
9. The modules which can communicate indicate the module which cannot communicate. You must clear the DTC 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.