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 5-10 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.
The following modules communicate on the class 2 serial data circuit:
| • | The body control module (BCM) |
| • | The communication interface module (OnStar®) |
| • | The digital radio receiver (DRR) |
| • | The driver door module (DDM) |
| • | The driver door switch (DDS) |
| • | The driver position module (DPM) |
| • | The fold top controller (FTC) |
| • | The head up display (HUD) |
| • | The HVAC control module |
| • | The inflatable restraint sensing and diagnostic module (SDM) |
| • | The instrument panel cluster (IPC) |
| • | The passenger door module (PDM) |
| • | The radio |
| • | The remote control door lock receiver (RCDLR) |
| • | The steering column lock control module (SCLCM) |
The BCM emulates all devices that are on high speed GMLAN serial data circuits to the modules on class 2 serial data circuit. The BCM transmits multiple Node Alive messages using source IDs that correspond to each of the GMLAN nodes. This is in addition to the BCM behaving itself as a real node. The following module communicate on high speed GMLAN serial data circuits and are emulated by BCM on class 2 serial data circuit:
| • | The electronic brake control module (EBCM) |
| • | The electronic suspension control module (ESC) |
| • | The engine control module (ECM) |
| • | The transmission control module (TCM) |
DTC Descriptor
This diagnostic procedure supports the following DTC:
DTC U1000 or U1255 Class 2 Data Link
DTC Symptom | DTC Symptom Descriptor |
|---|---|
00 | No additional information |
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 message containing a critical operating parameter has not been received within the last 5-10 seconds after establishing class 2 serial data communication.
Action Taken When the DTC Sets
The module uses a default value for the missing parameter.
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
The numbers below refer to the step numbers on the diagnostic table.
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A DTC U1001-U1254 "Lost Communications with XXX" with a history status may indicate the cause of U1000 or U1255. A non communicating GMLAN module can cause the DTC setting, besides other DTC in the range of U2105-U2175 or U0100-U0299.
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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.
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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.
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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.
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If all modules are communicating, the module which set U1000 or U1255 may have done so due to some other condition.
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If all modules are communicating, the module which set U1000 or U1255 is the likely cause of the malfunction.
Step | Action | Yes | No | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Schematic Reference: Data Communication Schematics Connector End View Reference: Master Electrical Component List | |||||||||||||||||||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | |||||||||||||||||
Did you record any DTCs in the range of U1001-U1254, or in the range of U2105-U2175, or in the range of U0100-U0299? | Go to Step 3 | ||||||||||||||||||
Does any module on the class 2 serial data circuit not communicate? | Go to Step 4 | Go to Step 13 | |||||||||||||||||
4 | Test the following circuits of the module that is not communicating for an open or short to ground:
Refer to the following:
Did you find and correct the condition? | Go to Step 9 | Go to Step 5 | ||||||||||||||||
5 |
Did you find and correct the condition? | Go to Step 9 | Go to Step 6 | ||||||||||||||||
6 |
Important: Inspect all connectors in the serial data communications circuit. Refer to the following:
Did you find and correct the condition? | Go to Step 9 | Go to Step 7 | ||||||||||||||||
7 | Inspect the harness connector of the module that is not communicating for poor connections and terminal tension at the following circuits:
Refer to the following:
Did you find and correct the condition? | Go to Step 9 | Go to Step 8 | ||||||||||||||||
Replace the module that is not communicating. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | Go to Step 11 | -- | |||||||||||||||||
9 |
Does the scan tool display any DTCs which do not begin with a "U"? | Go to Step 10 | |||||||||||||||||
Use the scan tool in order to clear the DTCs. Did you complete the action? | Go to Step 11 | -- | |||||||||||||||||
11 | Retrieve DTCs from the modules which had U1000 or U1255 set as a current DTC. Does the scan tool display DTCs which do not begin with a "U"? | Go to Step 12 | |||||||||||||||||
Use the scan tool in order to clear the DTCs. Did you complete the action? | System OK | -- | |||||||||||||||||
Did you record any other DTCs for the modules which had U1000 or U1255 set as a current DTC? | Go to Step 14 | ||||||||||||||||||
14 |
Does the scan tool display U1000 or U1255 set as a current DTC? | Go to Step 15 | Go to Diagnostic Aids | ||||||||||||||||
15 | Replace the module which had U1000 or U1255 set as a current DTC. Refer to Control Module References for replacement, setup, and programming. Did you complete the replacement? | System OK | -- | ||||||||||||||||
