Circuit Description
Modules connected to the high speed GMLAN serial data circuits monitor for serial data communications during normal vehicle operation. Operating information and commands are exchanged among the modules. The modules have prerecorded information about what messages are needed to be exchanged on the serial data circuits. The messages are supervised and also, some periodic messages are used by the receiver module as an availability indication of the transmitter module. Each message contains the identification number of the transmitter module. A module with a high speed GMLAN serial data circuits malfunction or which loses power during the current ignition cycle will have a Loss of Communication DTC set by other modules that depend on information from that failed module. If any of the high speed GMLAN circuits are shorted to battery voltage, ground or between them, no communication is possible on high speed GMLAN serial data bus, and a DTC U0073 is set.
DTC Descriptors
This diagnostic procedure supports the following DTCs:
• DTC U0100 Lost Communication With Engine Control Module (ECM)
• DTC U0101 Lost Communication With Transmission Control Module (TCM)
• DTC U0106 Lost Communication With Glow Plug Control Module (GPCM)
Conditions for Running the DTCs
• Voltage supplied to the modules is in the normal operating voltage range.
• The engine is running.
Conditions for Setting the DTC
A supervised periodic message that includes the transmitter module availability has not been received.
Action Taken When the DTC Sets
• The module uses a default value for the missing parameter.
• The malfunction indicator lamp (MIL) will illuminate on the second consecutive ignition cycle that the diagnostic runs and fails, for the type B DTCs. For a detailed description about DTC types, refer to Powertrain Diagnostic Trouble Code (DTC) Type Definitions for the 6.6L (LLY) engine.
Conditions for Clearing the MIL/DTC
The MIL/DTC clearing depends on DTC type. For information about DTC types, refer to Diagnostic Trouble Code (DTC) Types for the 6.6L (LLY) engine.
• DTC type B
- The MIL clears after 3 ignition cycles when the malfunction is no longer present.
- 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.
• DTC type C
- 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
• A poor connection at the inoperative module may cause this code to set.
• An improperly powered module may cause this code to set.
Test Description
The numbers below refer to the step numbers on the diagnostic table:
2.
A total loss of communication on high speed GMLAN serial data link should be diagnosed first.
10.
The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
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
Go to Diagnostic System Check - Vehicle
2
1. Install a scan tool.
2. Start the engine.
3. Retrieve DTCs from engine control module (ECM).
Is DTC U0073 set as current?
Go to DTC U0073
Go to Step 3
3
Important: Use the DTC descriptor list in order to determine which module is not communicating on high speed GMLAN serial data bus.
1. Turn the ignition ON, with the engine OFF.
2. Test the following circuits of the module that is not communicating on high speed GMLAN serial data bus for an open or a short to ground:
• The battery positive voltage input circuits
• The battery positive voltage output circuits
• The ignition voltage input circuits
• The ignition voltage output circuits
• The switched battery positive voltage circuits
Refer to the following:
• Control Module References for the applicable schematic
• Circuit Testing
• Wiring Repairs
Did you find and correct the condition?
Go to Step 9
Go to Step 4
4
1. Turn OFF the ignition.
2. Test the ground circuits of the module that is not communicating on high speed GMLAN serial data bus for an open.
Refer to the following:
• Control Module References for the applicable schematic
• Circuit Testing
• Wiring Repairs
Did you find and correct the condition?
Go to Step 9
Go to Step 5
5
Test the high speed GMLAN serial data circuits of the module that is not communicating on high speed GMLAN serial data bus for an open. Refer to Circuit Testing and Wiring Repairs .
Did you find and correct the condition?
Go to Step 9
Go to Step 6
6
Inspect the harness connectors of the module that is not communicating on high speed GMLAN serial data bus for poor connections and terminal tension at the following circuits:
• The battery positive voltage input circuits
• The battery positive voltage output circuits
• The ignition voltage input circuits
• The ignition voltage output circuits
• The switched battery positive voltage supply circuits
• The ground circuits
• The high speed GMLAN serial data circuits
Refer to the following:
• Control Module References for the applicable schematic
• Testing for Intermittent Conditions and Poor Connections
• Connector Repairs
Go to Step 9
Go to Step 7
7
Replace the module that is not communicating on high speed GMLAN serial data bus. Refer to Control Module References for replacement, setup, and programming.
Did you complete the replacement?
Go to Step 8
--
8
1. Install a scan tool.
2. Turn ON the ignition with the engine OFF.
3. Select the Display DTCs function for the module which was not communicating.
Does the scan tool display any DTCs which do not begin with a "U"?
Go to Diagnostic Trouble Code (DTC) List - Vehicle
Go to Step 9
9
Select the Display DTCs function for the modules which had the Loss of Communications DTC set.
Does the scan tool display any DTCs which do not begin with a "U"?
Go to Diagnostic Trouble Code (DTC) List - Vehicle
Go to Step 10
10
1. Use the scan tool in order to clear the DTCs.
2. Continue diagnosing or clearing the DTCs until all the modules have been diagnosed and all the DTCs have been cleared.
Did you complete the action?
System OK
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