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Test Description

The numbers below refer to the step numbers on the diagnostic table:

  1. 3:  If BCM detects an open in its class 2 circuit, the RCDLR takes control of the Run/Crank relay.
  2. 4:  A power moding code needs to be diagnosed first.
  3. 6:  The scan tool communicate with VCIM via class 2, so for high speed GMLAN data link diagnostic the VCIM will not be considered.
  4. 18:  The module which was not communicating due to an open in the high speed GMLAN serial data circuits may have set Loss of Communication DTCs for those modules that it was monitoring.
  5. 20:  The modules which can communicate indicate the module which cannot communicate. You must clear the DTC from these modules to avoid future misdiagnosis.
DTC U2105 To U2199: Lost Communication With Engine Control Module (ECM)

Step Action Yes No
Schematic Reference:  Data Link Connector (DLC) Schematics  
Connector End View Reference:  Computer/Integrating Systems Connector End Views  
1 Did you perform the Diagnostic System Check - Vehicle? Go to Step 2  Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Attempt to communicate with the Engine Control Module (ECM).
Were you able to communicate with ECM?		 Go to Step 11  Go to Step 3 
3 
  1. Turn ON the ignition, with the engine OFF.
  2. With the scan tool check the Run/Crank Relay Command parameter from BCM Data Display under Computer/Integrating System menu.
Does the scan tool display OFF?		 Go to Step 4  Go to Step 6 
4  Retrieve DTCs from Body Control Module.
Is the DTC B1440 set?		 Go to Go to Step 5 
5
  1. Turn the ignition OFF.
  2. Disconnect the BCM.
  3. Test the class 2 serial data circuit of the BCM for an open. Refer to and in Wiring Systems.
Did you find and correct the condition?		 Go to Step 19  Go to Step 16 
6  Attempt to communicate with electronic suspension control (ESC) module.
Were you able to communicate with ESC module?		 Go to Step 11  Go to Step 7 
7
  1. Turn OFF the ignition.
  2. Disconnect the vehicle communications interface module (VCIM).
  3. Bypass the VCIM installing jumpers between GMLAN serial data circuits at the VCIM connector:
    • Connect a jumper between high speed GMLAN bus (+) circuit to BCM and high speed GMLAN bus (+) circuit to the ESC module.
    • Connect a jumper between high speed GMLAN bus (-) circuit to BCM and high speed GMLAN bus (-) circuit to the ESC module.
  4. Attempt to communicate with ESC module.
Were you able to communicate with ESC module?		 Go to Step 8  Go to Step 10 
8 Test the high speed GMLAN serial data circuits of the VCIM for poor connections.
Did you find and correct the condition?		 Go to Step 20  Go to Step 9 
9 Replace the VCIM. Refer to
Did you complete the replacement?		 Go to Step 20  -
10
  1. Turn OFF the ignition.
  2. Test all the high speed GMLAN serial data circuits between BCM and ESC module for an open. Refer to the following:
Did you find and correct the condition?		 Go to Step 20  Go to Step 11 
11
IMPORTANT: Use the GMLAN ID table to determine the closest module to data link connector (DLC) which is not communicating.
Test the following circuits of the module that is not communicating, for an open or a short to ground:
  • The battery positive circuits
  • The battery voltage output circuits
  • The switched battery positive voltage circuits
Refer to the following:Did you find and correct the condition?		 Go to Step 20  Go to Step 12 
12
  1. Turn OFF the ignition.
  2. Test the ground circuits of the module that is not communicating, for an open. Refer to the following:
Did you find and correct the condition?		 Go to Step 20  Go to Step 13 
13 Test the high speed GMLAN serial data circuits of the module that is not communicating, for an open. Refer to the following:
Did you find and correct the condition?		 Go to Step 18  Go to Step 14 
14 Test the following circuits of the module that is not communicating, for poor connections:
  • The battery positive voltage circuits
  • The battery voltage output circuits
  • The switched battery positive voltage circuits
  • The ground circuits
  • The high speed GMLAN serial data circuits

Did you find and correct the condition?		 Go to Step 20  Go to Step 15 
15 Replace the module that is not communicating.
Did you complete the replacement?		 Go to Step 20  -
16 Inspect for poor connections at the harness connector of the BCM.
Did you find and correct the condition?		 Go to Step 20  Go to Step 17 
17 Replace the BCM.
Did you complete the replacement?		 Go to Step 20  -
18 
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. Retrieve DTCs from the module that was not communicating.
Does the scan tool display any DTCs which do not begin with a "U"?		 Go to the Diagnostic System Check - Vehicle in Vehicle DTC Information Go to Step 19 
19 Use the scan tool in order to clear the DTCs.
Did you complete the action?		 Go to Step 20  -
20  Retrieve DTCs from 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 the Diagnostic System Check - Vehicle in Vehicle DTC Information Go to Step 21 
21
  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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