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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 1779 (DATA COMMUNICATION SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESSCAN TOOL DOES NOT COMMUNICATE WITH LOW SPEED GMLAN DEVICEDIAGNOSTIC AIDS
2003 Chevrolet TrailBlazer 4.2 S, 4WD

Diagnostic Aids

2003 Chevrolet TrailBlazer 4.2 S, 4WDSECTION Diagnostic Aids
WARNING: This page is about a different car, the 2007 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

The following conditions may cause a total loss of low speed GMLAN data communication:

  • The low speed GMLAN serial data circuit is shorted to ground.
  • The low speed GMLAN serial data circuit is shorted to voltage.
  • An internal malfunction of a module on the low speed GMLAN network that causes a short to voltage or ground.
Scan Tool Does Not Communicate with Low Speed GMLAN Device

Step Action Yes No
Schematic Reference: Data Communication Schematics 
Connector End View Reference: Data Communication Connector End Views 
1 Did you perform the Diagnostic System Check - Vehicle? Go to Step 2  Go to Diagnostic System Check - Vehicle
2
IMPORTANT: Make sure the CANdi module works fine. When functioning properly, CANdi's LED flashes. In the event of a problem, the LED will be constantly illuminated.
  1. Turn the ignition ON, with the engine OFF.
  2. Attempt to communicate with each module on the low speed GMLAN serial data circuit using a scan tool with an in-line CANdi adaptor module properly installed.
Does the scan tool communicate with all modules on the GMLAN serial data circuits?		 Go to Testing for Intermittent Conditions and Poor Connections Go to Step 3 
3
  1. Disconnect the scan tool from the data link connector (DLC).
  2. Turn the ignition OFF.
  3. Disconnect all non-communicating modules on the low speed GMLAN serial data circuit.
  4. Turn the ignition ON, with the engine OFF.
  5. Test all of the low speed GMLAN serial data circuits between DLC and all the modules on the low speed GMLAN serial data circuit, for the following conditions:
    • An open
    • A short to ground
    • A short to voltage
Refer to the following:Did you find and correct the condition?		 Go to Step 9  Go to Step 4 
4
  1. Reconnect the module closer to DLC on the low speed GMLAN bus.
  2. Turn the ignition ON, with the engine OFF.
  3. Attempt to communicate with the module on low speed GMLAN bus.
Does the scan tool communicate with the module?		 Go to Step 5  Go to Step 6 
5 If the vehicle is equipped with two modules on low speed GMLAN serial data circuit, consider the disconnected module the non communicating one for the next step.
Is the vehicle equipped with two modules on low speed GMLAN?		 Go to Step 6  Go to Testing for Intermittent Conditions and Poor Connections
6
  1. Turn OFF the ignition.
  2. Disconnect the non communicating module if it is not already disconnected.
  3. Test the following circuits of the module that is not communicating for an open or high resistance:
    • The battery positive voltage input circuits
    • The battery voltage output circuits
    • The ignition voltage input circuits
    • The ignition voltage output circuits
    • The switched battery positive voltage supply circuits
    • The ground circuits
Refer to the following:Did you find and correct the condition?		 Go to Step 9  Go to Step 7 
7 Inspect for poor connections and terminal tension at the following harness connector circuits of the module that is not communicating:
  • 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 low speed GMLAN serial data circuits

Refer to the following:
Did you find and correct the condition?		 Go to Step 9  Go to Step 8 
8 Replace the module that is not communicating on the low speed GMLAN serial data circuit. Refer to Control Module References for replacement, setup and programming.
Did you complete the replacement?		 Go to Step 9  -
9
  1. Connect all modules and connectors.
  2. Connect the scan tool to the DLC.
  3. Turn the ignition ON, with the engine OFF.
  4. Attempt to communicate with the modules on low speed GMLAN bus.
Does the scan tool communicate normally with the modules on low speed GMLAN bus?		 System OK Go to Step 2 
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Stop DIY work and contact a certified mechanic immediately if any of the following apply:

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