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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 1604 (CELLULAR COMMUNICATION SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESDTC U1500TEST DESCRIPTION
2003 Chevrolet TrailBlazer 4.2 S, 4WD

Test Description

2003 Chevrolet TrailBlazer 4.2 S, 4WDSECTION Test Description
WARNING: This page does not describe the selected car, but rather 6 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Yukon, 2002 Chevrolet Tahoe, 2002 Chevrolet Suburban, and 2002 Cadillac Escalade EXT. However, it is still accessible from the selected car via links, so may be relevant.

The number below refers to the step number on the diagnostic table.

  1. 2:  By disconnecting the VCU you can check the 3 wire bus supply and carrier voltages supplied by the VIU.
DTC U1500

Step Action Value(s) Yes No
Schematic Reference:  OnStar Schematics  
1 Did you perform the Cellular Communications Diagnostic System Check? - Go to Step 2  Go to Diagnostic System Check - Cellular Communication 
2 
  1. Turn the ignition OFF.
  2. Disconnect the VCU connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Measure the voltage from the OnStar® serial data (+) circuit to a good ground.
Does the voltage measure greater than the specified value?		 4 V Go to Step 3  Go to Step 7 
3  Measure the voltage from the OnStar® serial data (-) circuit to a good ground.
Does the voltage measure greater than the specified value?		 4 V Go to Step 4  Go to Step 8 
4 Measure the voltage from the battery positive voltage circuit from the VIU to the VCU to a good ground.
Does the voltage measure greater than the specified value?		 10 V Go to Step 5  Go to Step 9 
5
  1. Turn the ignition OFF.
  2. Reconnect the VCU connector.
  3. Disconnect the VIU connector C1
  4. Remove the OnStar serial data return circuit from the connector at the Vehicle Interface Unit (VIU).
  5. Reconnect VIU connector C1.
  6. Connect a DMM between the OnStar serial data return circuit and a good ground.
  7. Turn the ignition ON.
  8. Set the DMM to the DC voltage scale and enable the MIN/MAX feature.
  9. Allow the meter to record for 45 seconds.
  10. Record the minimum and maximum values displayed on the DMM.
Does the maximum voltage measure less than the specified value?		 4 V Go to Step 10  Go to Step 6 
6 Does the minimum voltage measure greater than the specified value? .10 V Go to Step 11  Go to Step 13 
7 Test the OnStar® serial data (+) circuit for an open or short to ground. Refer to CIRCUIT TESTING and WIRING REPAIRS .
Did you find and correct the condition?		 - Go to Step 14  Go to Step 12 
8 Test the OnStar® serial data (-) circuit for an open or short to ground. Refer to CIRCUIT TESTING and WIRING REPAIRS .
Did you find and correct the condition?		 - Go to Step 14  Go to Step 12 
9 Test the battery positive circuit from the VCU to the VIU for an open or a short to ground. Refer to CIRCUIT TESTING and WIRING REPAIRS .
Did you find and correct the condition?		 - Go to Step 14  Go to Step 12 
10 Test the OnStar serial data return circuit for an open or a short to ground. Refer to CIRCUIT TESTING and WIRING REPAIRS .
Did you find and correct the condition?		 - Go to Step 14  Go to Step 13 
11 Test the following circuits for a short to voltage:
  • OnStar® serial data return
  • OnStar® serial data (-)
  • OnStar® serial data (+)

Refer to CIRCUIT TESTING and WIRING REPAIRS .
Did you find and correct the condition?		 - Go to Step 14  Go to Step 12 
12
IMPORTANT: Perform the OnStar® setup procedure.
Replace the VIU. Refer to OnStar Vehicle Interface Unit (VIU) Replacement .Did you complete the replacement?		 - Go to Step 14  -
13
IMPORTANT: Perform the OnStar® setup procedure.
Replace the VCU. Refer to OnStar Vehicle Communication Unit (VCU) Replacement .Did you complete the replacement?		 - Go to Step 14  -
14
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
Does the DTC reset?		 - Go to Step 2  System OK
RENDER: 1.0x

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.