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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 1638 (RETAINED ACCESSORY POWER SYSTEM)DIAGNOSTIC INFORMATION AND PROCEDURESDTC B1478TEST 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 7 other vehicles, including the 2002 GMC Yukon XL, 2002 GMC Yukon, 2002 Chevrolet Tahoe, 2002 Chevrolet Suburban, and 2002 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.

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

  1. 2:  Listen for an audible click when the RAP relay operates. Command both the ON and OFF states. Repeat the commands as necessary.
  2. 3:  Tests for battery voltage at the coil side of the RAP relay. The RAP fuse supplies power to the coil side of the RAP relay.
  3. 4:  Verifies that the BCM is providing ground to the RAP relay.
  4. 5:  Tests if ground is constantly being applied to the RAP relay.
  5. 10:  After replacement of the BCM you must calibrate the new module for proper operation.
DTC B1478

Step Action Yes No
Schematic Reference: Retained Accessory Power (RAP) Schematics 
1 Did you perform the Diagnostic System Check - Retained Accessory Power (RAP)? Go to Step 2  Go to Diagnostic System Check - Retained Accessory Power 
2 
  1. Install a scan tool.
  2. Turn ON the ignition, with the engine OFF.
  3. With a scan tool, command the RAP relay ON and OFF.
Does the RAP relay turn ON and OFF with each command?		 Go to Diagnostic Aids Go to Step 3 
3 
  1. Turn OFF the ignition.
  2. Remove the RAP relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the battery positive voltage circuit of the RAP relay with a test lamp.
Does the test lamp illuminate?		 Go to Step 4  Go to Step 8 
4 
  1. Connect a test lamp between the RAP relay control circuit and the battery positive voltage circuit of the RAP relay.
  2. With a scan tool, command the RAP relay ON and OFF.
Does the test lamp turn ON and OFF with each command?		 Go to Step 6  Go to Step 5 
5  Test the control circuit of the RAP relay for a short to voltage or a short to ground. Refer to CIRCUIT TESTING and WIRING REPAIRS .
Did you find and correct the condition?		 Go to Step 11  Go to Step 7 
6 Inspect for poor connections at the RAP relay. Refer to TESTING FOR INTERMITTENT AND POOR CONNECTIONS and CONNECTOR REPAIRS .
Did you find and correct the condition?		 Go to Step 11  Go to Step 9 
7 Inspect for poor connections and terminal damage at the harness connector of the BCM. Refer to TESTING FOR INTERMITTENT AND POOR CONNECTIONS and CONNECTOR REPAIRS .
Did you find and correct the condition?		 Go to Step 11  Go to Step 10 
8 Repair the battery positive voltage circuit of the RAP relay. Refer to WIRING REPAIRS .
Did you complete the repair?		 Go to Step 11  -
9 Replace the RAP relay.
Did you complete the replacement?		 Go to Step 11  -
10 
IMPORTANT: Replace the BCM. Refer to BODY CONTROL MODULE REPLACEMENT .
Did you complete the replacement?		 Go to Step 11  -
11
  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.