GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Blazer - 2WD 🢒 Vehicle Control Systems 🢒 Computer/Integrating Systems 🢒 DTC B1477 or B1478

Circuit Description

The retained accessory power (RAP) function allows selected vehicle functions to operate for 10 minutes after the ignition switch is turned OFF. The body control module (BCM) monitors the ignition switch position, the door jamb switches and the voltage on the RAP relay control circuit . The BCM will command the RAP relay on by grounding the RAP relay control circuit when all the doors are closed and the ignition switch is switched from ON to the OFF position.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC B1477 Retained Accessory Power Circuit Low
    • DTC B1478 Retained Accessory Power Circuit High

Conditions for Running the DTC

The system voltage must be between 9-16 volts.

Conditions for Setting the DTC
    • DTC B1477:
       - The BCM detects a short to ground or an open in the RAP relay control circuit when the RAP relay is not energized.
       - The condition above must be present for more than 1 second.
    • DTC B1478:
       - The BCM detects a short to battery in the RAP relay control circuit when it tries to energize the RAP relay.
       - The condition above must be present for more than 1 second.

Action Taken When the DTC Sets
    • DTC B1477:
       - The BCM stores DTC B1477 in memory.
       - The circuits powered by the RAP relay will not operate if the RAP relay control circuit is open.
       - The circuits powered by the RAP relay remain powered at all times if the RAP relay control circuit is shorted to ground.
    • DTC B1478:
       - The BCM stores DTC B1478 in memory.
       - The RAP relay can not be energized.

Conditions for Clearing the DTC
    • A current DTC clears when the malfunction is no longer present.
    • A history DTC clears when the ignition has been cycled 100 times, without a repeat of the malfunction.

Diagnostic Aids

If the DTC B1477 or B1478 is a history DTC, the fault may be intermittent. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.

The following conditions may cause an intermittent malfunction to occur:

    • An intermittent short to ground or to battery in the RAP relay control circuit.
    • The RAP relay coil is shorted to ground or to battery internally.
    • The BCM is shorted to ground or to battery internally.

Test Description

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

  1. During the RAP relay operation, listen for an audible click. Command the ON and the OFF states. Repeat the commands as necessary.

    2.

  2. This step verifies whether the RAP relay has a defect.

    3.

  3. This step tests the supply voltage circuit to the RAP relay coil.

  4. This step tests the control circuit of the RAP relay.

Step

Action

Yes

No

Schematic Reference: Retained Accessory Power (RAP) Schematics

Connector End View Reference: Master Electrical Component List in Wiring Systems

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. Command the retained accessory power (RAP) relay ON and OFF, with a scan tool.

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. Disconnect the RAP relay.
  3. Connect a test lamp between the relay coil circuits of the RAP relay.
  4. Command the RAP relay ON and OFF, with a scan tool.

Does the test lamp turn ON and OFF with each command?

Go to Step 6

Go to Step 4

4

Test the supply voltage circuit of the RAP relay coil for a short to ground or an open. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 10

Go to Step 5

5

  1. Disconnect the body control module (BCM).
  2. Test the RAP relay control circuit for a short to voltage, ground or an open.
  3. Turn ON the ignition, with the engine OFF, when testing for a short to voltage. Refer to Circuit Testing and to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 10

Go to Step 7

6

Inspect for poor connections at the RAP relay. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 10

Go to Step 8

7

Inspect for poor connections at the harness connector of the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and to Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 10

Go to Step 9

8

Replace the RAP relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 10

--

9

Replace the BCM . Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming.

Did you complete the replacement?

Go to Step 10

--

10
  1. Reconnect all previously disconnected components.
  2. Use the scan tool in order to clear the DTCs.
  3. Operate the vehicle within the Conditions for Running the DTC.

Does the DTC reset?

Go to Step 2

System OK