GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 Canyon Pickup - 2WD 🢒 DTC 🢒 DTC B1477 or B1478

Circuit Description

The body control module (BCM) interfaces with the vehicle's power distribution system through the retained accessory power (RAP)/adaptive cruise control (ACC) relay. The RAP/ACC relay allows the BCM to control the application of power to the feed circuits of the windshield wipers and power windows. When the ignition switch is in either the ACC or the ON position, the BCM energizes the RAP/ACC relay by providing ground to the RAP/ACC relay control circuit.

The BCM turns OFF the RAP/ACC relay when the ignition switch is in the START position. The RAP feature therefore disables during engine cranking.

When the ignition switch turns from ON or ACC to OFF, with all of the vehicle doors closed, the BCM leaves the RAP/ACC relay energized for 10 minutes, or until any vehicle door opens.

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 is between 9-18.5 volts.

Conditions for Setting the DTC
    • DTC B1477:
       - The BCM detects an open or a short to ground in the RAP/ACC relay control circuit when the RAP/ACC relay is not energized.
       - The condition above must be present for more than 1 second.
    • DTC B1478:
       - The BCM detects a short to voltage in the RAP/ACC relay control circuit when the BCM is trying to energize the RAP/ACC 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 windshield wipers and power windows will not operate if the relay control circuit is open.
       - The windshield wipers and power windows will operate at all times if the relay control circuit is shorted to ground.
    • DTC B1478:
       - The BCM stores DTC B1478 in memory.
       - The windshield wipers and power windows will not operate.

Conditions for Clearing the DTC
    • A current DTC clears when the ignition is turned OFF.
    • A history DTC clears when the ignition is 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/ACC relay control circuit.
    • The RAP/ACC 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/ACC relay operation, listen for an audible click. Command the ON and the OFF states. Repeat the commands as necessary.

    2.

  2. Verifies whether the RAP/ACC relay has a defect.

    3.

  3. Tests the control circuit of the RAP/ACC relay.

    4.

  4. Tests the ground circuit of the RAP/ACC 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 Retained Accessory Power (RAP) Diagnostic System Check?

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. Command the RAP relay ON and OFF with the scan tool.

Does the RAP/adaptive cruise control (ACC) 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/ACC relay.
  3. Connect a test lamp between the relay coil circuits of the RAP/ACC relay.
  4. 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 4

4

Test the control circuit between the body control module (BCM) and the RAP/ACC relay for a short to voltage, ground or an open.

Turn the ignition ON, 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 5

5

Test the ground circuit of the RAP/ACC relay coil for a short to voltage or an open.

Turn the ignition ON, 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/ACC 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/ACC relay. Refer to Relay Replacement in Wiring Systems.

Did you complete the replacement?

Go to Step 10

--

9

Important: Perform the module setup procedure, if required.

  1. Replace the BCM . Refer to Body Control Module Replacement in Body Control System.
  2. Perform the setup procedure for the BCM. Refer to Body Control Module (BCM) Programming/RPO Configuration in Body Control System.

Did you complete the replacement?

Go to Step 10

--

10
  1. Use the scan tool in order to clear the DTCs.
  2. Operate the vehicle within the Conditions for Running the DTC.

Does the DTC reset?

Go to Step 2

System OK