GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2007 🢒 Canyon Pickup - 2WD 🢒 DTC 🢒 DTC B1477 or B1478

Circuit Description

The body control module (BCM) controls the retained accessory power (RAP) relay. The RAP 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 relay by providing ground to the RAP relay control circuit.

When the ignition switch turns from ON or ACC to OFF, with all of the vehicle doors closed, the BCM leaves the RAP 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 9-16 volts.

Conditions for Setting the DTC

DTC B1477

    • The BCM detects an open or a short to ground 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 voltage in the RAP relay control circuit when the BCM is trying 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 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 malfunction is no longer present.
    • A history DTC clears when the module ignition cycle counter reaches the reset threshold, 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 .

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. Verifies whether the RAP relay has a defect.

    3.

  3. Tests the control circuit of the RAP relay.

    4.

  4. Tests the coil supply voltage circuit of the RAP relay.

Step

Action

Yes

No

Schematic Reference: Power Moding Schematics

Connector End View Reference: Master Electrical Component List

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

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 retained accessory power (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. 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

  1. Turn OFF the ignition.
  2. Disconnect the body control module (BCM).

Test the control circuit between the BCM and the RAP 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 .

Did you find and correct the condition?

Go to Step 10

Go to Step 5

5

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

Refer to Circuit Testing and Wiring Repairs .

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 Connector Repairs .

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 Connector Repairs .

Did you find and correct the condition?

Go to Step 10

Go to Step 9

8

Replace the RAP relay. Refer to Relay Replacement .

Did you complete the replacement?

Go to Step 10

--

9

Replace the BCM. Refer to Control Module References for replacement, setup, and programming.

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 1

System OK