GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 2005 🢒 Impala 🢒 Vehicle Control Systems 🢒 Computer/Integrating Systems 🢒 DTC B1478

Circuit Description

The body control module (BCM) interfaces with the vehicles power distribution system through the retained accessory power (RAP) relay. The RAP relay allows the BCM to control the application of power to the feed circuits to the sunroof and the power windows. When the ignition switch is in either the ACC or the RUN position, the BCM energizes the RAP relay by applying battery voltage to the control circuit.

The BCM turns OFF the RAP 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 LOCK or OFF, with all of the vehicle doors closed, the BCM leaves the RAP relay energized for 10 minutes, or until any vehicle door opens and the BCM detects a ground signal in 1 of the door ajar circuits.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC B1478 Retained Accessory Power Circuit High

Conditions for Running the DTC

The BCM must be commanding the RAP relay to energize.

Conditions for Setting the DTC
    • The BCM detects a short to battery in the RAP relay control circuit when the BCM is trying to de-energize the RAP relay.
    • The condition above must be present for more than 300 milliseconds.

Action Taken When the DTC Sets
    • The circuits powered by the RAP relay remain powered at all times, if the RAP relay control circuit is shorted to battery voltage.
    • The BCM stores DTC B1478 in memory.

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.

Test Description

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

  1. Listen for an audible click when the RAP relay operates. Command both the ON and OFF states. Repeat the commands as necessary.

  2. Test for voltage at the coil side of the RAP Relay.

  3. This step verifies that the BCM is providing ground to the RAP relay.

    4.

  4. This step tests if ground is constantly being applied to 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. With a scan tool, command the retained accessory power (RAP) Relay ON and OFF.

Does the RAP Relay turn ON and OFF with each command?

Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems

Go to Step 3

3

  1. Turn OFF the ignition.
  2. Disconnect the RAP relay.
  3. Turn ON the ignition, with the engine OFF.
  4. Probe the control circuit of the RAP relay with a test lamp that is connected to a good ground.
  5. With a scan tool command the RAP relay ON and OFF.

Does the test turn ON and OFF with each command?

Go to Step 4

Go to Step 5

4

  1. Connect a test lamp between the control circuit of the RAP relay and the ground 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 8

Go to Step 10

5

Does the test lamp remain illuminated with each command?

Go to Step 7

Go to Step 6

6

Test the control circuit of the RAP relay for an open. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

7

Test the control circuit of the RAP relay for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 13

Go to Step 9

8

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

Did you find and correct the condition?

Go to Step 13

Go to Step 11

9

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

Did you find and correct the condition?

Go to Step 13

Go to Step 12

10

Repair the ground circuit of the RAP relay. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 13

--

11

Replace the RAP relay.

Did you complete the replacement?

Go to Step 13

--

12

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

Did you complete the replacement?

Go to Step 13

--

13
  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