GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 HUMMER 🢒 2007 🢒 H3 🢒 Power and Signal Distribution 🢒 Data Communications 🢒 DTC B1372, B1373, or B1374
Table 1: Correct Ignition Switch Inputs

Circuit Description

The body control module (BCM) monitors the ignition inputs Ignition Off/Run/Crank, Ignition 1 Run/Crank and the Ignition Accessory signals supplied from the ignition switch. The BCM uses the sequence that the signals appear, their voltage levels and the engine run flag (ERF) status to determine the power mode. Battery voltage is supplied to the ignition switch by a fuse in the underhood fuse block. The Run/Crank voltage circuit energizes the Run/Crank relay. The following table shows the correct ignition switch signals:

Correct Ignition Switch Inputs

Ignition Switch Position

BCM Input and Scan Tool Display

Ignition Accessory

Ignition 1 Run/Crank

Ignition Off/Run/Crank

OFF

Inactive

Inactive

Active

ACC

Active

Inactive

Inactive

ON

Active

Active

Active

START

Inactive

Active

Active

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC B1372 Device Ignition 1 ON and START Circuit Low
    • DTC B1373 Device Ignition 1 ON and START Circuit High

Conditions for Running the DTC

Battery voltage is 9-16 volts.

Conditions for Setting the DTC

DTC B1372

The BCM detects that the Run/Crank voltage circuit is open or shorted to ground for at least 1 second.

DTC B1373

The BCM detects that the Run/Crank signal is shorted to voltage for at least 1 second.

Action Taken When the DTC Sets
    • The BCM stores DTC B1372 or B1373 in memory.
    • The BCM determines a fail-safe power mode dependent on the last valid power mode detected and the state of the ERF.

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.

Step

Action

Yes

No

Schematic Reference: Body Control System Schematics and Power Distribution Schematics

Connector End View Reference: Data Communication Connector End Views

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. Observe the Ignition 1 Run/Crank parameter with the scan tool.
  3. Turn the ignition to all positions and compare the parameter to the correct ignition switch inputs table above.

Does the Ignition 1 Run/Crank parameter display correctly?

Go to Testing for Intermittent Conditions and Poor Connections

Go to Step 3

3

  1. Place the ignition switch in the OFF position, leaving the key in the ignition.
  2. Disconnect the body control module (BCM) harness connector 2.
  3. Probe the Run/Crank circuit with a test lamp connected to a good ground.
  4. Probe the Run/Crank circuit with a test lamp connected to B+.

Does the test lamp illuminate for either test?

Go to Step 7

Go to Step 4

4

  1. Turn the ignition switch to the RUN position.
  2. Probe the Run/Crank circuit at the BCM connector 2 with a test lamp connected to a good ground.
  3. Turn the ignition switch to CRANK.

Does the test lamp illuminate in both RUN and CRANK?

Go to Step 5

Go to Step 8

5

  1. Turn the ignition OFF.
  2. Disconnect the BCM harness connector 1.
  3. Probe the Run/Crank circuit in the connector with a test lamp connected to a good ground.
  4. Probe the Run/Crank circuit with a test lamp connected to B+.

Does the test lamp illuminate for either test?

Go to Step 6

Go to Step 10

6
  1. Disconnect the Run/Crank relay. Refer to Relay Replacement .
  2. Test the Run/Crank circuit between the BCM and the Run/Crank relay for a short to ground or short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 11

7
  1. Disconnect the ignition switch harness connector.
  2. Test the Run/Crank circuit for a short to ground or short to voltage. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 9

8
  1. Disconnect the ignition switch harness connector.
  2. Test the Run/Crank circuit for an open or a high resistance. Refer to Circuit Testing and Wiring Repairs .
  3. Test the battery positive voltage circuit of the ignition switch for an open or a high resistance. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 9

9

Inspect for poor connections at the harness connector of the ignition switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 12

10

Inspect for poor connections at the BCM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 14

Go to Step 13

11

Replace the Run/Crank relay. Refer to Relay Replacement .

Did you complete the replacement?

Go to Step 14

--

12

Replace the ignition switch. Refer to Ignition and Start Switch Replacement .

Did you complete the replacement?

Go to Step 14

--

13

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

Did you complete the replacement?

Go to Step 14

--

14
  1. Reconnect all disconnected connectors.
  2. Turn the ignition ON, with the engine OFF.
  3. Clear the DTCs with the scan tool.
  4. Operate the vehicle within the Conditions for Running the DTCs as specified in the supporting text.

Does DTC B1372 or B1373 reset?

Go to Step 2

System OK