GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The purpose of the accessory power module (APM), integral to the starter/generator control module (SGCM), is to control the amount and direction of energy flowing between the 42v DC and 14v DC Buses via the APM, bi-directional 42v DC to 14v DC converter. During all of the APM functions the voltage is closely monitored between the internal voltage and the 12-volt underhood battery for voltage correlation faults.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0A8B 14-Volt Power Module System Voltage

Conditions for Running the DTC

    • Engine is running for 30 seconds.
    • The SGCM has received voltage on the ignition 0 voltage circuit.
    • The SGCM has turned ON and no configuration DTC is present.
    • This diagnostic shall run every 100 milliseconds.

Conditions for Setting the DTC

APM voltage is not equal to 12-volt battery voltage +/-2 volts.

Action Taken When the DTC Sets

    • The powertrain control module (PCM) stores the DTC information into memory when the diagnostic runs and fails.
    • The malfunction indicator lamp (MIL) will not illuminate.
    • The driver information center (DIC), if equipped, may display a message.

Conditions for Clearing the DTC

    • A current DTC Last Test Failed clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other non-emission related diagnostic.
    • Clear the DTC with a scan tool.

Step

Action

Values

Yes

No

Schematic Reference: Hybrid Controls Schematics

Connector End View Reference: Hybrid Controls Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle

2

  1. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.
  2. With a scan tool observe the starter/generator control module (SGCM) DTC list.

Is the DTC set as current DTC?

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Go to Step 3

Go to Testing for Intermittent Conditions and Poor Connections

3

With a scan tool, compare the 12-volt Converter Output Data from the SGCM data list, with the Desired Accessory Power data in the HPCM data list.

Is the data parameter within the specified value?

+/-2 V

Go to Step 5

Go to Step 4

4

Test the battery positive circuit from the Underhood Fuse Block to the SGCM including the 175-amp fuse for the following:

    • An open
    • A high resistance
    • A short to ground

Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

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Go to Step 7

Go to Step 5

5

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

Did you find and correct the condition?

--

Go to Step 7

Go to Step 6

6

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

Did you complete the replacement?

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Go to Step 7

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7

  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