GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The starter/generator control module (SGCM) unit is an electronic device that works in conjunction with the electrical subsystem to efficiently perform DC/DC and DC/AC electrical energy conversion and support vehicle functions. The primary mission of the SGCM unit is to:

    • The auxiliary power outlets auxiliary power outlet (APO) function will convert 36-volt DC bus power to 120 volts A/C. If the value of APO mode is OFF, then the APO function will be disabled and the APO converter/inverter will not transfer or consume any power.
    • The APO output A/C voltage will represent the A/C voltage measured at the output of the APO inverter. This signal will have the following specifications:
       - Range 120 volts AC, +5 percent, -8 percent, at 20A
       - Frequency 60 Hz +/- 0.25 Hz
       - Provide 120 volts AC at 60 Hz energy for ON and OFF board electrical equipment.

The following DTCs are for external 120-volt A/C harness failures, excessive current loads or internal system failures within the SGCM.

    • B139A Output voltage of the converter is too low to meet current APO mode.
    • B139B Value of APO current is less than 60 amps.
    • B139C Value of APO current is greater than 60 amps.
    • B139E APO commanded ON, but the converter remains OFF.
    • B139F APO commanded OFF, but the converter remains ON.
    • B140A Internal SGCM circuit to monitor the ground fault detection.
    • B140C APO is disabled due to improper DC bus input voltage or excessive internal temperatures.

DTC Descriptors

This diagnostic procedure supports the following DTCs:

    • DTC B139A DC/AC Converter Input Voltage Low
    • DTC B139B DC/AC Converter Current Sensor Low
    • DTC B139C DC/AC Converter Current Sensor High
    • DTC B139D DC/AC Converter Output (Single or 1) Current High
    • DTC B139E DC/AC Converter Stuck OFF
    • DTC B139F DC/AC Converter Stuck ON
    • DTC B140A DC/AC Converter Output (Single or 1) Ground Fault Performance
    • DTC B140B DC/AC Converter Output (Single or 1) Ground Fault
    • DTC B140C DC/AC Converter Performance

Conditions for Running the DTC

    • Engine running for 30 seconds
    • The SGCM has received voltage on the ignition 0 voltage circuit.
    • The SGCM has turned ON and no configuration DTC are present.
    • This diagnostic will be run every 1.12 ms.
    • Reference voltage is stable and accurate.

Conditions for Setting the DTC

    • B139A DC/AC converter input voltage low.
    • B139B DC/AC converter current sensor low.
    • B139C DC/AC converter current sensor high.
    • B139D DC/AC converter output 1 current sensor high.
    • B139E DC/AC converter stuck OFF.
    • B139F DC/AC converter stuck ON.
    • B140A DC/AC converter output 1 ground fault circuit performance.
    • B140B DC/AC converter output 1 ground fault detected.
    • B140C DC/AC converter performance.

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, 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

Yes

No

Schematic Reference: Starting and Charging Schematics

Connector End View Reference: Engine Electrical Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

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?

Go to Step 3

Go to Testing for Intermittent Conditions and Poor Connections

3

Inspect for poor connections at the 120-volt, orange harness connector of the SGCM and auxiliary power outlets (APO). Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs .

Did you find and correct the condition?

Go to Step 6

Go to Step 4

4

With a DMM test the circuits of the 120-volt, orange harness for an open or short to ground. Refer to Circuit Testing and Wiring Repairs .

Did you find and correct the condition?

Go to Step 6

Go to Step 5

5

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

Did you complete the replacement?

Go to Step 6

--

6

  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