GM Service Manual Online
For 1990-2009 cars only

Circuit Description

At each power-up the transfer case shift control module runs a self-test on the following functions:

    • EEPROM Checksum
    • ROM Checksum
    • RAM Checksum
    • RAM Malfunction

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC C0550 ECU Malfunction

Conditions for Running the DTC

    • The ignition is ON.
    • The system voltage is 9-18 volts.

Conditions for Setting the DTC

    • Critical operational parameters stored in EEPROM have failed Checksum test indicating invalid data.
    • Masked ROM code has failed Checksum test indicating that masked ROM data is not currently valid.
    • ROM code has failed Checksum test indicating that ROM data is not currently valid.
    • One or more ROM locations have failed operational test indicating that some portions of RAM are not functional.

Action Taken When the DTC Sets

    • The SERVICE 4WD indicator will be latched on and the system will be disabled.
    • The transfer case control switch will display the current position LED while DTC C0550 is current, but further shifting will be disabled.

Conditions for Clearing the DTC

    • Replace the transfer case shift control module.
    • Perform the Diagnostic System Check - Vehicle.

Test Description

The number below refers to the step number on the diagnostic table.

  1. This step replaces the transfer case shift control module.

Step

Action

Yes

No

Schematic Reference: Transfer Case Control Schematics

Connector End View Reference: Transfer Case Control Connector End Views

1

Did you perform the Diagnostic System Check - Vehicle?

Go to Step 2

Go to Diagnostic System Check - Vehicle

2

Replace the transfer case shift control module. Refer to Control Module References for replacement, setup and programming.

Is the repair complete?

System OK

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