For 1990-2009 cars

### Diagnostic Instructions

• | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.

• | Review Strategy Based Diagnosis for an overview of the diagnostic approach.

• | Diagnostic Procedure Instructions provides an overview of each diagnostic category.

### DTC Descriptor

DTC C0550: ECU Malfunction

### Circuit/System Description

At each ignition ON, the transfer case shift control module runs an internal self-test on the following functions:

• | EEPROM Checksum

• | ROM Checksum

• | RAM Checksum

• | RAM Malfunction

### Conditions for Running the DTC

• | The ignition is ON.

• | The system voltage is between 10-16 volts.

### Conditions for Setting the DTC

• | The parameters stored in EEPROM have failed the Checksum test as invalid data.

• | The parameters stored in ROM have failed the Checksum test as invalid data.

• | One or more RAM locations have failed operational test indicating that some portions of RAM are not functional.

### Action Taken When the DTC Sets

• | The SERVICE 4WD indicator remains illuminated for the remainder of the current ignition cycle.

• | Transfer case shifting is disabled.

### Conditions for Clearing the DTC

• | Replace the transfer case shift control module.

• | Perform the Diagnostic System Check - Vehicle .

### Diagnostic Aids

A low battery condition or charging system that is faulty could set this DTC. Refer to Battery Inspection/Test and Charging System Test .

### Reference Information

#### Schematic Reference

Transfer Case Control Schematics

#### Connector End View Reference

Component Connector End Views

#### Description and Operation

Transfer Case Description and Operation

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

1. Ignition ON, verify that DTC C0550 is not present.
2. Operate the vehicle within the conditions for running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

### Circuit/System Testing

1. Ignition OFF, disconnect the transfer case shift control module electrical connector C1 and C2.
2. Verify that a test lamp illuminates between B+ and C1 terminal 1.

⇒ | If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

3. Verify that a test lamp illuminates between B+ and C1 terminal 9.

⇒ | If the test lamp does not illuminate, test the ground circuit for an open/high resistance.

4. Verify that a test lamp illuminates between C1 terminal 8 and ground.

⇒ | If the test lamp does not illuminate test the B+ circuit for a short to ground or an open/high resistance.

5. Ignition ON, verify that a test lamp illuminates between C2 terminal 10 and ground.

⇒ | If the test lamp does not illuminate test the ignition 1 voltage circuit for a short to ground or an open/high resistance.

6. If all circuits test normal, replace the transfer case shift control module.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

Control Module References for transfer case shift control module replacement, setup, and programming.