Front Axle Will Not Engage

Front Axle Will Not Engage

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides an overview of each diagnostic category.

Circuit Description

To control the front axle assembly, a front axle actuator will engage or disengage the front axle to transfer case. The front axle actuator motor consists of a permanent magnetic motor, a worm gear controlled plunger, a front axle switch and electronic control circuits all within the actuator assembly.

The front axle actuator consists of the following circuits:

* An ignition voltage circuit
* A ground circuit
* The front axle control circuit
* The front axle switch circuit

When a shift to 4Hi or 4LO is requested, the transfer case shift control module engages the front axle by grounding the front axle control circuit through a current limiting driver. If the front axle is not able to engage when in 4HI or 4LO, the mode indicator in the switch button will continue to flash.

Diagnostic Aids

* A mechanical condition inside the front axle could prevent the shift fork from moving, causing the motor to be stuck in one position. Refer to Symptoms - Front Drive Axle (Symptoms - Front Drive Axle).
* A low battery condition or charging system that is faulty could contribute to the front axle to not engage. Refer to Battery Inspection/Test (Battery Inspection/Test) and Charging System Test (Charging System Test).

Reference Information
Schematic Reference

Transfer Case Control Schematics (Electrical Diagrams)
Connector End View Reference

Component Connector End Views (Connector End Views By Name)
Description and Operation

Transfer Case Description and Operation (Description and Operation)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Verification

1. Ignition ON, engine OFF, with a scan tool command the front axle to the Unlocked and Locked positions. The axle should turn ON and OFF with each command.
2. If the vehicle passes the Circuit/System Verification, then refer to Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections).

Circuit/System Testing

1. Ignition OFF, disconnect the front axle actuator electrical connector.
2. Verify that a test lamp illuminates between B+ and terminal E of the front axle actuator connector.

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

3. Ignition ON, verify that a test lamp illuminates between terminals C and E of the front axle actuator connector.

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

4. Ignition OFF, connect the front axle actuator electrical connector.
5. Ignition ON, connect a DMM by back probing connector C1 terminal 4 at the transfer case shift control module and ground. The DMM should measure between 10-12 volts.

• If less than the specified range, test the axle actuator control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the front axle actuator.

6. Push the 4 HI button on the accessory switch. The DMM display should switch from 10-12 volts to less than 1 volt.

• If the DMM fails to switch to less than 1 volt, replace the transfer case shift control module.

7. Ignition OFF for 30 seconds.
8. Ignition ON, connect a DMM by back probing connector C2 terminal 12 at the transfer case shift control module and ground. The DMM should measure less than 1 volt.

• If the DMM measures more than 1 volt test the axle switch signal circuit for a short to B+. If the circuit tests normal, replace the front axle actuator.

9. Push the 4 HI button on the accessory switch. The DMM display should switch from less than 1 volt to 10-12 volts.

• If the DMM voltage display fails to switch to 10-12 volts, test the axle switch signal circuit for a short to ground, or an open/high resistance. If the circuit tests normal, replace the front axle actuator.

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

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Control Module References (Programming and Relearning) for transfer case shift control module replacement, setup, and programming.
* Electric Motor Actuator Replacement (Service and Repair)