The Isuzu T150 transfer case contains a 2/4 wheel indicator switch and a separate neutral switch. Each switch is provided 12 volts on the signal circuit by the transfer case shift control module. A separate ground circuit is provided to both switches. A fault in the 2WD/4WD signal circuit or the neutral switch signal circuit will cause the 4WD high or 4WD low mode indicator to flash. Also, a fault in the front axle actuator or actuator circuits can cause the 2WD mode indicator to flash. In 2WD mode, the 2/4 wheel indicator switch and the neutral switch are open and the signal circuits are at B+. In 4WD high or low mode, the transfer case 2/4 wheel indicator switch is closed, and the neutral switch is open.

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

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

Scan Tool Reference

Control Module References for scan tool information

1. Ignition ON, select the 4WD high and low mode button one at a time. The mode indicator on the 4WD high and low button, should be ON steady, after the shift is completed.
2. If the vehicle passes the Circuit/System Verification, then refer to Testing for Intermittent Conditions and Poor Connections .

1. Ignition ON, engine OFF. Set the parking brake.
2. Shift the transfer case to 4WD high and the transmission into neutral.
3. Raise the vehicle on a hoist.
4. With a scan tool, command the front axle actuator to engage. While spinning a front tire, the front propshaft should turn.

⇒	If the front propshaft does not spin, refer to Front Axle Will Not Engage .

5. Ignition OFF, disconnect the transfer case 2/4 wheel indicator switch electrical connector.
6. Ignition OFF, verify that a test lamp illuminates between B+ and terminal 2 of the transfer case 2/4 wheel indicator switch connector.

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

7. Ignition ON, with a DMM measure for 10-12 volts on terminal 1 of the transfer case 2/4 wheel indicator switch connector and ground.

⇒	If the voltage is not within the specified range, test the 2WD/4WD signal circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the transfer case shift control module.

8. Ignition OFF, disconnect the transfer case neutral switch electrical connector.
9. Ignition OFF, verify that a test lamp illuminates between B+ and terminal 2 of the transfer case neutral switch.

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

10. Ignition ON, with a DMM measure for 10-12 volts on terminal 1 on the transfer case neutral switch and ground.

⇒	If the voltage is not within the specified range, test the neutral switch signal circuit for a short to ground, or an open/high resistance. If the circuit tests normal, replace the transfer case shift control module.

Important: You must perform the Circuit/System Testing first.

1. Ignition OFF, remove the transfer case 2WD/4WD wheel indicator switch from the transfer case.
2. Connect a DMM across terminals 1 and 2 on the switch. Measure for less than 1 ohm of resistance with the switch plunger depressed.

⇒	If the resistance is more than the specified value, replace the transfer case 2WD/4WD wheel indicator switch.

3. Remove the transfer case neutral switch from the transfer case.
4. Connect a DMM across terminals 1 and 2 on the switch. Measure for less than 1 ohm of resistance with the switch plunger depressed.

⇒	If the resistance is more than the specified value, replace the transfer case neutral switch.

5. Test for continuity on the transfer case neutral switch between terminal 1 and the switch housing and also between terminal 2 and the switch housing.

⇒	If continuity is detected, replace the transfer case neutral switch.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.