Accessory Switch Transfer Case Mode Indicator Flashes in 2WD Mode
Accessory Switch Transfer Case Mode Indicator Flashes in 2WD Mode
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
The Isuzu T150 transfer case contains a 2/4 wheel indicator switch and a separate neutral switch. Each switch is provided ignition voltage 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 can cause the 2WD 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+ (+0.0/-1.0 V). In 4WD high, or 4WD low, the transfer case 2/4 wheel indicator switch is closed, and the neutral switch is open.
Diagnostic Aids
* A low battery condition or charging system that is faulty could produce this fault. Refer to Battery Inspection/Test (Battery Inspection/Test) and Charging System Test (Charging System Test).
* A TRUE B+ has enough current to light a test light.
* A DIAGNOSTIC B+ is a low current circuit that DOES NOT have enough current to light a test light.
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
Special Tools
* EL-49742 - Battery Voltage B+ and Ground Source Harness
* J 35616 - GM-Approved Terminal Test Kit
Circuit/System Verification
1. Ignition ON, select the 2WD mode button on the accessory switch. After the shift is completed, the mode indicator on the selected button should be ON steady.
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
Note: When test probing harness terminals, it is imperative to use the proper test probe from the most current version of the J 35616 - GM-Approved Terminal Test Kit. Use of an improper test probe may deform the terminal, resulting in intermittent open conditions.
1. Ignition ON, engine OFF.
2. Parking brake engaged. Transmission in Neutral.
3. Shift the transfer case to 2WD.
4. Raise the vehicle on a hoist.
5. With a scan tool, command the front axle actuator to disengage. While spinning a front tire, the front propshaft should not turn.
• If the front propshaft spins, refer to Front Axle Will Not Disengage (Front Axle Will Not Disengage).
6. Ignition OFF. Disconnect the connector at the transfer case 2/4 wheel indicator switch.
7. Ignition ON, engine OFF.
8. Using the YE probe J 35616-33, connect a test light between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator connector pin 1 and a good ground, such as the ground terminal of the EL-49742 - harness , black terminal. The test light should NOT illuminate.
• If the test light illuminates test the affected circuit for a short to voltage. If all circuits and connections test normal, test the transfer case shift control module (TCCM) for proper operation.
9. Using the YE probes J 35616-33, test for a diagnostic B+ (+0.0/-1.0 V) between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator connector pin 1 and a good ground, such as the ground terminal of the EL-49742 - harness , black terminal.
• If the voltage is less than the specified range, test the affected circuit for an open/high resistance, short to ground, or short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.
10. Using the YE probes J 35616-33, test for a diagnostic B+ (+0.0/-1.0 V) between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator connector pin 1 and the ground circuit, 2WD/4WD wheel indicator connector pin 2.
• If the voltage is less than the specified range, test the ground circuit for an open/high resistance or short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.
11. Ignition OFF. Disconnect the connector at the transfer case neutral switch.
12. Ignition ON, engine OFF.
13. Using the YE probe J 35616-33, connect a test light between the neutral switch circuit, neutral switch connector pin 1 and a good ground, such as the ground terminal of the EL-49742 - harness , black terminal. The test light should NOT illuminate.
• If the test light illuminates test the affected circuit for a short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.
14. Using the YE probes J 35616-33, test for a diagnostic B+ (+0.0/-1.0 V) between the neutral switch circuit, 2WD/4WD wheel indicator connector pin 1 and a good ground, such as the ground terminal of the EL-49742 - harness , black terminal.
• If the voltage is less than the specified range, test the affected circuit for an open/high resistance, short to ground, or short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.
15. Using the YE probes J 35616-33, test for a diagnostic B+ (+0.0/-1.0 V) between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator connector pin 1 and the ground circuit, 2WD/4WD wheel indicator connector pin 2.
• If the voltage is less than the specified range, test the ground circuit for an open/high resistance or short to voltage. If all circuits and connections test normal, test the TCCM for proper operation.
16. Go to Component Testing.
Component Testing
Transfer Case 2/4 Wheel Indicator Switch
Note: 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. Using the YE probes J 35616-34, test for less than 1 ohm between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator switch pin 1, and the ground circuit, 2WD/4WD wheel indicator switch pin 2, with the plunger NOT depressed.
• If greater than the specified value, test the affected circuits for an open/high resistance condition. If all circuits and connections test normal, replace the transfer case 2/4 wheel indicator switch.
3. Using the YE probes J 35616-34, test for greater than 1 M ohm between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator switch pin 1, and the ground circuit, 2WD/4WD wheel indicator switch pin 2, with the plunger depressed.
• If less than the specified value, test the affected circuits for a shorted together condition. If all circuits and connections test normal, replace the transfer case 2/4 wheel indicator switch.
4. Test for continuity between the following circuits and the 2/4 wheel indicator switch housing.
* 2WD/4WD signal circuit, 2WD/4WD wheel indicator switch pin 1
* Ground circuit, 2WD/4WD wheel indicator switch pin 2
• If continuity is detected, replace the transfer case 2WD/4WD wheel indicator switch.
5. Go to Transfer Case Neutral Switch.
Transfer Case Neutral Switch
Note: You must perform the Circuit/System Testing first.
1. Ignition OFF, remove the transfer case neutral switch from the transfer case.
2. Using the YE probes J 35616-34, test for greater than 1 M ohm between the neutral switch signal circuit, neutral switch pin 1, and the ground circuit, neutral switch pin 2, with the plunger NOT depressed.
• If less than the specified value, test the affected circuits for a shorted together condition. If all circuits and connections test normal, replace the transfer case neutral switch.
3. Using the YE probes J 35616-34, test for less than 1 ohm between the neutral switch signal circuit, neutral switch pin 1, and the ground circuit, neutral switch pin 2, with the plunger depressed.
• If greater than the specified value, test the affected circuits for an open/high resistance condition. If all circuits and connections test normal, replace the transfer case neutral switch.
4. Test for continuity between the following circuits and the transfer case neutral switch housing.
* Neutral switch signal circuit, neutral switch pin 1
* Neutral switch ground circuit, neutral switch pin 2
• If continuity is detected, replace the transfer case neutral switch.
5. Go to Circuit/System Testing.
Transfer Case Shift Control Module (TCCM)
1. Ignition OFF. Disconnect the connector at the transfer case 2/4 wheel indicator switch.
2. Ignition ON, engine OFF.
3. Using the YE probe J 35616-33, connect a test light between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator connector pin 1 and a good ground, such as the ground terminal of the EL-49742 - harness , black terminal. The test light should NOT illuminate.
• If the test light illuminates test the affected circuit for a short to voltage. If all circuits and connections test normal, replace the TCCM.
4. Using the YE probes J 35616-33, test for a diagnostic B+ (+0.0/-1.0 V) between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator connector pin 1 and a good ground, such as the ground terminal of the EL-49742 - harness , black terminal.
• If the voltage is less than the specified range, test the affected circuit for an open/high resistance, short to ground, or short to voltage. If all circuits and connections test normal, replace the TCCM.
5. Using the YE probes J 35616-33, test for a diagnostic B+ (+0.0/-1.0 V) between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator connector pin 1 and the ground circuit, 2WD/4WD wheel indicator connector pin 2.
• If the voltage is less than the specified range, test the ground circuit for an open/high resistance or short to voltage. If all circuits and connections test normal, replace the TCCM.
6. Ignition OFF. Disconnect the connector at the transfer case neutral switch.
7. Ignition ON, engine OFF.
8. Using the YE probe J 35616-33, connect a test light between the neutral switch circuit, neutral switch connector pin 1 and a good ground, such as the ground terminal of the EL-49742 - harness , black terminal. The test light should NOT illuminate.
• If the test light illuminates test the affected circuit for a short to voltage. If all circuits and connections test normal, replace the TCCM
9. Using the YE probes J 35616-33, test for a diagnostic B+ (+0.0/-1.0 V) between the neutral switch circuit, 2WD/4WD wheel indicator connector pin 1 and a good ground, such as the ground terminal of the EL-49742 - harness , black terminal.
• If the voltage is less than the specified range, test the affected circuit for an open/high resistance, short to ground, or short to voltage. If all circuits and connections test normal, replace the TCCM.
10. Using the YE probes J 35616-33, test for a diagnostic B+ (+0.0/-1.0 V) between the 2WD/4WD signal circuit, 2WD/4WD wheel indicator connector pin 1 and the ground circuit, 2WD/4WD wheel indicator connector pin 2.
• If the voltage is less than the specified range, test the ground circuit for an open/high resistance or short to voltage. If all circuits and connections test normal, replace the TCCM.
11. Go to Component Testing.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Control Module References (Programming and Relearning) for control module replacement, setup, and programming
* Transfer Case Neutral Switch Replacement (Service and Repair)
* Transfer Case 2/4 Indicator Switch Replacement (Service and Repair)