Transfer Case - Nvg 236/246-NP8 (2-SPEED Automatic): Overview GMC Sierra 2500

Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Lack of communication may be due to a partial malfunction of the Class 2 serial data circuit or due to a total malfunction of the Class 2 serial data circuit. The specified procedure will determine the particular condition.
2. 3) - A fuse failure will cause no communication. This step checks for this condition.
3. 4) - This step determines if the fuse blows due to a short to ground on the battery positive voltage circuit.
4. 5) - This step determines if there is a short to ground in the battery positive circuit causing the fuse to blow.
5. 6) - This step refers to the table for DTC C0306 for further diagnosis if a transfer case mode change is causing the fuse to blow.
6. 7) - The presence of DTCs indicates that the transfer case shift control module has detected a fault. Go to symptoms for other concerns.
7. 8) - The presence of DTCs which begin with "U" indicates some other module is not communicating. The specified procedure will compile all the available information before tests are performed.
8. 9) - Vehicle speed is provided to the transfer case shift control module via Class 2 and can affect the setting of DTC C0300. Perform the diagnostics for powertrain DTCs P0502 and P0503 first.
9. 10) - Perform diagnostics for DTC C0300 and C0305 first before addressing other DTCs. DTC C0300 and C0305 can be responsible for setting other DTCs.

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Scheme 68

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests for proper operation of the transfer case mode select switch.
2. 3) - Tests for proper voltage supply to the transfer case mode select switch.
3. 4) - Tests the transfer case mode select switch for proper resistance values in all switch states.
4. 5) - Tests the switch signal circuit for a short to voltage.
5. 6) - Tests the switch signal circuit for an open or short to ground.
6. 7) - Tests the switch signal circuit for a short to the 8 V Reg circuit.
7. 8) - Determines whether the 8 V Reg circuit is greater than or less than the specified voltage.
8. 9) - Tests the 8 V Reg circuit for a short to voltage.
9. 10) - Tests the 8 V Reg circuit for an open or short to ground.

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Scheme 70

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests to see if the malfunction is intermittent.
2. 3) - Tests the internal resistance of the propshaft speed sensor.
3. 4) - Tests to see if the propshaft speed sensor can generate a sufficient amount of AC voltage.
4. 5) - Tests the RSS LO circuit for a short to ground, a high resistance, or an open.
5. 6) - Tests the RSS HI circuit for a short to ground, a high resistance, or an open.
6. 7) - Tests the RSS HI circuit for a short to voltage.
7. 8) - Tests the RSS LO circuit for a short to voltage.
8. 9) - Tests the RSS HI circuit for a short to the RSS LO circuit.

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The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests to see if the malfunction is intermittent.
2. 3) - Tests the internal resistance of the propshaft speed sensor.
3. 4) - Tests to see if the propshaft speed sensor can generate a sufficient amount of AC voltage.
4. 5) - Tests the FSS LO circuit for a short to ground, a high resistance, or an open.
5. 6) - Tests the FSS HI circuit for a short to ground, a high resistance, or an open.
6. 7) - Tests the FSS LO circuit for a short to voltage.
7. 8) - Tests the FSS HI circuit for a short to voltage.
8. 9) - Tests the FSS LO circuit for a short to the FSS HI circuit.

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The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Listen for an audible motor noise when the encoder motor operates. Command both the ON and OFF states. Repeat the commands as necessary.
2. 3) - Tests for a short to ground in the Motor Control A circuit.
3. 4) - Tests for a short to ground in the Motor Control B circuit.
4. 5) - Tests for a higher than normal resistance in the Motor Control A and B circuits through the module.
5. 6) - Tests for a short to ground in the encoder motor windings.
6. 7) - Tests for continuity across the motor circuit. Resistance readings vary depending on the location of the brush contact inside the motor assembly.
7. 8) - Tests Motor Feed A circuit for a short to ground.
8. 9) - Tests Motor Feed B circuit for a short to ground.
9. 10) - Tests Motor Feed A and B circuits for an open or high resistance.

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Scheme 76

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Listen for an audible motor noise when the encoder motor operates. Command both the ON and OFF states. Repeat the commands as necessary.
2. 3) - Tests Motor Feed A or the transfer case shift control module for a short to voltage.
3. 4) - Tests Motor Feed B or the transfer case shift control module for a short to voltage.
4. 5) - Tests Motor Feed A circuit for a short to voltage.
5. 6) - Tests Motor Feed B circuit for a short to voltage.

Scheme 77

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Listen for an audible motor noise when the encoder motor operates. Command both the ON and OFF states. Repeat the commands as necessary.
2. 3) - Tests the for a higher than normal resistance in the Motor Control A and B circuits and through the module.
3. 4) - Tests the Motor Feed A circuit for an open or high resistance.
4. 5) - Tests the Motor Feed B circuit for an open or high resistance.
5. 6) - Tests for continuity across the motor circuit.

Scheme 78

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Listen for an audible motor noise when the encoder motor operates. Command both the ON and OFF states. Repeat the commands as necessary.
2. 3) - Tests the Motor Ground circuit for an open.

Scheme 79

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Measures for proper voltage on the lock feed circuit.
2. 3) - Measures the resistance through the lock control solenoid.
3. 4) - Measures for a short to ground in the transfer case lock solenoid.
4. 5) - Tests the transfer case lock circuit for a short to ground.
5. 6) - Tests the Transfer Case Lock circuit for an open.
6. 7) - Repairs the Brake Feed circuit for an open.

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The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Measures the resistance across the encoder motor lock.
2. 3) - Tests the Transfer Case Lock circuit for a short to voltage.

Scheme 81

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - This step determines if the transfer case control module is supplying the correct 8-volt reference
2. 3) - This step inspects for proper voltage on the four encoder signal circuits.
3. 4) - This step tests for an open or high resistance on the encoder signal ground circuit.
4. 5) - This step tests the encoder signal circuits for an open, short to ground, or short to power.
5. 6) - This step inspects for a binding sector shaft.
6. 7) - This step disassembly and overhaul for a binding sector shaft.

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The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests the operation of the front axle switch.
2. 3) - Tests the electrical control of the front axle switch.
3. 4) - Tests the Front Axle Control circuit for a short to voltage.
4. 5) - Inspects for poor connections at the front axle switch/actuator.
5. 6) - Inspects for poor connections at the harness connector of the transfer case shift control module.

Scheme 83

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Helps determine if the DTC set due to a mechanical or an electrical fault.
2. 3) - Tests the integrity of the Motor Control circuit.
3. 4) - Determines whether the Encoder Motor is moving to the proper position.

Scheme 84

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Helps determine if the transfer case speed sensors are working properly.
2. 3) - Tests to see if the DTC will reset.
3. 4) - Inspects for mismatches and whether speed sensors are wired correctly.
4. 5) - Inspects the transfer case for mechanical failures.

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The numbers below refer to the step numbers on the diagnostic table.

1. 3) - Tests whether the transfer case shift control module is sensing any change in all of the encoder channels.
2. 4) - Helps determine whether the shift is prevented due to a mechanical fault.
3. 5) - Tests whether the transfer case shift control module is sensing any change in any of the encoder channels.
4. 6) - Tests if proper reference voltage is being supplied to the encoder from the suspect circuit.
5. 7) - Helps determine whether a short between 2 or more encoder circuits exist.
6. 8) - Helps determine whether the shift is prevented due to a mechanical fault.
7. 9) - Tests for the electrical energizing of the encoder motor.
8. 10) - Tests the suspect circuit(s) for an open or a short to power.
9. 11) - Tests for short between the suspect circuits.

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

1. 2) - Replacement of the transfer case shift control module.

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The numbers below refer to the step numbers on the diagnostic table.

1. 3) - Tests whether the condition is module, system or condition related.
2. 4) - Tests whether the condition has been corrected or identified as specified in the supporting text.
3. 5) - Replaces the transfer case shift control module.

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Symptoms - Transfer Case

Important: Perform the Transfer Case Diagnostic System Check before using the Symptom Tables in order to verify that all of the following are true

1. There are no DTCs set.
2. The control module(s) can communicate via the serial data link.
3. Review the system operation in order to familiarize yourself with the system functions. Refer to «Transfer Case Description & Operation»(ref-175701-S39268570442005042600000) for system operation.

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests the electrical function of the front axle actuator. Helps determine between mechanical and electrical malfunctions.
2. 3) - Tests for proper power and ground at the front axle actuator.
3. 4) - Tests the operation of the front axle control circuit.
4. 5) - Helps determine if the transfer case shift control module is able to monitor the front axle switch.
5. 6) - Tests the front axle switch circuit for an open or high resistance.
6. 7) - Measures the voltage between the front axle feed circuit and ground.
7. 8) - Tests the front axle control circuit for an open or high resistance.
8. 9) - Inspects the front axle for a mechanical malfunction.
9. 10) - Tests for open fuse in the front axle feed circuit.

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Scheme 91

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests the electrical function of the front axle actuator. Helps determine between mechanical and electrical malfunctions.
2. 3) - Tests for proper and ground at the front axle actuator.
3. 4) - Tests the operation of the front axle control circuit.
4. 5) - Tests the electrical function of the front axle actuator control circuit.
5. 6) - Tests for proper supply voltage to the front axle actuator.
6. 7) - Tests for open fuse in the front axle feed circuit.
7. 8) - Tests the front axle control circuit for an open or high resistance.
8. 9) - Tests the front axle switch circuit for an open or high resistance.
9. 10) - Helps determine if the front axle actuator has a mechanical malfunction.
10. 11) - Tests the front axle switch circuit for a short to voltage.
11. 12) - Inspect the front axle for a mechanical malfunction.

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The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests the functionality of the Mode/Range buttons.
2. 3) - Tests the modules ability to operate the transfer case modes.

Scheme 94

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests the functionality of the Mode/Range buttons.
2. 3) - Tests the modules ability to operate the transfer case modes.

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The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Helps determine whether the front axle switch is functioning properly.
2. 3) - Helps determine whether the front axle switch is locking and unlocking.
3. 4) - Tests transfer case clutch performance.

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

1. 2) - Determines whether the failure is the result of a faulty transfer case shift control module or the instrument cluster.

Scheme 97

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

1. 2) - Determines whether the failure is the result of a faulty transfer case shift control module or the instrument cluster.

Scheme 98

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests the transfer case electrical control of the lamp circuits.
2. 3) - Helps determine a short to ground in the suspect lamp circuits.
3. 4) - Test the suspect lamp circuits for a short to ground.
4. 5) - Repair short to ground in the suspect lamp circuit.

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The numbers below refer to the step numbers on the diagnostic table.

1. 2) - This step determines if any DTCs have set.
2. 3) - Tests for an open or high resistance in the motor windings.
3. 4) - Tests for an open or high resistance in the lock solenoid windings.
4. 5) - This step determines if the module is sending voltage through the motor control A circuit.
5. 6) - This step determines if the module is sending voltage through the motor control B circuit.
6. 7) - This step determines if the reason voltage is not get from the transfer case shift control module to the encoder motor through motor control A and B circuits is due to an open or high resistance in these circuits.
7. 8) - This step determines if the proper 8 V reference signal is coming from the transfer case shift control module.
8. 9) - This step determines if the module is producing the correct voltage through the P, A, B, and C channels.
9. 10) - Tests the encoder signal ground circuit for an open or high resistance.

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Scheme 101

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Determines the position of the Front Axle Switch.
2. 3) - Helps determine if the front axle will not engage.
3. 4) - Helps determine if the front axle will not disengage.

Scheme 102

The numbers below refer to the step numbers on the diagnostic table.

1. 2) - Tests the transfer case electrical control of the lamp circuits.
2. 3) - Helps determine module failures.
3. 4) - Tests the effected lamp circuit for an open or high resistance.
4. 5) - Tests the lamp feed circuit for an open or short to ground.

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1. Raise the vehicle.
2. Remove the transfer case shield mounting bolts from the transmission support (3500 series shown).
3. Remove the transfer case shield from transmission support.

Transfer Case Description & Operation

The NVG 236/246 transfer case features a 4 button shift control switch located on the instrument panel. When the vehicle has the ignition key in the RUN position, the transfer case shift control module starts monitoring the transfer case shift control switch to determine if the driver desires a new mode/gear position. At a single press of the transfer case shift control switch, the lamp of the new desired position will begin flashing to inform the driver that the transfer case shift control module has received the request for a new mode/gear position. The lamp will continue to flash until all shifting criteria has been met and the new mode/gear position has been reached, or has been engaged. Once the new mode/gear position is fully active, the switch indicator lamp for the new position is will remain ON constantly.

During normal driving situations the transfer case can operate in the Auto 4WD mode. In the Auto 4WD mode the transfer case shift control module monitors rear wheel slip speed, based on the inputs from both the front and rear propshaft speed sensors. When the vehicle experiences a rear wheel slip condition, the transfer case shift control module sends a pulse width modulated (PWM) signal to an electronic motor, which is the transfer case encoder motor. This motor rotates the transfer case sector shaft, applying a clutch pack. This clutch pack is designed to deliver a variable amount of torque, normally delivered to the rear wheels, and transfers it to the front wheels. Torque is then ramped up to the front wheels until the front propshaft speed sensor matches that of the rear propshaft speed sensor. Torque is then ramped down until torque is completely removed from the front wheels or until rear wheel slip is once again detected. The process would then repeat.

The NVG 236/246 transfer case has the added feature of also providing the driver with 3 manual mode/gear positions

1. 4HI - 4 Wheel Drive high range
2. 2HI - 2 Wheel Drive high range
3. 4LO - 4 Wheel Drive low range

The driver may choose to select any of these mode/gear positions while driving the vehicle. However, the transfer case will not allow a shift into or out of 4LO unless the following criteria has been met

1. The engine is running.
2. The automatic transmission is in Neutral, clutch depressed on manual transmissions.
3. The vehicle speed is below 5 km/h (3 mph).

This transfer case also has a Neutral position. A shift to the Neutral position allows the vehicle to be towed without the rare axle rotating the transfer case main shaft and the transmission output shaft. Neutral position may be obtained only if the following criteria has been met

1. The engine is running.
2. The automatic transmission is in Neutral, clutch depressed on manual transmissions.
3. The vehicle speed is below 5 km/h (3 mph).
4. The transfer case is in 2HI mode.

Once these conditions have been met, press and hold both the 2HI and 4LO buttons for 10 seconds. When the system completes the shift to neutral, the red neutral lamp will illuminate.

View the list of major components that make up the automatic transfer case (ATC) system below.