The NVG 233 transfer case features a three 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 will remain ON constantly.

The NVG 233 transfer case provides the driver with 3 manual mode/gear positions:

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:

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

The front axle indicator switch is mounted to the front axle assembly. When 4WD has been selected by the driver and all conditions have been met to complete the shift, the transfer case encoder motor will engage the front axle gears closing the front axle switch. This sends ignition voltage from the 4WD fuse to the PCM to indicate the front axle has been engaged.

The encoder is mounted to the transfer case encoder motor assembly and is replaced only as an assembly. The encoder converts the sector shaft position (representing a mode or range) into electrical signal inputs to the transfer case shift control module. The module detects what position the transfer case is in by monitoring the 4 encoder channels (P, A, B, and C). These inputs translate into 2HI, 4HI, and 4LO or whether the motor is still in transition between gears.

The transfer case encoder motor consists of a permanent magnet (PM) DC motor and gear reduction assembly. It is located on the left hand side of the transfer case. When activated it turns the sector shaft of the transfer case (clockwise or counter clockwise) to shift the transfer case and to apply the clutch that applies the front propshaft. The encoder motor is controlled with a pulse width modulated (PWM) circuit provided by the transfer case shift control module. This circuit consists of a driver on both the Motor Control A and Motor Control B circuits. The encoder motor is bi-directional to allow the motor to shift the transfer case from 2HI or 4HI to 4LO positions.

The transfer case shift control module receives the following functions:

The transfer case shift control module receives input signals from the following sources:

The transfer case shift control module outputs signals to the following components:

In order to ensure the electronic shift system is operating properly, the transfer case shift control module continually performs diagnostics checks on itself and other parts of the electronic shift system when the ignition switch is in the run position. The following are different types of system checks that the transfer case shift control module continually performs.

RAM/ROM Check

The transfer case shift control module compares the current internal operating program with a stored operating program. This comparison allows the shift control module to evaluate if the shift control module's RAM and ROM are operating properly. Should the shift control module detect a problem with the internal operating program, a diagnostic trouble code (DTC) of 4 is stored indicating the shift control module's memory is damaged.

Data Memory Retention Check

The transfer case shift control module checks the stored diagnostic trouble code memory to see if the memory has changed since the ignition switch was last positioned to OFF. This self test determines if the RAM standby power supply has been interrupted. If the transfer case shift control module detects a RAM standby power supply interruption, (the loss of stored diagnostic trouble code) the shift control module stores a DTC of 1 indicating RAM standby power failure. This condition occurs when the shift control module is disconnected from the wiring harness or battery power is removed.

Encoder Signal Check

The transfer case shift control module checks the encoder signal in order to verify that the transfer case is in either 2HI, 4HI, or 4LO.

Encoder Switch Monitor Check

While the transfer case shifts, the encoder within the encoder motor assembly is monitored for the proper operating sequence. If during a shift, the encoder changes to an incorrect position, an error counter in the transfer case shift control module starts to count the number of times the encoder fails. If the encoder fails eight times, the shift control module produces a DTC of 2 indicating an encoder fault. When this happens, the shift control module outputs a signal in order to default the rail shift pattern in the encoder in order to allow for only shifts into 2HI and 4LO. To protect against transient, random encoder faults caused by vibration, contamination, electrical noise, etc., the error counter reduces the count by one each time a good encoder value is detected. The encoder must fail 25 percent of the time for the shift control module to store a DTC of 2 and indicate a damaged encoder.

Motor and Relay Voltage Checks

Whenever the encoder motor is turned ON or OFF the motor and the electrical circuits are tested both in the de-energized and energized condition. If one or both of the voltage relays fail to detect the proper voltage after energizing or de-energizing, the shift is aborted by the shift control module and a DTC of 3 is stored in order to indicate a faulty shift control module motor circuit. If one or both relays fail to detect the proper voltage after de-energizing, both relays are turned ON by the shift control module, (even with the ignition switch positioned OFF) in order to prevent the motor from running. The shift control module then stores a DTC of 3 to indicate a faulty shift control module motor circuit. All of the status lamps blink three times, stop, and repeat the blinking continuously in order to alert the driver that the electronic shift system requires immediate repair. The battery will drain when this condition occurs, regardless of the ignition switch position.