How can I tell whether the transfer case control module is defective and needs to be replaced?

You need to take it to a qualified repair shop that can check DTC'S - diagnostic trouble code's in the transfer case control module . Code's will point to a diagnostic direction .

The NVG 226 transfer case features a rotary 4 mode shift control switch located on the instrument panel. When the ignition key is in the RUN position, the transfer case shift control module monitors the transfer case shift control switch to determine if the driver desires a new mode/range position. At a single turn 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/range position. The lamp will continue to flash until all shifting criteria has been met and the new mode/range position has been reached, or has been engaged. Once the new mode/range position is fully active, the switch indicator lamp for the new position 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 control actuator lever 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 ramped up to the front wheels until the front propshaft speed sensor matches that of the rear propshaft speed sensor. Torque is ramped down to the front wheels. The process would repeat if rear wheel slip is detected again. The NVG 226 transfer case has the added feature of also providing the driver with 3 manual mode/range positions:

DTC B0770
DTC B0775
DTC B0780
DTC B0785
DTC B0790
DTC B2725
DTC C0300
DTC C0305
DTC C0306
DTC C0321
DTC C0327
DTC C0359
DTC C0374
DTC C0379
DTC C0550

As you can see this transfer case control module can set a number of different DTC'S

Before trying to replace the switch ,are any lights flashing ? 2hi , 4lo, 4hi etc... Do you know which transfer case yours has ?
Transfer Case - BW 4482-NR4
Transfer Case - NVG 246-NP8
How many buttons ?
The BW 4482 transfer case features a 3-button shift control switch located on the instrument panel.
The NVG 246 EAU transfer case features a 4 button shift control switch located on the instrument panel
Before you replace the switch and finding out that it isn't the problem . The switch is a low voltage input to the tansfer case shift control module (A COMPUTER ) Could have diagnostic trouble codes stored .
DTC B0790
DTC B2725
DTC C0306
DTC C0327
DTC C0329
DTC C0359
DTC C0550
Auto part stores can not read these codes .

The BW 4482 transfer case features a 3-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/range 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/range position. The lamp will continue to flash until all shifting criteria have been met and the new mode/range position has been reached, or has been engaged. Once the new mode/range position is fully active, the switch indicator lamp for the new position will remain ON constantly. In addition, the switch includes a VSES request button which sends a voltage signal to the transfer case shift control module. The transfer case shift control module in turn sends a request via the class 2 data bus to the ABS control module which controls the VSES system.

The NVG 246 EAU transfer case features a 4 button shift control switch located on the instrument panel. When the ignition key is in the RUN position, the transfer case shift control module monitors the transfer case shift control switch to determine if the driver desires a new mode/range 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/range position. The lamp will continue to flash until all shifting criteria has been met and the new mode/range position has been reached, or has been engaged. Once the new mode/range position is fully active, the switch indicator lamp for the new position will remain ON constantly.
DTC B2725
Circuit Description
The range/mode switch circuit consists of 5 modes. The transfer case shift control module supplies 5 volts to the switch through the 5 volt reference circuit. The current travel through the resistor of the currently active mode. The current is then returned to the transfer case shift control module through the switch signal circuit.
The transfer case shift control module constantly monitors this signal voltage to determine the condition of the mode switch circuit.
When each of the modes are selected they complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch input to determine whether the Neutral, AWD, 2HI, 4HI, or 4LO mode was selected by the driver.

You may want to take this to a repair shop .

Two speed automatic or is it two speed selectable ? Either case the first thing would be have it check for DTC'S - diagnostic trouble codes . Both are computer controlled .
DTC B2725 ATC Mode Switch Circuit Malfunction
DTC C0300 Rear Speed Sensor Malfunction
DTC C0305 Front Speed Sensor Malfunction
DTC C0308 Motor A/B Circuit Low
DTC C0309 Motor A/B Circuit High
DTC C0310 Motor A/B Circuit Open
DTC C0315 Motor Ground Circuit Open
DTC C0323 T-Case Lock Circuit Low
DTC C0324 T-Case Lock Circuit High
DTC C0327 Encoder Circuit Malfunction
DTC C0362 4LO Discrete Output Circuit High
DTC C0367 Front Axle Control Circuit High
DTC C0374 General System Malfunction
DTC C0376 Front/Rear Shaft Speed Mismatch
DTC C0387 Unable to Perform Shift
DTC C0550 ECU Malfunction
DTC C0611 VIN Information Error

This for the two speed selectable
The transfer case shift control module normally has battery voltage supplied to it at all times through the battery positive voltage circuit (CKT 40). When the ignition is ON, the transfer case shift control module conducts a self-test to determine if it retains the same memory it had when the ignition was last turned OFF. This self-test indicates whether the RAM standby power supply to the transfer case shift control module pin C6 was interrupted, which would result in clearing DTC's from memory. If the self-test fails the transfer case shift control module sets DTC 1.

You can check codes without a scan tool on this two speed selectable system . Do you know what the DLC is ? Under the steering wheel , connector to plug a scan tool into ?
Important: Check the transfer case shift control module for DTCs. The transfer case shift control module is not a class 2 device. DTCs are displayed on the transfer case shift control switch. Use the following four steps to obtain DTCs.
Turn OFF the ignition for 10 seconds.
Connect PIN 13 on the data link cable (DLC) to a ground. The DLC is located in the cab under the instrument panel on the driver's side.
Turn ON the ignition, with the engine OFF.
Observe the transfer case shift control switch select buttons for flashing codes.
DTC 001
DTC 002
DTC 003
DTC 004

12590219 or 12590220 are the part numbers. GM has issued a technical bulletin that most failures are caused by bad connections rather than defective control module. If code c0550 is present in module or the scanner can't communicate with module are the only reason to replace the control module.

Sorry ,but your best bet would be to take your vehicle to a qualified repair shop that has the diagnostic tool that check's for DTC'S - diagnostic trouble code's in the 4X4 control module . Light blinking is suppose tell you that you need to take it in for sevice ,not for you to play with something you have no idea how it work's . You probably couldn't even tell me which 4X4 system your vehicle has . Transfer Case - BW 4482-NR4
Transfer Case - NVG 246-NP8
The BW 4482 transfer case features a 3-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/range 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/range position. The lamp will continue to flash until all shifting criteria have been met and the new mode/range position has been reached, or has been engaged. Once the new mode/range position is fully active, the switch indicator lamp for the new position will remain ON constantly. In addition, the switch includes a VSES request button which sends a voltage signal to the transfer case shift control module. The transfer case shift control module in turn sends a request via the class 2 data bus to the ABS control module which controls the VSES system.

The NVG 246 EAU transfer case features a 4 button shift control switch located on the instrument panel. When the ignition key is in the RUN position, the transfer case shift control module monitors the transfer case shift control switch to determine if the driver desires a new mode/range 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/range position. The lamp will continue to flash until all shifting criteria has been met and the new mode/range position has been reached, or has been engaged. Once the new mode/range position is fully active, the switch indicator lamp for the new position will remain ON constantly.

Looking at DTC'S is a far better place to start then crawling around on the ground looking at thing's you have no idea how there work.
DTC B0790
DTC B2725
DTC C0300
DTC C0305
DTC C0306
DTC C0321
DTC C0327
DTC C0359
DTC C0374
DTC C0379
DTC C0550
DTC B2725 ATC Mode Switch Circuit Malfunction
The range/mode switch circuit consists of 5 modes. The transfer case shift control module supplies 5 volts to the switch through the 5-volt reference circuit. The current travel through the resistor of the currently active mode. The current is then returned to the transfer case shift control module through the switch signal circuit.
The transfer case shift control module constantly monitors this signal voltage to determine the condition of the mode switch circuit.
When each of the modes are selected they complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch input to determine whether the Neutral, AWD, 2HI, 4HI, or 4LO mode was selected by the driver.
Transfer Case Shift Control Switch Indicator Flashes, then Returns to Previous Mode
The transfer case shift control module controls the transfer case lock solenoid. The transfer case lock solenoid is released by being energized. This is accomplished by grounding the lock solenoid control circuit during gear shifts, and in the AUTO 4WD mode. Locking action is applied when power or ground are taken away from the lock solenoid, the transfer case motor is prevented from moving. The system is capable of providing a 2HI, 4HI and 4LO lock-up without the need of additional vehicle power to hold the transfer case in these positions.
The transfer case motor is a bi-directional, permanent magnet, DC motor. When energized, through motor control A or motor control B, the ground is provided by the opposing motor control circuit and then grounded through the transfer case shift control module ground circuit, the motor, through a series of gears, rotates a shaft which moves the mode and range forks to shift the transfer case between the following ranges: 4HI, AUTO 4WD, 2HI, Neutral, and 4LO.
If there is a fault in the brake control circuit, motor control A and B circuits are shorted together, or a motor control driver is damaged in the transfer case shift control module, the transfer case shift control module goes into a shift block mode. The transfer case module will also go into shift block mode if there is a mechanical concern causing the shift lever detent shaft to bind, or the transfer case shift control module is not seeing a signal from the Park/Neutral position (PNP) switch. The shift block mode causes the mode indicator to flash for a period of time when another mode is selected. After that period of time has ended, the indicator reverts back to the previous mode selection.
Without factory service info. to guide you through a set diagnostic procedure you probably don't have a chance of fixing this yourself .

.DTC C0326 08: Transfer Case Lock System Performance Signal Invalid.
The transfer case shift control module controls the transfer case lock solenoid. The transfer case lock solenoid is released by being energized. This is accomplished by grounding the lock solenoid control circuit during gears shifts, and in the AUTO 4WD mode. Locking action is applied when power or ground are taken away from the lock solenoid, the transfer case motor is prevented from moving. The system is capable of providing a 2HI, 4HI and 4LO lock-up without the need of additional vehicle power to hold the transfer case in these positions. This DTC detects a lock solenoid that is mechanically unable to hold when in the locked state.

If your transfer case is bad (broken fork, gear, shaft or something) It could engage both the low and the high ratio gears at the same time. The vehicle will not move if this happens even if you put your foot in it and try to power out of it. Two ratios cannot be engaged at the same time. Just by your description, it sounds like something like a shift fork or shaft has broken loose from the selection gear and this is allowing the gear to slide on its own unchecked, randomly engaging and dissengaging ratios. I would have a trustworthy shop take a look right away. Any reputable shop could diagnos a transfer case problem within an hour. The shop should charge 1 hour diagnostic time to tell if it is bad and what needs to be done to correct it. If they need to take longer than that then have them tell you why, and it better be good.

Good luck, and if you have any further questions- just ask!

Is the switch lit up , light flashing ? Did you have the system checked for DTC'S - diagnostic trouble codes . Should have had this done before replacing parts . TCM ? your replaced the transmission control module ? Or the transfer case shift control module ? If no codes ,check B+ power and grounds for the control module . Find a wiring diagram so you know which wires are what etc.....Videos on youtube on basic automotive electrical testing , testing inputs an outputs for control modules . This system can set a number of codes !
DTC B2725
DTC C0300
DTC C0305
DTC C0308
DTC C0309
DTC C0310
DTC C0315
DTC C0323
DTC C0324
DTC C0327
DTC C0362
DTC C0367
DTC C0374
DTC C0376
DTC C0387
DTC C0550
DTC C0611

The front axle control circuit consists of a Front Drive Axle Clutch Solenoid which is electronically controlled by the transfer case shift control module. This solenoid opens and closes a vacuum control valve allowing vacuum to apply and release a clutch pack inside the front axle housing. The transfer case shift control module determines whether or not the clutch pack has been applied or released based on a signal from the front axle switch mounted on the front axle housing assembly. The front axle is normally locked or engaged during AUTO 4WD, 4WD HI, and 4WD LO modes. The front axle is unlocked or disengaged only during 2 HI or neutral modes.

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 will remain ON constantly.

The NVG 233 transfer case features a 3 button shift control switch, located on the instrument panel. When the ignition is in the RUN position, the transfer case shift control module starts monitoring the transfer case shift control switch, to determine if a new mode/gear position has been selected. At a single press of the transfer case shift control switch, the lamp of the new position begins flashing to inform the driver that the transfer case shift control module has received the request for a new mode/gear position. The lamp continues to flash until all shifting criteria has been met and the new mode/gear position has been reached, or has engaged. Once the new mode/gear position is fully active, the switch indicator lamp for the new position remains ON constantly.

Which 4X4 system does your's have ? Three or four button ?

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Manual or electronic t case?

Section 07-07: Transfer Case, 4x4 System, Service 1994 Bronco/Econoline/F-Series Workshop Manual
DESCRIPTION AND OPERATION Electronic Shift Operation The transfer case is equipped with a magnetic clutch, similar to an air conditioning compressor clutch, which is located inside the transfer case adjacent to the lockup collar. The clutch is used to spin up the front drive system from zero to vehicle speed in milliseconds. This spin-up allows the shift between 2-high and 4-high to be made at vehicle speeds listed in the vehicle owner's guide. The spin up engages the front lock hubs. When the transfer case rear and front output shafts reach synchronous speed, the spring-loaded lockup collar mechanically engages the mainshaft hub to the chain drive sprocket and the magnetic clutch is then deactivated. Shifts between 4-high and 4-low can only occur with the transmission safety switches closed. The vehicle's speed must also be within specified limits as determined by the transfer case speed sensor (3 mph or under). Electronic Shift Control System
This system consists of a two-switch control system, an electronic control module, an electric shift motor (7G360) with an integral shift position sensor, and a speed sensor. Switch Control System
There are two control switches located on the upper right corner of the instrument panel for fingertip shift control. The two switches are mounted horizontally and are identified as "4x4" and "LOW RANGE". Two indicator bar lamps are located in the instrument cluster.



When either of the two switches on the instrument panel control are depressed, the first thing that happens is that the electronic control module receives input from the transfer case shift position sensor to verify what position the transfer case is in (2H, 4H or 4L). Next, the electronic control module looks at the speed sensor and the park/neutral position switch or clutch pedal position switch (depending on transmission type). If all conditions are correct to allow the desired shift, the electronic control module will tell the electric shift motor to execute the shift. After the shift has been made and the electric shift motor is off, the control module again looks at the shift position sensor to make sure the proper shift has been accomplished. Finally, the dash indicator light and the pushbutton light on the control panel will be illuminated by a signal from the electronic control module indicating the desired function has been completed.
Electronic Control Module
The electronic control module, located on the passenger side, controls the operation of the transfer case in response to inputs to the pushbutton control by the vehicle operator, the speed sensor, shift position sensor and the park/neutral position switch. Speed Sensor
The speed sensor, mounted on the rear of the transfer case , tells the electronic control module the speed of the vehicle to allow range shifts (in or out of 4L) to occur. Shift Position Sensor
The shift position sensor, an integral part of the electric shift motor , tells the electronic control module the shift position of the transfer case . Electric Shift Motor
The electric shift motor , mounted externally at the rear of the transfer case , drives a rotary helical cam which moves the 2W-4W shift fork and 4H-4L reduction shift fork to the selected vehicle drive position.

Checking the 4X4 control module for DTC'S - diagnostic trouble codes is what you should done . Need a factory or professional type scan tool , no an OBD2 code reader . Looking at factory service repair info. description of how it works , an testing is how to diagnose . Not guessing an throwing parts at it .
DTC B2725
The accessory switch circuit consists of 4 modes. The transfer case shift control module supplies 5 volts to the switch through the switch signal circuit. The current travels through the resistor of the currently active mode. The current on the switch signal circuit is monitored by the transfer case shift control module. The voltage on the switch signal circuit will vary depending on which mode button is pressed by the driver.
The transfer case shift control module constantly monitors the switch signal circuit to determine the condition of the accessory switch circuit.
When each of the modes are selected they complete a circuit through their own specific resistor. The transfer case shift control module continuously monitors the switch signal circuit to determine whether the Neutral, 2HI, 4HI, or 4LO mode was selected by the driver.

DTC C0327 Encoder Circuit Malfunction
The transfer case shift control module has 4 encoder circuits, and each circuit is supplied 12 volts. These 4 circuits are A, B, C, and P, and each circuit runs to a switch located inside the encoder/motor assembly. When a particular encoder circuit is active, the switch is closed, and the 12 volts are grounded by the switch inside the encoder.
The transfer case shift control module supplies 12 volts on all the circuits. The module can interpret the location of the transfer case shift position when the circuits are grounded.
This DTC detects a fault when it compares the shift sequence/strategy to a chart located in read only memory (ROM), within the electronic control unit (ECU), within the transfer case shift control module. This is accomplished by the module reading the different combinations of encoder signals being grounded while others remain high depending on what mode or range the transfer case is in.

DTC C0550 ECU Malfunction
At each power-up, the transfer case shift control module runs a self-test on the following functions:
• EEPROM Checksum
• ROM Checksum
• RAM Checksum
• RAM Malfunction

I suggest your take your vehicle to a qualified repair shop .