I have a 2004 astro van and the transfer case control module is only passing the encoder motor 4.5 volts.

Which transfer case does it have ????

Is the service light on ?
SERVICE indicator (4WD/AWD) Lamp
The SERVICE indicator (4WD/AWD) lamp is an integral part of the cluster and cannot be serviced separately. This lamp is used to inform the driver of the vehicle of malfunctions within the automatic transfer case (ATC) system. The SERVICE indicator (4WD/AWD) lamp is controlled by the transfer case shift control module via CKT 1567.
Circuit Description
The ignition switch supplies Ignition 3 voltage to the transfer case shift control module at connector C1 terminal E10 using the 4WD Fuse. When the ignition switch is first turned to the RUN position, the transfer case shift control module responds by flashing the SERVICE indicator (AWD/4WD) lamp for approximately 2 seconds, then turning it OFF, while performing tests on the ATC system. The transfer case shift control module will continue to perform tests on the ATC system during a normal drive cycle.

Hooking up a factory or some other professional scan tool that can view DTC'S - diagnostic trouble code's in the 4X4 control module would be the best way to find the problem.

DTC B0768 Service Indicator Circuit High
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 C0357 Park Switch Circuit High
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

There are three different transfer case's ! Which of these does your olds have ?

Transfer Case - NVG 136-NP4 (One Speed Automatic)
Transfer Case - NVG 233-NP1 (Two Speed Selectable)
Transfer Case - NVG 236 (Two Speed Automatic)

All three have an electronic control module ! When the light is on DTC'S are stored in the transfer case shift control module .

SERVICE indicator (AWD/4WD) lamp
The SERVICE indicator (AWD/4WD) lamp is an integral part of the instrument cluster and cannot be serviced separately. This lamp is used to inform the driver of the vehicle of malfunctions within the automatic transfer case (ATC) system. The SERVICE indicator (AWD/4WD) lamp is controlled by the transfer case shift control module via a Class 2 serial data message.

There are three speed sensors on the Automatic Transfer Case (ATC), two on the rear output shaft and one on the front output shaft. Each speed sensor is a Permanent Magnet (PM) generator. The PM generator produces a pulsing AC voltage. The AC voltage level and number of pulses increases as speed increases.
Vehicle Speed Sensor
One of the two speed sensors on the rear output shaft is the vehicle speed sensor (VSS). It is an input to the vehicle control module (VCM). The VCM sends this information to the transfer case shift control module via the Class 2 Serial Data bus.
Rear Propshaft Speed Sensor
The transfer case shift control module converts the pulsating AC voltage from the rear propshaft speed sensor to a Rear Propshaft Speed in RPMs to be used for calculations. The Rear Propshaft Speed can be displayed with a scan tool.
Front Propshaft Speed Sensor
The transfer case shift control module converts the pulsating AC voltage from the front propshaft speed sensor to Front Propshaft Speed in RPM to be used for calculations, and to monitor the difference between the front and rear propshaft sensor speed. It is also used in the AUTO 4WD mode of operation to determine the amount of slip and the percent of torque to apply to the front axle. The Front Propshaft Speed can be displayed with a scan tool.

Transfer Case Encoder Motor
The transfer case encoder motor is a permanent magnet D.C. motor and gear reduction assembly. The encoder motor is controlled with a Pulse Width Modulated (PWM) signal provided by the transfer case shift control module. It is located on the left hand side (drivers side) of the transfer case. When activated it turns the sector shaft of the transfer case to shift the transfer case. This circuit consists of a power supply relay (Motor Feed A circuit ) that supplies voltage to the motor. The Motor Control B circuit is a PWM driver that varies the duty cycle to control the amount of current through the motor to ground.

There are a number of codes that can set .
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 C0374 General System Malfunction
DTC C0376 Front/Rear Shaft Speed Mismatch
DTC C0550 ECU Malfunction
DTC C0611 VIN Information Error

You don't want to just start replacing parts , that could cost $$$$$$ . You could check connections at all the sensors an the control module , but other then that ,well . Your best bet , take it to a qualified repair shop .

No 4X4 switch on the instrument panel ?
Automatic Transfer Case
The single speed automatic transfer case system shifts automatically from rear wheel drive to four wheel drive when the transfer case shift control module (TCSCM) senses the difference in rotation speed of the front propeller shaft and the rear propeller shaft . Two vehicle speed sensors, one located at the rear output shaft area of the transfer case, and the other, located at the front output shaft area of the transfer case, send front and rear propeller shaft rotation speed information to the TCSCM.
When the front and rear output shaft rotations remain the same, the TCSCM sends pulse width modulation signals to the encoder/motor to keep the transfer case in rear wheel drive. When the front and rear output shafts rotations are different, the shift control module sends pulse width modulation signals to the encoder/motor to engage the transfer case into four wheel drive.
The system will automatically transfer a percentage of torque to either the front or rear axles as required to stop all wheel slip. When the front and rear output shafts rotations return to the same speed rotation, the shift control module sends pulse width modulation signals to the encoder/motor to position the transfer case back into rear wheel drive.
When the transfer case is engaged into four wheel drive, a lamp indicator on the instrument cluster will light. The automatic transfer case system is completely automated, no transfer case control button switching system is required.

Do you know what DTC'S - diagnostic trouble code's are ?
Auto part stores can't read these , your best bet take it to a qualified repair shop . One that has the knowledge an diagnostic equipment .
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 C0374 General System Malfunction
DTC C0376 Front/Rear Shaft Speed Mismatch
DTC C0550 ECU Malfunction
DTC C0611 VIN Information Error
DTC C0895 Device Voltage Malfunction
Without proper factory service an repair info. an diagnostic tools

Hi, it sounds like your front diff engages but your transfer case is not. raise one of your front tires off the ground and see if it will lock up when you go to 4hi. If it does then the problem is your encoder motor that hangs down under your transfer case.

Your brake an CV joint issue can't an won't cause your 4X4 problem . The 4x4 system has a control module which supply's B+ voltage to the switch . Checking for codes in the 4X4 control module should have been the first step . Your best bet , take it to a qualified repair shop . You don't have the knowledge or test equipment to fix this.
SERVICE indicator (4WD/AWD) Lamp
The SERVICE indicator (4WD/AWD) lamp is an integral part of the cluster and cannot be serviced separately. This lamp is used to inform the driver of the vehicle of malfunctions within the automatic transfer case (ATC) system. The SERVICE indicator (4WD/AWD) lamp is controlled by the transfer case shift control module via CKT 1567.

Transfer Case Shift Control Module
The transfer case shift control module uses the VIN information for calculations that are required for the different calibrations used based on axle ratio, transmission, tire size, and engine. The system does not know which calibration to use without this information.
The direct battery supply line (CKT 1640) provides the power to the module and motor circuitry.
Transfer Case Encoder Motor
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 (drivers 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. The encoder motor is controlled with a pulse width modulated (PWM) circuit within the transfer case shift control module. This circuit consists of a driver on both the Motor A and Motor B circuits. The encoder motor is bi-directional to allow the motor to shift the transfer case from 2HI or 4HI to NEUTRAL and 4LO positions.
Do you know what DTC'S are ?
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 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

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 .

Probably a bad transfer case encoder motor. Common problem on Bravadas. Also make sure your transfer case uses only "blue" GM Auto Trac II fluid. Flush once after driving a week with "blue" if you don't find "blue" fluid when origianlly checking. Just temporarily plugging a new encoder motor to harness should get the light to go out. Other less likely problem could be bad AWD control module or fuse.

You will end up destroying the transfer case, The transfer case is electronic and not mechanical as the older ones and the speed sensors will throw off the transfer case, if you lose traction to the rear end in the rain, snow or around a corner. The transfer case may activate and deactivate and without the front drive shaft could result in the transfer case to literately explode, also know as grenade. At that point, your looking at a transfer case and may be a transmission. You van runs on RWD unless the rear wheel start to lose traction and then the encoder motor activates the transfer case turning your astro in too a AWD. This was done to save gas, the older AWD transfer case was casing the idle arm to wear out prematurely and the gas economy was 17 MPG and the new AWD system that you have brought the van to 22 MPG. Why do you want to remove it??? if you do i guess you can disconnect the wire harness to the Encoder motor to the transfer case. Good luck and keep me posted.

1. Inspect Transfer Case Control Module (TCCM) and encoder/motor wiring, terminal contact, grounds.

2. Install updated TCCM calibration.

3. Inspect for TCCM water intrusion.
4. Replace transfer case encoder/motor. 1. Check voltage on pin A, Light Blue wire at the engage switch.

2. The voltage should remain near 1.0v with no buttons pushed, compare to the value on the scanner when the problem happens.

3. Back close to the switch by hand and check for a glitch.

4. Wiggle tug test harnesses at the switch and TCCM.

5. Check terminal to pin grip. could be your transfercase control module locater under drivers side of dash. or the encoder motor

How may buttons for the 4X4 two , three , or four ? Do the lights flash ? Any messages - service four wheel drive system ? Which transfer case does it have ?

Transfer Case - NVG 246-NP8 (Two Speed Automatic)
The transfer case shift control module uses the VIN information for calculations that are required for the different calibrations used based on axle ratio, transmission, tire size, and engine. The system does not know which calibration to use without this information.
Transfer Case Encoder Motor
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 (drivers 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. The encoder motor is controlled with a pulse width modulated (PWM) circuit within the transfer case shift control module. This circuit consists of a driver on both the Motor Feed A and Motor Feed B circuits. The encoder motor is bi-directional to allow the motor to shift the transfer case from 2HI or 4HI to NEUTRAL and 4LO positions.
Transfer Case Encoder
The encoder is mounted to the transfer case encoder motor assembly and is replaced as an assembly. The encoder converts the sector shaft position (representing a mode or range) into electrical signals inputs to the transfer case shift control module. The module can detect what position the transfer case is in by monitoring the 4 encoder channels (P, A, B, and C). These inputs translates into AUTO 4WD, 2H, 4H, NEUTRAL, and 4L or in transition between gears.
Transfer Case Motor Lock
The transfer case motor lock is used to provide a 2H, 4H, and 4L lock-up feature. When the lock circuit is energized, the transfer case encoder motor is allowed to turn. When the transfer case is placed 2H, 4H, or 4L the motor lock circuit has no power provided to it and the lock is applied. This assures that the transfer case remains in the current gear position. When AUTO 4WD is selected the motor lock remains applied until an adaptive mode (torque is applied to the front propshaft) is required. During an adaptive mode the motor lock circuit is energized and the motor lock is released, enabling the encoder motor to turn and apply torque to the front propshaft.


SERVICE indicator (4WD/AWD) Lamp
The SERVICE indicator (4WD/AWD) lamp is an integral part of the cluster and cannot be serviced separately. This lamp is used to inform the driver of the vehicle of malfunctions within the automatic transfer case (ATC) system. The SERVICE indicator (4WD/AWD) lamp is controlled by the transfer case shift control module via a Class 2 message or by a Sevice Indicator Control Circuit.

The first thing to do before assuming it's a loose wire would be to have it checked for codes , auto zone , advance etc... can not check this codes .
DTC B0768 Service Indicator Circuit High
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 C0357 Park Switch Circuit High
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

If you know nothing about this , your best bet would be to take it to a qualified repair shop .