You can check the vacuum lines and diaphram that are located underneath the battery shelf. Comes out really easy. Check that the lines are not broken. There is a vacuum line that runs to a solenoid in the center of the fire wall, make sure they are not broken there, and there is an actuator cable that runs to the transfer case. Sometimes it is as easy as replacing the vacuum hose, the actuator cable, or the diaphram under the battery shelf went bad. If you ever had a battery leak acid because it went bad.....that acid goes under the shelf and eats those lines. Hope this helps.
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Which do you have ?
Transfer Case - NVG 236 (Two Speed Automatic)
Transfer Case - NVG 233-NP1 (Two Speed Selectable)
The NV236 transfer case is an automatic transfer case with three drive ranges. Shifting from rear wheel drive to four wheel drive is done automatically when the transfer case shift control module receives wheel rotating slip information from the speed sensors. The transfer case shift control module then engages the transfer case motor/encoder to position the transfer case from rear to four wheel drive. When the transfer case shift control module receives information that the wheel rotation is the same on both axles, the transfer case shift control module sends position information to the motor/encoder to put the transfer case back into rear wheel drive. The three drive ranges are 2HI, 4HI, and 4LO. The 2HI position is for normal driving with the rear wheels pushing the vehicle. The 4HI, position is for driving through sand, snow, mud, gravel, or heavy rain at normal or slightly below normal speeds. The 4HI engages the front axle to allow for four wheel drive. The front axle is pulling the vehicle, and the rear axle is pushing the vehicle. The 4LO range is used for off road driving, heavy snow, deep mud, or shallow fording. The 4LO is similar to the 4HI except that the gearing in the transfer case is set for low speed and high torque. The NV236 is manufactured at Syracuse New York by New Venture Gear a division of New Process Gear for General Motors.
Is the switch light on or flashing ? If you have automatic 4x4 there are several DTC'S that can set >
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
There are 2 models here the push button will flash if the system is not engaging or disengaging properly . If front drive shaft is turning. That is with car raised and supported . if front drive axles do not turn then the axle actuator is not engaging axle shaft passenger side. Remove actuator and manually engage shifting collar with a screwdriver still does not turn then inner axle is twisted.
2 speed electric shift transfer case problems #1 jack up truck place on 4 jack stands. Start and select 4 high position in drive release brake ad watch front driveshaft does it turn "yes" problem is axle actuator on passenger side of differential not engaging axle shaft. "no" apply brakes bring to a stop engage low range does low range work "yes" internal transfer case problems "no" electric transfer case shifting motor failure.
Not good! there is a sensor in your transfer case that senses that you are now in four wheel drive if your transfer case does not lock in it will set a code and the light will flash and default back to two wheel drive. check the fluid level in the transfer case and front differential for a sign of internal failure. that is causing the grinding sound.
Depending on the year, it is not posted, it may be a bad transfer case motor. The clicking you are hearing is the motor trying to engage transfer case & front axle. It looks like a windshield wiper motor & when it gets power & works properly it engages front wheels. Again, depending on the year, may be 4 wheel drive actuator or switches bad on front axle or transfer case.
that vacume line goes back to the transfer case, the end that goes behind the motor hooks to a steel tube so it does not get hot from the exhaust., my guess it should be hooked up so it will feed vacume to the vacume ball so it can lock in the axels. if you start the car and shift into 4 wheel drive and listen under hood you should hear a hissing sound, that is vacume, if you can't hear it there, you may have to craw under the right side to hear it, probably came unhooked underneith the car
The four wheel drive systems in these vehicles are simple. The push button controls the four wheel drive computer. The computer tells the encoder motor (on transfer case) to shift. When the transfer case shifts there is a vaccum switch on the transfer that is activated which sends vaccum to the accuator under the battery. The accuator pulls a cable to lock the front axle in. This covers all s series trucks and suv's with push button four wheel drive but not with auto or all wheel. Now to answer some questions. If your push button is lit up but will not go in to four high you should try to reset the four wheel drive computer. To reset pull the courtesy light and atc fuses for sixty seconds install fuses then cycle key on and off 5 times. If your transfer case shifts but your front end does not lock in check for vaccum at the switch on the transfer case in and out. If no vaccum out replace switch motormite number 49315 at parts store. If you have vaccum out check accuator.