Before changing the actuator check the vacuum lines at the vacuum switch on the left side of the transer case if they are wet with transmission fluid the the switch is bad and that will prevent 4wd from engaging.
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no, if the computer doesn't detect a signal from the t case motor it wont even try to shift. the motor sometimes gets clogged with broken down aluminum. I would remove and inspect to see if it is turning freely or just replace it.
What do the lights on the 4x4 selector switch do when you switch to A4WD or $HI or 4LO positions? Do they blink and revert to previous selection? Has the Transfer Case fluid been changed at 50K intervals? There are actuators on the Transfer Case and Front Differential that control the application of power to the front wheels. These actuators are controlled by the TCCM (Transfer Case Control Module). It takes a qualified 4 wheel drive technician to diagnose these systems.
Fab only gave part of an answer. Check the vacuum lines on the driverside firewall. Also check your heater/AC unit make sure the air deflector will change from defrost to vent. If it stays on defrost no matter where you put the knob then you have a disconnected or leaking vacuum line to the dash. If that works and you don't see any open hoses then remove the battery and battery tray. You'll find the actuator there. Remove the vaccum line that goes to it, connect the battery or jumper cables so that you can start the engine. Put it in 4HI and feel for a vacuum at the line. If you do then put it on the actuator and see if it pulls on the cable. If there is no vacuum at that line then the switch at the transfer is bad and isn't sending vacuum to the actuator. If you do have vacuum and the cable didn't pull then change the actuator.
This is something you can do yourself if you have ever worked on a vehicle before. The part you need is called a front drive actuator valve. It costs about $ 50.00, GM part # 15165891.
Here is some more infor about it:
Main 4wd Components: TCCM - Transfer Case Control Module - Located behind the glove box. This is the electronic 'brains' behind the shifting. Switch - The push button switch in the dash - driver input/indicator. Encoder Motor - Located on the transfer case - This is what actually shifts the transfer case, and tells the TCCM what position it is in. Vacuum Switch - Also located on the transfer case - This is what sends vacuum to the vacuum actuator (see below) when the transfer care is in 4HI or 4LO. Vacuum Actuator - Located under the battery tray - This is what engages or disengages the front driveshaft. Front Axle Switch - Located on the front axle - This is what tells the TCCM when the front axle is engaged.
If you want to try and replace it yourself, please let me know how it goes.
Check the fuse to your 4wd at the fuse panel at the end of the dash drivers side. If the fuse is good, put the transmission in neutral and listen for the Transfer case encoder motor to work when you push the button on the dash for 4wd hi then to 2wd. If you do not hear it working, inspect the wire harness at the Encoder motor to make sure there not damaged in anyway due to corrosion or disconnected.
Here's some info on how the 4wd system operates on newer Blazers for all those searching for the cause of the flashing dash switch light and no engagement.
4WD System Operation on 98 up (2nd Generation) Blazers/Jimmies
The front drive shaft always turns on “shift on the fly” systems so the drive train is at the same speed in the front as the rear or the whole thing would break when engaging 4Hi or Autotrac when moving. InstaTrac (NV243 case or NP1) has 3 buttons. AutoTrac (NV246 case or NP8) has 4 buttons and clutch packs in the transfer case and is the only system for on pavement driving! Prior were the NV231 (87-93) lever and NP233 (94-97) electric motor. Your VIN # is the only sure way the dealer can get correct parts/listings! 1. Select mode on dash switch 2. Signal goes to the Transfer case control module (TCCM) or computer located in the cab on the passenger side behind the kick panel. 3. If you want 4 Lo the TCCM commands the encoder motor on the transfer case to switch a lower gear in (requires putting the shifter in Neutral and be stopped or only going 2-3mph). Note that if 4 Hi selected no action done in the transfer case. The transfer case is chain coupled and has the low and high gear. You can put all systems into Neutral by pushing the 2Hi and 4Lo button at the same time when the key is in the run position (or running but not off or acc). 4. The left front (drivers side) CV axle is always engaged to the front differential, but the right side is not coupled in 2wd. There is an actuator located under the battery tray that when commanded is vacuum actuated and pulls a cable from it to the right side CV axle coupler engaging that axle. A sensor is on that axle that tells the TCCM the axle position. 5. On 99up there is an additional vacuum switch on the firewall that switches the vacuum on and signals the TCCM also. 6. The dash light on the selector switch flashes while all this is occurring and will not switch out of 2wd unless all parts report correctly – same on the switch to any mode or back to 2wd.
So, there are a lot of possible failure areas with actuators/computer/motor/sensors/vac hoses and a special scanner is required to read the codes when the Service 4WD light comes on. But, a few things can easily be done before going to the dealer. 1. Check all vacuum hoses from the intake to the drivers side reservoir can, then to the extra switch (99up) and down to the under battery actuator. Small hose is 5/32 and larger is 7/32”. 2. See if the actuator pulls the front axle cable to engage it. If not and all hoses are good the diaphragm may be leaking on the actuator. If it engages but the light is still flashing remove the indicator switch on the front axle (only electrical plug in there) and manually push it in with the connector plugged in and see if the light stops, if so replace. 3. Listen to the encoder motor clunk when 4 Lo selected. If not working look at the wiring going to it for any cuts/breaks and repair if any found. Another indicator switch on top of this case. The fluid in the AutoTrac case should be the newer light blue Autotrak –II synthetic P/N 12378508 ($7 qt) not the red Dexron III. 4. If the light keeps flashing after this then disconnect the battery for 30 minutes for a TCCM reset, just make sure you turn off your radio first. 5. Still not working – take it to the dealer and stop at the bank on the way for $$$$! 12577412 is the part number for the Transfer Case Control Module (TCCM), or you can do the Salvage Yard for $15.00 if that's the part that has gone bad.
your problem sounds like either the transfer case control module located where you hear the clicking sound, or the actuator on the transfer case itself.
try unplugging the transfer case control module and then plug it back in, try engaging 4wd again (make sure your in neutral, coasting at 1-2 mph might help as well) if it still doesn't work check the actuator itself to see if the actuator is trying to engage. if the actuator is trying to engage but doesn't, take the actuator out and check it for wear or damage. if the actuator seems fine and it responds to the buttons when pressed i'd go after the transfer case control module as the culprit. ran into that a few times on those s-10's. 3 out of 4 that i've seen were fixed with a new tccm. 1 of the 4 needed an actuator, check your local junk yard for a working tccm or actuator.