If you are getting a message that says "Service 4 wheel drive" and the switch light blinks after attempting to make a shift then odds are about 60% that you have a faulty encoder sensor that's part of the shift motor on the transfer case or the push button switch on the dash is faulty. If you can get someone with a scan tool to read the service codes then the code will direct you to which one it is. You could also have some other problem but those two items are what I see fail most often.
SOURCE: My 98 chevy blazer won't shift into 4wd.
Most common problem is the vacuum diaphragm that is so inconviently mounted UNDER the battery tray.The cable that locks in the front diff is attached to it and vacuum pulls it in/out.
SOURCE: s10 pickup 4wheel drive wont shift from 4hi to 2hi
A good ground is very imporant on these vehicles
SOURCE: chevrolet truck 4wd switch blinks
If they are blinking I believe it means that there are fault codes present in the pcm. You need to get the computer scaned to see if there are an codes present. If you live near an Autozone or Checker autoparts they can scan it for free.
SOURCE: I have a 2001 Chevy 2500 4X4. Four wheel drive not
Sounds like the chain in the transfer case has broke or the linkage is out of adjustment
SOURCE: 4wd 4HI and 4LO not working
Check the fuse to the 4x4, if the fuse is good then check the 4x4 electric motor actuator on the transfer case. You should hear it work from 2wd, 4hi to 4low and back to 2wd with the push of the switch. if you don't hear it working then check the wire harness at the actuator motor, look for any damages like corrosion or cracked wire connector clips. If the 4x4 actuator is working on the transfer case, then the front hub 4x4 actuator may have gone bad and that's located on the front differential. It's common for the clip there to corrode and break and is an easy fix with a new collar pin, at worst it's the diaphragm on the vacuum actuator that is located under the battery tray. There is a vacuum switch on the top of the transfer case, driver side but when that fails, your front hub stay's locked in the on position. Good luck and keep me posted and hope it's a simple fuse that has gone bad. You can pick up all these parts at Auto Zone or Parts Source and they will have the step by step instructions for you to get the job done with common tools.
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