My daughter has a 1999 Blazer with the three button drive switch on the dash. Last week she called me and said she had put it in 4WD while driving in snow. When she got to clear road she tried to shift it back into 2WD. It would not go.

From reading this forum, I determined the likely cause was a faulty vacuum switch. I replaced it and it still would not go into 2WD. I removed the battery and released the cable from the vacuum pot. The pot fully retracted as if it still had vacuum. I got under the truck and removed the hose from the switch. I heard the vacuum pot release. I checked the cable for free movement and it seemed OK. I then hooked up the cable and started the truck. I then was able to get it into 2WD.

Now it will not switch to 4HI. I can get it to go to 4LO. I have checked to see if the transfer case is switching by running the truck and putting the case into 4LO. With the vehicle in park I can not turn the front driveshaft. When I press the 4HI button the lights flash but it goes into 2WD. In this mode I can turn the drive shaft.

It seems as if the transfer case is not shifting into 4HI. What should I look for?

 

Your daughters truck has the NV233 transfer case in it. As such, THIS THREAD has a lot of pertinent information into the control of that transfer case. Even though the OP states that it is for the 89-94, the application is much wider than that and the links I had provided are good reads as well. THIS THREAD gets into how to check the TCCM for codes which may point you in the proper direction.

 

Here is an update. I got it to shift into 4WD. Now it will not go back to 2WD. I checked for codes and got none.

When the truck is in 4WD HI and 4WD LO I can not turn the output shaft from the transfer case.

With the truck running and in park I had someone press the 2WD button while I turned the output shaft. When the button was depressed I heard the case shifting and could turn the output shaft by hand.

However, the case would not fully lock into 2WD. After a few moments I could hear the case shift back into 4WD and I could no longer turn the shaft.

It seems as if something is preventing the transfer case from locking into 2WD.

 

Most likely, this is caused by an encoder fault where the encoder does not sense that the encoder motor has reached the proper position for 2HI. This is likely the cause of the problem when trying to get to 4HI in your prior posts. The encoder position sensors (hall effect sensors) are supposed to sense the position of a variety of magnets that are attached to the main encoder motor shaft (what turns when you select a new mode/range).

You can back probe the encoder channel wires and see if the states are going through the proper values while you are changing the mode. The details of what the proper values are is given in the RSGear.com links provided in one of the links above.

 

Thanks for the suggestions. I don't have time to look at it until this weekend. I have been able to get it to shift if I disconnect the battery for 30 minutes and reconnect. However, it does not last. So depending on the weather forecast, I put it in 2WD or 4WD for the next day.