Hi,
My friend has an '01 Blazer with Auto-4WD, 4HI, 4LO. The 4x4 will not engage. If 4HI is pressed, the 4HI button flashes and we hear some noises, like it's trying to engage, but it never does. After waiting a minute, driving it, putting it in neutral... nothing. Then I press the 2HI button and the 2HI button lights solid. The 4LO behaves the same. So it appears that it just will not engage. No fault indicator lights on theinstrument panelare lit. I was reading some threads and it sounds like the vacuum actuator may be the problem?? The battery is located in the front corner, passenger side. Would the actuator be under the battery?

Some history:
Last winter the 4x4 behaved the same way, but after messing with it and going between 2HI and 4HI, it finally engaged. After that it had no problems the entire winter. The 4x4 was never used during the summer, now this winter it is not working again.

We might try to tackle this tonight so any ideas or tips would be greately appreciated! Any suggestions and I'll post back my findings. Thanks alot!!!

 

I have heard tale of the encoder sticking. Browse around the search here to look for similar problems. There are some that are quite knowledgeable here.

 

If you don't mind not having the auto4wd, you could try my locking cable fix.

Here's my how-to

 

That is if the problem is in the front axle...

With the Auto4wd option, you need to have a scan tool (Tech2) to access the TCCM for getting the codes.