I have a 2001 Blazer that would not go into Auto 4WD or 4Hi with the push of the button. Symptom was just a blinking 4WD button light for about 20-30 seconds and then revert back to 2WD. After research on this site (BTW, thanks to Admins for having & maintaining this site), I found the problem:

On my vacuum switch/solenoid (located on top center of the firewall), the top hose had broken in half where it makes the bend to attach to the nipple on the switch. I simply cut off the broken hose portion, reinforced remaining end of the hose with some Rescue Tape (self-adhering silicone tape to prevent future break at the bend), reattached to the nipple and I was good to go ! Here is a pic:

3 Notes:

1. I had to have the Blazer rolling for 4WD to engage....just sitting in the garage with the engine running was not enough.

2. That top hose is clearly deteriorating from age, heat, and under-hood dirt/grime thus ideally should be replaced entirely.

3. In the course of the repair, I also checked the vacuum diaphram under the battery tray, lubed the cable and put rubber conditioner on the diaphram boot. The battery tray was fairly rusted, so I cleaned/sanded and gave it a quick new paint job while it was out.

Hope this is helpful to someone down the line ! Thanks again to the everyone who keeps this site alive and well !

Bill