Hello, The 4 wheel drive stopped working in my 1999 Jimmy (same car as a Chevy Blazer, just made by GMC). I got 103,000 miles on the car. I've had the car since it was new and always used the 4 wheel drive at the proper times (snow, ice..ect.). It'* always worked perfect. I've kept the fluid fresh. It has GMC auto trak 2 fluid in it. It'* got the so-called "automatic transfer case".
When I press the 4-hi button, the 4-hi light blinks like it'* supposed to, but normally, it only blinks for a few seconds, and then comes on steady. Now it just keeps blinking and the car stays in 2 wheel drive. When I press the 2-wheel drive button, the 2-hi light comes on steady as it should. I've already tried shifting into all gears and reverse while trying to get it to go into 4-hi. But it just won't go into 4-hi.
When i press the button'*, I can hear the accuator underneath the car making what seems to be the usual 3 to 4 second humming and click noises it'* always made, so I'm not sure where to start looking for the problem. I gotta have my 4 wheel drive working, thats one of the main reasons I got the car, cause I like having 4x4 for bad weather.
Does anybody have an idea where I should start looking to find the cause?

I started looking around under the hood. I was going to take the battery and battery holder out to look at the vacuum device that pulls the differential cable. but I started looking at hoses in general and i found this disconnected hose in the attached picture just hanging there. I looked and looked and it'* probably right in front of me, but I cannot find where it'* supposed to attach to. It may not have anything to do with my 4 wheel drive problem, but obviously I need to reconnect it. Can somebody pop the hood on their Blazer or Jimmy, and let me know where the heck this hose is supposed to be attached to please? Even better, if you have the time, please take a picture of the connection. It'* on the drivers side at the back not far from firewall. Mines a 1999. thank you, Mike

ok, i found out plugging the un-attached vacuum line with a screw fixes the 4 wheel drive problem. and i found out where the un-attached vacuum line went to.....it goes to a vacuum line nipple that leads nowhere. the nipple is on a flange that sticks out an inch or 2 from the inner wheel well. from that point the nipple seems to go nowhere.....it does not seem to go into the wheel well or anything.....i guess its just a dead-end vacuum nipple.....why didnt they just put a cap on the vacuum line??? ....who knows......well, im relieved now, cause i been looking at the cost of 4-wheel drive component parts, thinking something serious was wrong. thanks...see attachment