Four Wheel Drive does not engage. But...
 

Hello Blazer Forum,

My wife owns a 1998 4x4 Blazer. When engaging the 4 wheel drive the front wheels do not engage in 4H or 4L but when engaging in 4L the rear tires are in low but the front differential still does not engage.
Where should I start troubleshooting, I think it is odd that when selecting 4L the transfer case does shift to low gear but does not engage the front 4 wheel drive.

Thank you for your help,
John

Take a look at the vacuum actuator under the battery tray, the cable from it to the front diff, and the engagement fork for the passenger side axle. Your problem is likely one of those.

Hello AJBert,
Interesting the cable was disconnected from the actuator so I thought it was going to be an easy fix. Reconnected and started the car to watch the action. Once running and 4H NOT selected the vacuum module pulled the cable off again. So for some reason there is vacuum prior to selecting four wheel drive.

I just purchased a 4WD vacuum switch and will replace once I get home in a few weeks.
Not sure this is the culprit but for 20 bucks a good place to start. Any other suggestions?
thank you.