I just got this 93 Blazer. 4x4 goes into hi but not low and has trouble going out of 4x4. Air is stuck on defrost. I am new to Blazers so I am trying to learn as I trouble shoot these problems.

I was looking for vacuum lines and leaks and came across 3 hoses that have been plugged with screws. These are located near the firewall on the passenger side. You can see the transmission fill neck in the picture.

The far left is the largest diameter hose. The one is the middle is the top view of a t-fitting with a screw plugged into the lower end.

Any ideas of what each one goes to?

 

This may help a little. The greyed out lines you see are all connected together thru T fittings. The very start of this line goes into a tube sticking out of the plenum close to the distributer on the passenger side. It then goes to the yellowish T fitting. The bottome T line (red arrow) and the very small black plastic line (bottome red arrow) fit together. It then goes down to a large Hex Shape fitting. It branches off to the greyed out line with the 90 elbow and at the bottom (blue arrow) of the Hex fitting it goes into a T fitting which one line (aqua arrow) goes off to the right and the other Tees off to the left, all fastened to the firewall. My pic may be a little conffusing because I have the distributor out and the plenum off. The first large line in your pic may be the line in my pic that is directly behind the large 12v red line at the top of the pic. This line is plugged from the factory and is fastened to the firewall. There are two vacumn lines running along the right side on the fenderwell. One goes to the Vapor Canister (large round metal ball) the other runs up to the front and underneath the battery holder. Check too make sure they are there and are connected.
The book says any problems with the Hi-Lo, in-out etc...the "electronic transfer switch" needs adjusting. I don't know how to do that, perhaps someone else does. Hope this helps some....

 

There should be three lines going toward the passenger fender two 5/32 lines and one 7/32 line. The lines are marked.

7/32 line should be plugged.

1st 5/32 line goes to a T-fitting.
At the T it goes into a 3/8 line which goes toward the top off the hood, and also goes into a 5/32 line that goes to the 4x4 actuator located under the battery tray.

2nd 5/32 line goes to the big round ball

Hope this helps