4x4 shifting to 4 hi issue

[Marcinko]

Hi guys-
I spent the morning searching for this issue and am getting nowhere I apologize if I missed the answer.

I have a 1996 Chevy s-10 Blazer 4dr 4x4 with 4.3liter V6. Last night I realized it has been a while since I went into 4x4 to make sure every worked and sounds good. I placed the truck in neutral and hit the dash button to shift to 4x4 high...it blinked and shifted back to 2 high. I tried a couple more times and nothing. I then tried 4x4 Lo and it worked perfect. Tried to go from 4 lo from 4 hi and it blinked and went back to 4 lo.

I am no expert on this system, but I do know it operates using vacuum pressure. Problem is I have never tested vacuum on this truck and would not know the first place to begin.

What could be preventing the truck from shifting into 4 hi, but allows the switch between 2hi and 4 lo?

Appreciate the help and sorry in advance if I missed the answer in another post.

[Will Help]

Get a vacumn gage and hook it up directly to an intake manifold vacumn port. Gage should read 17 to 21 in at idle. If it is lower, you could have a vacumn leak in a line or the manifold gaskets itself. Also check for a bad vacumn canister. They are usually made of plastic and can crack and loose vacumn.

[Marcinko]

Thanks for the response. Stupid question - Where is the intake manifold vacumn port and vacumn canister located?

[Will Help]

Manifold vacumn ports are places where one or more vacumn lines is attached at a single pipe or junction of multi lines coming directly out of the mainfold. Some lines ar plugged off with a little rubber stopper. You may have to put a "T" in a line inorder not to unplug a necessary module/sensor.

The vacumn resevoir used to be a steel can much like a juice can. Newer version is usually a round black plastic ball, don't rust out, that is mounted either on the firewall or somewhere along the right inner fender under the hood.

[Marcinko]

I do see a variety of lines, with some "T"s in them. One of the lines runs to a plastic ball, about the size of a grapefuit, that is bolted to my hood. It is located on the driver side of the truck and at first glance looks to be in poor shape. Running as far as I could under the hood, tracing eah line, I did come across some questionable cracking.

If that plastic ball sounds like the resovoir to you then I think I am on the right track. I'll have to head up to Checker or Autozone to see if there is a vacum gauge available for rent, unless their inexpensive.

I'll go this route and report back on my vacuum pressure....thanks for your help.

[Will Help]

That should be it.