When the vehicle is in motion, the front driveshaft turns in 2WD and 4WD, this is what makes "shift on the fly" possible. In 2WD the front driveshaft is not powered by the transfer case, it is driven by the left front wheel. A conclusive way to determine if the transfer case is engaging, is to select 2WD, then simultaneously raise both front wheels off the ground. The front driveshaft should turn by hand. Next, select 4WD, now the front driveshaft should not turn. If it passes those tests, the encoder motor is in fact shifting the transfer case and operating the vacuum switch. You have already confirmed that the axle actuator works when vacuum is directly applied to it, which means that the problem is a vacuum line issue that new lines, (or fix the old ones) will correct, or you got a faulty vacuum switch

Just an FYI on the vacuum switch.... it's a 2 position switch, it either connects manifold vacuum to the actuator, or vents the actuator to atmosphere.

 

So...now i've replaced the full length of vacuum lines from the manifold tree to the transfer case vacuum switch and from the switch to the actuator, the vacuum switch and still I' don't have 4x4 because the actuator isn't pulling in...so I go direct from the manifold to the actuator and bingo it all works...next I disconnect both vacuum lines from the TC vacuum switch and plug them together and bingo...it works again...next I plug both vacuum lines directly to the TC vacuum, bypassing the "boot" used to join all three vacuum lines to the TC vacuum switch and fire the vehicle up, go through the paces and voila!!! it all works!!!! so...after chasing everything down...one final piece turns out to be the "boot" connector and that is not a generally available part...even the GM dealer I just spoke with doesn't know if I can get the part so I will have to try to put all three vacuum lines directly onto the switch which there is NOT a lot of room to do that...each of the lines squeezes up against the other as you try to crowd all three lines onto switch vs. using the boot...but hey, at least it now all works...now let's see if the fuel economy gets back to normal.

 

Cool, now you know what the problem is. Might try a boneyard for the 3 way fitting.

 

All back together...bypassed the 3-way connector and put the vacuum lines right on the transfer case vacuum switch...not an easy task!!!! But everything is working great now!!!
Will report after the week on how the MPG is doing.

Captain, Thank you SO much for all your help!!

 

Yeee Haaa! Good job! Glad you got her. Thanks for posting back!

 

That's great! My hands were cramped for a week! I took the easy way out and slathered the whole thing with liquid electric tape. This has worked for a year but I'm sure it will go. If anyone knows a source other then a late model wreck please chime in. Thank's.

 

This is the field test I used on my 02 sensors, and it worked like a charm! Hope it helps!
http://www.youtube.com/watch?v=vdn4Dk5PSwc

 

Ok...after running with the repairs (vacuum lines, transfer case valve) for a couple weeks I can happily report that the mileage has returned to normal... better than 16-17 mpg around town and it looks like 21 or so on the highway...will no better when we travel a lot more highway...most trips are around town. Thanks for all the help...this forum is great!

 

That is great! 21 on the hwy is a GIFT.... and thanks for the report back, alot of people just fade away..............

 

I just had to post here and give props to Captain that was very informative I couldn't believe I sat here and read this whole thread at once Thank You.

I been working on my brothers 96 chevy blazer v6 4.3 4dr and I'm getting horrible gas mileage as well. I been researching these forums, now I have a very good reference. Also, I damaged the EGR tube while removing the exhaust manifold. Basically my EGR tube/pipe is a crinkled. I'm going to try to diagnose the actuator before the EGR tube/pipe first because I noticed some fluid on the actuator switch when I reinstalled everything back when swapping my brother's transmission. Plus his heating vents are acting kind of weird with minimal fan flow and it feels like it's sticking but I'm not sure if a vacuum can effect the flow of the heater vents.

Anyways

Question to Deputyski If you're still around, did you end up with fluid on your HVAC system? or did you notice your heating system acting funky at all?