I've searched this and other forums about 4X4 engagement problems, and learned quite a bit, but still have not found the cause of my problem.

First, it's a 3 button system. We have determined that the stepper motor on the driver's side does move when the dash switches are pressed. We removed the battery and found that the vacuum diaphram is intact and works when vacuum is applied. The shifter cable moves with the vacuum diaphram. I also traced the three vacuum lines to the top of the transmission and found no bad hoses or disconnected hoses. I did find a blocked-off hose (with a bolt stuck in) leading to the area on the driver's side fender that, I believe, was at one time, connected to the vacuum canister under the fender. Whey someone would disconnect this is a mystery to me. There is strong vacuum on this line when the engine is running.

I think the problem is that no vacuum gets to the vacuum diaphram listed above. Does the vacuum switch on top of the trans control this? I can barely reach this switch from under the Blazer, let alone check operation.

One clue to the problem is that the heater controls will not switch the heater doors for on-the-floor heat.

 

That line needs to be connected to the canister for all of the vacuum powered accessories to work properly so I would start with that.

The vacuum switch on the transfer case does control the flow of vacuum to the actuator under the battery.

What happens when you push the 4HI or 4LO buttons? Do the lights just blink then go back to a solid 2HI light?

Have you tried to pull the codes from the TCCM as described in the one of the tech articles?

 

We will figure out a way to reach the vacuum canister. I have small hands and can't even locate it under the fender. I can't think of a reason the previous owner would have pulled this line off and blocked it with a bolt. In any case, when engine vacuum is high, the 4WD system should turn on with or without the canister, but it doesn't.
Also, the heater doors should work if there is vacuum at the controls when the vehicle is idling standing still.

When you press the 4HI button, it lights and stays on. When you press the 4LO button, it blinks and switches the 4HI button light on. Neither causes the 2HI light to come on.

We have not yet scanned for codes.

 

The vacuum reservoir is not located down low. It is up near the top of the fender.

Click here for a picture - found using the search feature with the term as "vacuum+reservoir+fender"...

With the 3-button system, you can get the codes using a paper clip - OBD2 NV233 Diagnostics

 

Thanks for the easy-to-access info. I know the vacuum canister needs to be connected as the next step.

The transfer case stepper motor is working and the vacuum diaphram under the battery and transfer case shifter cable is functional. The lights on the three switches appear normal but I need to scan OBDII for codes. If there are no codes, I assume the problem would have to be the vacuum switch on top of the transfer case or vacuum lines to it. Is this switch replaceable without removing the transfer case?

It appears the 4WD function on these trucks is what gives owners the most trouble. There are more posts about 4WD than any other technical item.

 

It is true that there are more 4wd problems than pretty much anything else, but most problems are easy to fix. However, one thing to note is that most people don't read, but rather just post even though the information is there to help them fix their problem. For other technical issues, a lot of the time I find that people have actually used the information before posting and haven't had to ask.

Check for the codes as described in the 2nd link I posted above and let us know what they are.