Blazer Transfer Case Switch
Around the same time I was testing my knowledge of the infamous
transfer case switch on S-series vehicles, using my 1996 Sonoma as
a pawn, I knew something was up with my 2004 Blazer's HVAC
system. Throughout the winter of 2010-11, I began noticing that air
flowed out the defroster vent when the controls were set to floor
vents. I was still getting heat to my feet, but also warm, dry air into my
eyeballs. I had a pretty good idea
what the problem was.

So, I started tearing into the vacuum lines under the hood. First thing
I checked was the front hub locker vacuum actuator. Like the
Sonoma, it was located under the battery tray. The actuator tested
fine for vacuum and had no oil inside. Next up, I checked the vacuum
supply cannister to see if any transmission fluid had made its way
there. Unlike the Sonoma, with its round ball hanging from the hood,
the Blazer's vacuum cannister was almost entirely hidden between
the inner and outer body walls near the anti-lock brake booster/motor.
The only evidence of its existence was a vacuum hose connecting to
what appeared to be a piece of black plastic stuck to the inner wall.
It's not easy to remove. The metal brackets securing the anti-lock
brake apparatus had to be removed, the fuse box had to be
temporarily set aside, and the cannister itself then had to be pulled
out of its mount holes. The plastic rivets GM used to secure the
cannister were apparently designed to be "one and done." Once they
go in, it's about impossible to get them out without breaking them off
(which I did). That's ok though...a little duct tape did the trick when it
was time to reinstall. Turns out the cannister had no fluid inside.

So apparently I caught the transfer case vacuum switch failure much
sooner on the Blazer than the Sonoma. While there was evidence of
fluid in the vacuum lines, there was much, much less of it.
Doin' It Again
Above: the vacuum actuator for the Blazer, and the hole it fits into.
Below: Had to remove a few brackets to get the anit-lock brake components
out of the way. The bracket on the far left was the most interesting. It fits
inside the inner and outer body walls. Its bolt holes align with the two round
holes on the black bracket on the far right. If you didn't know what to look for,
you'd never know the vacuum canister is there.
Above: A better shot of the vacuum cannister.
Accessing the recirculation actuator and the heater slave valve actuator
is as simple as removing the glove box. The connector block for the colored
vacuum hoses is also back here. Both of these actuators tested fine for
vacuum. When I blew out the lines with compressed air, only a small amount of
fluid came out. The heater slave valve actuator is an extra 4th actuator that
isn't present in my Sonoma. This was added sometime in the late 1990s.
Above: Here is the mode door actuator and the defroster vent actuator. These
are accessible from under the dash hear the accelerator pedal. Unlike the
Sonoma, it's difficult to see the action of the mode door itself. But not having
to drop the dash was one of the better design changes on these later-2nd
generation S-series vehicles. The defroster vent actuator has red and blue
vacuum lines attached. The mode door actuator has a yellow and brown lines

These actuators are not the easiest to remove, because of the plastic clips that
hold them in place. They were clearly designed for ease of installation, not
removal (just like the vacuum cannister).

The vacuum hose connection behind the AC/heat controller was not making a
good connection, resulting in a hiss that wouldn't go away. I replaced the
controller with a new one and I regained control of the vents. However, I still
get air coming out of the defroster vent when the control knob is set for floor
vents. Also, air still comes out of the floor vents even when the heat & A/C is
shut off. That will be a project for another time...not a big enough deal to
worry about for now. With the transfer case vacuum switch replaced, I averted
some potentially more significant problems down the road.
Spring 2011
Helpful hint: you do not have to remove the glove box to access this
area. See that hook in the upper left? Just lift up on it and the glove
box door will swing all the way down and out of the way.
Photo credit: Mike (a/k/a RareBreedZ)
There are a few parts
removed to make this visible.
4WD actuator under battery tray