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
Doin' It Again
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.
Here is the vacuum actuator for the Blazer, and the hole it fits into.
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
Accessing the
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.
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.
Here's a better shot of the vacuum cannister.
Photoo credit: Mike (a/k/a
RareBreedZ) at
There are a few
parts removed to
make this visible.