Above left: Mode actuator removed for testing. With vacuum applied to the top
inlet, the lever pulls up on the mode door, which sends air to the floor. Vacuum
applied to the lower inlet forces the lever down and sends air up to the dash
vents. Like all the actuators, this one tested ok for holding vacuum, which
leads me to believe I'm either lucky, or the rubber diaphragms are more
durable than most people think. This actuator held very little fluid when I
suck-tested it with my Mity-Vac.
Above right: The actuators all seemed to hold decent vacuum. The MityVac was
a great help in diagnosing vacuum leaks. My problem seemed to be more about
dry rot in a couple of the vacuum lines, than failing actuators. I replaced all
HVAC actuators anyway, since they don't cost too much.
Above: Here's the main wiring harness for the steering column. The GM
engineers really did not want this harness to accidentally disconnect. While
most harnesses use various types of clips which snap together when the
harness ends are joined, this one has a 7mm bolt to keep it firmly in place.
Makes sense, since there's a lot going on within these wires.

The yellow harness in the next photo below even had an extra "safety" clip
(orange). These wires control the airbag. When airbags fail to deploy, car
makers tend to lose lawsuits (very expensive lawsuits).
Here's the actuator that controls the defroster vent. When vacuum enters the
blue line, the actuator lever pulls down and the defroster flap (brown door)
opens. When open, the defroster vent is actually blocked, which causes the air
to be directed to the dash vents.
Most of the HVAC parts we're interested in are located behind the
dash, which makes this job somewhat unpleasant. It starts with
removing all the many plastic trim pieces which conceal the underside
of the dash. The best way to do this is to get yourself a 7mm socket
driver (or better yet, a cordless drill with a socket adapter) and start
unscrewing. If you're smarter than me, you'll carefully segregate all of
the screws and bolts, label them in Ziplock baggies, and take a
bunch of digital photos. When the job is done, you won't remember
where they all go...trust me.

The trim pieces I removed for this job included some of the parts seen
here. All the under-the-dash plastic came out, as did the trim around
the instruments. I removed the radio, the heat and A/C controller, the
ash tray, and the glove box door. The last two components might not
have needed to be removed, but I was doing this for the first
time...didn't really know what I was doing.

It's important to disconnect any electrical harnesses that may interfere
when the dash is moved. The GM designers were kind enough to
make the dash pivot forward for access to the components behind it.
This is nice, because the whole dash can remain in the vehicle for this
job. However, if there's not enough slack in the wires, some of the
harnesses may be stretched when the dash pivots forward. Some of
those harness clips are tough to figure out, but a small flat-blade
screwdriver and a set of metal picks helped out greatly. A small
retractable mechanic's mirror is also helpful, as it's difficult to see
behind some of the harness connectors that must be disconnected
before certain trim pieces can be removed.

Once all the trim pieces on the front and underside of the dash are
removed, the defroster grille and the dash speaker covers must be
removed. My Sonoma has 4 metal screws under these grilles which
secure the dash to the firewall. The defroster grille is tricky - this is
one of the most commonly broken pieces of dash removal. The grille
is held in by several metal clips and must be carefully pried out.

Most of the various S-series owner websites have "Stickies" in their
forums that cover dash removal procedures. These are often
contained in heater core replacement discussions, where the entire
dash has to be removed from the vehicle. The website I gleaned the
most info from was ZR2USA.com, which was focused on the ZR2
version of S-series vehicles (like my Blazer).
Diggin' into the Dash
My next task was dropping the steering wheel. This is necessary to
allow the dash to pivot forward on its hinges. I removed my front seat
to give more room to move around; plus, the steering wheel needs to
be dropped to the floor to allow the dash to pivot forward far enough.
It's pretty simple...4 bolts and one small trim piece around the bottom
of the seat. Looking back on it, I would have dropped the steering
column before removing the trim around the instrument gauges. The
trim fits pretty tightly around the column.

Dropping the steering wheel, for me, was a better alternative than
removing it altogether. The dash could pivot forward and rest against
the steering column, leaving me free to work behind the dash on my
own. Also, the 1996 Sonoma has its shifter lever on the steering
column, which meant I didn't have to remove it or detach anything
from it. The various wiring harnesses do need to be disconnected,
however.

The column is attached with four 15mm nuts: two under the dash and
two where the steering shaft attaches to the frame. There's also a nut
and bolt through the shaft coupler, which I removed; not sure if this
was necessary.
Above: Steering column nuts removed; column pulled out about an inch to
clear the studs in the red part of the frame in the photo at left.

As mentioned previously, I also removed this bolt from the shaft coupling near
the firewall, which may have been what allowed me to pull on the column just
enough so that it cleared the studs and could drop to the floor (which is
possible because of the universal joint in the shaft).

Note to the wise: get a cordless impact driver. Priceless.

Below: With the steering column dropped, you can begin detaching any wiring
harnesses that may not have enough slack when the dash is pivoted forward.
After the screws are removed from under the defroster and dash speaker
grilles, there are two 10mm bolts under each corner of the dash, which upon
removal, will allow the entire dash to pivot forward. This is possible because of
metal pegs under each corner of the dash, which act as a hinge when all the
screws and bolts are removed. In this photo, the dash has pivoted forward and
rests on the steering column.
Here's a sampling of what you'll see with the dash tilted forward:
Spot the actuators: blue line is attached to the defroster vent actuator; orange
line goes to the recirculation actuator (opens a vent that pulls in air from inside
the cab). The gray line is the vacuum supply line. The heater core is in the
black plastic container under the colored vacuum lines.
This is the mode actuator, on the left side of the defroster vent door. It is
attached to a door that directs air either to the defroster vent, the floor vents,
or both. The mode actuator has two vacuum lines attached to it: one to pull the
lever, and one to push it. The actuator arm was a real pain to unhook from the
mode door below. There's very little clearance to detach the arm.
Above: For pretty much any inside cab work, removing the seats frees up an
incredible amount of space. The steering column cannot be dropped without
removing the driver's seat, which is attached by 4 studs in the floor.

Also, you never know what might pop up under the seats. I found 42 cents!
Almost made this job worth the effort.

I kind of wish I had taken a lot more digital pictures during this project...there
are at least 25 screws, nuts and bolts to remove. Plus, I did it over two
weekends. What you think you'll remember is often very different from what
you will actually remember.
Behind the HVAC dash
controls is the plug at left,
which distributes vacuum
to the various actuators.
This is a common source
of transmission fluid.
Leaking fluid typically
drips down into the ash
tray. Seen here is
evidence of fluid in the
lines, particularly on the
top left holes. The orange
line had little evidence of
oil contamination,
probably due to the fact
that it controls the
recirculation vent. I almost
New parts: 3 actuators,
vacuum cannister, HVAC
controller, and some new
vacuum line.
With all vacuum lines cleaned and/or replaced, and the new parts installed, I
not-so-simply reassembled everything in reverse of disassembly. When I was
done, all I had left was this. The trim piece came from the driver's seat, and
was too much of a pain to reassemble, for what it was worth. The rest of the
parts? Who knows. Everything fit together fine.
In my research before tackling this project, I found quite a bit of
useful information, some of it relevant to my 1996 Sonoma and some
more applicable to later models. One of the better sources of info
came from a discussion at s10forums.com (I saved a PDF version  in
case this thread ever disappears).

Here's more examples of what I found:
Vacuum system diagrams
Above: Similar diagram as my 1996 Sonoma, except for the vacuum line going
to the cruise control module. My cruise control module does not use vacuum.
Above: This is a diagram of later versions of the vacuum actuators behind the
dash. My 1996 Sonoma has only 3 actuators. I believe the heater slave valve
actuator was added later.
Below: HVAC schematic from 1998 and newer S-series vehicles. Borrowed
without permission from Minnesota Fred. Click on the image above for a larger
view.
Update January 2012
Snow took quite awhile to arrive in the Midwest in
2011-12, but when it did, the Blazer's 4WD wouldn't
work. After much use of my Mity Vac to trace leaks, it
became clear that the hoses weren't connecting tightly
to the transfer case vacuum switch. The transfer case
would engage the switch, but it couldn't maintain
The duct tape
solution
(this view is looking into the hose
connector that slides over the 3
prongs of the switch)
Update March 2012
Here's a word to the wise: stay away from the Dorman vacuum switch.
The one I got failed a year after I installed it, and let huge amounts of
transfer case fluid contaminate the vacuum system. The vacuum
reservoir was so full of fluid that I threw it out and put in a new one.
Two of the actuators in the dash, as well as some of the vacuum lines
under the hood, were leaking around the line connectors. I was not
happy. The Blazer now has a GM vacuum switch.
never use the "MAX" air conditioning setting, so that line didn't get
much chance to be contaminated with fluid.

With my MityVac, I was able to test the actuators by applying vacuum
to each line, before digging into the dash. All actuators seemed to
function ok, and this was further proven when I pulled out the
actuators and bench tested each of them. Various online forums have
much discussion on this topic, with many people finding that the
actuators no longer worked after fluid entered the system. This wasn't
the case with me, but I still wanted to get behind the dash and
properly clean out all of the vacuum lines.

The photo below shows a vacuum line connector located in the dash.
I disconnected the lines and blew compressed air through each one.
It takes some time, but that's about the only way to clean them out.
The colored vacuum lines were all in good condition, with no evidence
of leaking. Not bad for an old truck....
Above: Here's another slightly different version of the 1998 and newer HVAC
diagram. This one is helpful because it tells you which actuators should be
receiving vacuum under the various settings.
Above is a schematic showing the actuators and vacuum lines behind the dash.
(click on diagram for larger view)
Above: This shows how the various doors inside the dash work when vacuum
is applied to the actuators (click on diagram for larger view).

Below: this diagram illustrates vacuum actuators inside the dash, showing
which ports get vacuum when the HVAC control knob is in its various settings.
More Diagrams - 1998 and Newer
Sonoma Transfer Case Switch
enough vacuum to power the 4WD actuator under the battery tray.

I used a Dorman aftermarket vacuum switch with the Blazer, and
apparently the 3 tubes sticking out of it weren't quite large enough in
diameter to fit snugly inside the hose connector. It was an easy fix,
though. I simply wrapped pieces of duct tape (what else?) around the
tubes, and they then fit nice and tight into the hose connector.
Problem solved.