Sonoma Transfer Case Switch
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.
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).
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.
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 cash 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 refer to the most is
ZR2 USA, which is 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.
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.
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.
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 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.
New parts: 3 actuators, vacuum
cannister, HVAC controller, and
some new vacuum line. Not sure
the old HVAC controller was bad,
but I replaced it anyway.
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
This diagram is similar to my 1996 Sonoma, except for the vacuum line going to
the cruise control module. My cruise control module does not use vacuum.
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.
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 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.
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.
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....
HVAC schematic from 1998 and newer S-series vehicles. Borrowed without permission from Minnesota Fred. Click
on the image above for a larger view.
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.
Here's a schematic showing the actuators and vacuum lines behind the dash.
(click on diagram for larger view)
Here's a schematic showing the actuators and vacuum lines behind the dash.
(click on diagram for larger view)
This shows how the various doors inside the dash work when vacuum is applied to the
actuators (click on diagram for larger view).
More Diagrams - 1998 and Newer
Here is a diagram of the vacuum actuators inside the dash. This shows which ports get
vacuum when the HVAC control knob is set to its various settings.