Sonoma Transfer Case Switch
The 4x4 actuator is sometimes overlooked when purging the HVAC system of fluid. My actuator was very full
of oil - much more than is shown here. However, it did still hold vacuum very well. Since it's much easier to
replace this actuator than any behind the dash, I elected to continue using it. That thin, coiled wire near the
diaphragm acts as a clip that holds the cable in the diaphragm housing.

Some guys give up on the vacuum system altogether and solve the 4x4 engagement problem in creative
Here is one example (also available in PDF format in case the link does not work).
One of the most common complaints of S-series owners is that over time, the HVAC system stops working correctly.
Airflow refuses to come out of the proper vents and a noticeable hissing originates behind the heating and air controls
on the dash panel.  Anyone new to S-series ownership is usually shocked to find out that this is caused by…the
transfer case vacuum switch? Yes, it's true. Actually, the root cause goes a little deeper than that, but a bad vacuum
switch on the transfer case of four-wheel-drive (4WD) vehicles is the major culprit.

So…what the heck does the engagement of 4WD have to do with the HVAC system? More than you might think. The
reason for this has to do with GM’s use of engine vacuum as a source of energy to make things do stuff. The physics
of internal combustion engines, if you are interested, includes the production of vacuum. S-series vehicles, as well as
many others, make use of this vacuum to control various doors that open and close behind the dash. These doors
direct warm or cool air into certain vents inside the cab. The doors are controlled by vacuum-driven actuators which
The round-ish looking thing with the silver lever sticking out of it is the defroster
actuator. When vacuum is applied to the blue hose, the arm pulls open the defroster
vent door and air is directed to the defrost vent.
move them back and
forth in various
directions. Vacuum is
also used to engage
an actuator which
pulls on a cable that
locks the front axles
on four wheel drive
vehicles. The
activation is triggered
by linkage inside the
transfer case, which
upon engagement of
the transfer case,
opens a vacuum
passage that
supplies vacuum to
the actuator that
pulls the cable that
locks the front axles
(got that?). The
vacuum passage
opens and closes by
way of a 3-pronged
vacuum switch that
sits on top of the
transfer case. Seals
inside the switch
keep the fluid inside
the transfer case out
of the vacuum lines.

However, it's possible for the transfer case to "over-fill" itself with fluid. How? The junction of the transfer case and the
transmission is contains a seal, which is supposed to prevent the automatic transmission fluid (ATF) inside the
transmission from making its way into the transfer case.  These seals can fail over time. When that happens, ATF
from the transmission will leak into the transfer case. The transfer case is not supposed to be completely full of ATF
(the transfer case also uses ATF for fluid). When it does get full, ATF can push its way past the vacuum switch and
into the rubber vacuum lines that connect to the switch. Once the fluid makes its way into the lines, vacuum sucks the
fluid all through the vacuum system.  This is bad. The reason it’s bad is because ATF  and HVAC actuators do not
play well together. The rubber parts get soft and don’t seal vacuum well. Fluid begins to leak into unsightly places.
Over longer periods of time, transmission fluid levels drop. None of this is good.

After many years, GM engineers apparently caught on to what had to have been an odd correlation between the
ordering of replacement vacuum actuators and transfer case vacuum switches at the same time. The exact date of
recognition is unknown, but (supposedly) the switch has now been redesigned so that fluid won’t get past it (or at
least not as easily). Unfortunately for the tens of thousands of us who own S-series vehicles with the old transfer case
switches, we gotta deal with the consequences when the transfer case input shaft seal fails.

Those consequences usually include some or all of the following:

Noticeable hissing in the HVAC controller on the dash
Somewhere in the HVAC system, there’s a leak. Since it often seems like the sound is originating from behind the
control selector, the initial conclusion from anyone not familiar with the transfer case switch connection might be to
simply replace the $144 selector/temperature control unit and call it good. Sometimes it
is good, but usually not for

Lack of control of HVAC functions
The hissing might actually be coming from the vacuum-powered actuators, buried deep within the guts of the
dashboard. Transmission fluid tends to weaken the rubber diaphragms inside the actuators, which along with age,
may cause a rupture of the diaphragm and loss of vacuum control. The hissing may be the sound of air being sucked
through holes in the diaphragm. As the ruptures grow, eventually the vehicle’s vacuum capacity won't be sufficient to
move the actuator arms. When that happens, the HVAC system will seem to have a mind of its own. Air will come from
different vents than what is indicated on the control knob, or no air will come at all in certain settings. The control
knob itself will seem to move from setting to setting with less resistance.

Transmission fluid in the ash tray (yes…the ash tray)
This, the strangest phenomenon, is caused by fluid making its way to small, multi-colored vacuum lines connected to
the controller. The control knob just happens to be located directly above the ashtray. Leaky fluid conveniently drops
straight down into the ash tray, instead of your carpet, which is kind of nice.

Drop in Transmission Fluid Level
When the transfer case input shaft seal fails, ATF moves from the transmission to the transfer case. The transfer
case fills up with excess fluid, which depletes the ATF in the transmission. The transmission fluid dipstick may not
show much of a drop in fluid level over short periods of time, but eventually you'll see noticeably lower fluid levels.

One thing to keep in mind, however, is that over time, a bad input shaft seal will continuously over-fill the transfer
case with ATF. This will probably "test" the transfer case vacuum switch, as its seals are tasked with keeping ATF out
of the vacuum lines. So replacing only the vacuum switch and not the input shaft seal will possibly bring you back to
the same problem again.

An easy way to check the transfer case input shaft seal is to check for excess ATF in the transfer case. Simply
unscrew the
filler hole (upper bolt) at the back of the transfer case:
Fixin’er Up

Half of the problem is pretty easy to solve – replacement of the aforementioned transfer case vacuum switch. The
other half - replacement of the transfer cash input shaft seal - isn't as easy. That won't be covered here, since I
haven't done it on either of my vehicles as of this writing.

GM tech bulletin from several years ago noted that the vacuum switch has been redesigned to prevent ATF from
pushing its way past the switch seals and corrupting the vacuum lines. Accessing the switch is a bit awkward (it’s a GM
vehicle after all), but the task is manageable for the average weekend mechanic like me.

But oh, if it were only that easy.

After replacing the transfer case switch, the vacuum lines need to be purged of fluid (or replaced altogether, if you
really want to go all-out). Compressed air is a fairly effective method of ridding the vacuum lines of most fluid,
assuming you can locate all the lines. So let’s start with the transfer case switch and work our way towards the engine
Check out the
old-school cassette
tape player, circa
1996 (with Auto-
Reverse and Dolby
noise reduction!). I
was too cheap to add
the CD player option
when I ordered my
Sonoma (what can I
say, I was young).
The HVAC controls
are pretty simple, but
when transmission
fluid enters the
vacuum lines, the
mode selector will
eventually stop
working correctly.
The photo at left shows the
transfer case vacuum switch
with its hose connector
detached. The switch is
located on the top of the
transfer case, which is
directly underneath the cab.
When the vacuum switch
goes bad, the hose
connector will be dripping
with transmission fluid when
separated from the switch.

The vacuum switch (GM part
number 89059420) can be
bought for about $30 at
discount parts warehouses
such as
GM Parts Direct.
New transfer case vacuum
switch from
GM Parts
Direct. It screws into a hole
on top of the transfer case,
using a 7/8" wrench.
Eventually you'll want to blow out all the lines (there are many others within the HVAC system), but for now, let's start
with the vacuum "source" line and the 4WD actuator line.
Vacuum cannister:
"reserve" vacuum supply
Vacuum source line
<-- from engine --->
Supply line to
HVAC and transfer
<--- case switch
<--- "Tee" connector - splits
HVAC and transfer case lines
(HVAC line not visible)
To transfer case
vacuum switch --->
Blowing out the transfer case vacuum line is as simple as disconnecting the rubber hose and shooting compressed air
through it. With the 3-way hose plug disconnected from the transfer case vacuum switch (
very important), you'll see a
splattering of transmission fluid under the transfer case.

Next, we tackle the 4WD actuator, located under the battery tray. The tray is held on by the clamp that retains the
battery, and two bolts on the front of the tray. With the tray removed, the actuator is visible.
The front hub
locks  when the
vacuum actuator is
engaged by the
transfer case
switch. The
actuator pulls on a
cable that locks the
front hub and
allows the front
wheels to be driven
by the drive shaft.
When oil enters the
vacuum lines, a
common symptom is
loss of four wheel
drive. The
transmission fluid
can soften the
rubber diaphragm
and cause vacuum
At right is the line to the 4x4 actuator
under the battery tray. It takes a decent
amount of air to blow out every bit of oil,
and even then there will still be oil residue
left in the lines. The only perfect solution
is to replace the vacuum lines. I took a
calculated risk and continued to use the
line pictured here, as well as the vacuum
source hose to the transfer case switch.
These were the two longest vacuum lines
in the system and appeared to be in
better shape than some of the shorter
lines near the engine and vacuum
canister. These lines seemed to have
been more affected by engine heat and
were beginning to show some serious
wear, especially where they connected to
the vacuum cannister, the one-way valve
near the cannister, and the "tee"
connector that splits the HVAC and
transfer case vacuum lines.
Sixteen years of engine heat was
not kind to some of the vacuum
lines under the hood (click on photo
for larger view).
When transmission fluid enters the vacuum lines, eventually it settles
somewhere. As seen above, the 4WD actuator was one of those places.
Another common location for oil to collect is the vacuum cannister. On my
1996 Sonoma, the cannister was attached to the hood (later S-series models
would move the cannisters into the driver's side front fender well area - see
here). The cannister is designed to hold a reserve supply of vacuum
when the engine isn't producing enough of it to adequately control the
vehicle's various vacuum-driven systems. My cannister was heavy with
transmission fluid. In fact, it was so full of fluid that I trashed it and bought a
new one.

When fluid fills the cannister, its volume to hold vacuum decreases. The
reserve supply of vacuum is depleted, and the vacuum system becomes less
At this point, the cleansing of the vacuum lines and replacement of the transfer case vacuum switch probably seems
fairly uncomplicated. Unfortunately, the fun is just we get to enter the inside of the cab and see
what's behind the dashboard
Replacing the HVAC actuators.....continue reading
Updated June 2012
The photo above is from the transfer case of my Blazer, which had similar issues (read about it here). This is actually
coming out of the
fill hole, not the drain hole. About 16 ounces of excess fluid poured out. On the Sonoma, there was
a bit more than that
Yeah, that's a lot of fluid. About half a gallon's worth. The transfer case was about as full as it could possibly be.
Inset: Click on
photo for
larger view of
the hose
Transfer case vacuum switch -->
(hoses disconnected)
hoses (3 lines)
Thank you, Canadian
Steve and
for explaining to me the
mechanics of all this
After replacing the vacuum switch, the vacuum lines will need to be cleared of
transmission fluid. The best way to do this is to open up the hood, disconnect the
vacuum lines, and blow compressed air through them. The photo above shows 3
vacuum lines connecting to the vacuum switch. One line comes from the vacuum
source (the engine), another line goes to an actuator that locks the front hub when
4WD is engaged, and the third line is a vent line that actually splits into two lines - one
that goes back to the transfer case and another line that vents to the atmosphere (this
line runs back up under the hood and ends near the transmission fluid dipstick).
Here's a cut-away
view of the transfer
case vacuum
switch. The two
rubber seals are
shown here, as well
as the spring-loaded
ball (the spring has
been removed).
Failure of those
seals can cause a
couple problems. If
they're doing an
especially bad job of
sealing, the vacuum
system may not be
able to provide
enough vacuum at
all times. If the
transfer case input
shaft seal is bad,
ATF will push past
the seals and
corrupt the vacuum