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 ways.
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

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.
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.
These doors direct warm or cool air into certain vents inside the cab.
The doors are controlled by vacuum-driven actuators which
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

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

The photo below 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.
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 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"
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 nice splattering of
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
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 leaks.
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
Sixteen years of engine heat
was not kind to some of the
vacuum lines under the
(click on photo for larger
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 pics
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

When fluid fills the cannister, its volume to hold vacuum decreases.
The reserve supply of vacuum is depleted, and the vacuum system
becomes less effective.
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
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
for larger view
of the hose
Transfer case vacuum switch -->
(hoses disconnected)
Vacuum hoses
(3 lines)
Thank you, Canadian
Steve and
for explaining to me the
mechanics of all this
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).

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.
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 system.
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.
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