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 switch? Yes, it's true. Those of us who
thought the electronic, push-button 4WD option was an excellent feature may eventually change their tune.
So…what the heck does the engagement of four wheel drive 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
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 switch? Yes, it's true. Those of us who
thought the electronic, push-button 4WD option was an excellent feature may eventually change their tune.
So…what the heck does the engagement of four wheel drive 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
| 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. |
doors are controlled by
vacuum-driven actuators
which swing them back
and forth in various
directions. Vacuum is
also used to engage an
actuator which pulls on a
cable that locks the front
hub 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 hub (got
that?). The vacuum
passage opens and
closes by way of a
3-pronged vacuum
switch that sits on top of
the transfer case. When
the switch stops
functioning properly, fluid
inside the transfer case
is able to push its way
past the vacuum switch
and into the rubber
vacuum lines that
connect to the switch.
Once the fluid makes its
vacuum-driven actuators
which swing them back
and forth in various
directions. Vacuum is
also used to engage an
actuator which pulls on a
cable that locks the front
hub 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 hub (got
that?). The vacuum
passage opens and
closes by way of a
3-pronged vacuum
switch that sits on top of
the transfer case. When
the switch stops
functioning properly, fluid
inside the transfer case
is able to 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 back to its source, where it is then distributed all throughout the rest of the vacuum lines in the
vehicle. This is bad. The reason it’s bad is because the fluid is the same as is used in your transmission. HVAC actuators do not like
this stuff. Seals get soft and don’t seal 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.
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 long.
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 vacuum switch fails, it allows negative pressure (vacuum) to enter the transfer case. This vacuum is able to suck transmission
fluid past the input seal where the transfer case is connected to the transmission. The transfer case fills up with excess fluid, which
depletes the fluid in the transmission. The transmission fluid dipstick may not show much of a drop in fluid level over short periods of
time, but eventually, if you let the vacuum switch problem linger (like I did), you'll see noticeably lower fluid levels.
One thing to keep in mind, however, is that a drop in transmission fluid and a transfer case over-fill situation may be related to a bad
input seal at the junction of the transfer case and the transmission. A new vacuum switch won't fix that. So it's important to still keep an
eye on the fluid level in the transfer case. If you replace the vacuum switch and still see transfer case fluid coming out of the fill hole,
there might be more to it than a bad vacuum switch.
To see if you have excess fluid in the transfer case, simply unscrew the filler hole at the back of the transfer case:
vehicle. This is bad. The reason it’s bad is because the fluid is the same as is used in your transmission. HVAC actuators do not like
this stuff. Seals get soft and don’t seal 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.
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 long.
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 vacuum switch fails, it allows negative pressure (vacuum) to enter the transfer case. This vacuum is able to suck transmission
fluid past the input seal where the transfer case is connected to the transmission. The transfer case fills up with excess fluid, which
depletes the fluid in the transmission. The transmission fluid dipstick may not show much of a drop in fluid level over short periods of
time, but eventually, if you let the vacuum switch problem linger (like I did), you'll see noticeably lower fluid levels.
One thing to keep in mind, however, is that a drop in transmission fluid and a transfer case over-fill situation may be related to a bad
input seal at the junction of the transfer case and the transmission. A new vacuum switch won't fix that. So it's important to still keep an
eye on the fluid level in the transfer case. If you replace the vacuum switch and still see transfer case fluid coming out of the fill hole,
there might be more to it than a bad vacuum switch.
To see if you have excess fluid in the transfer case, simply unscrew the filler hole at the back of the transfer case:
Fixin’er Up
The root cause of the problem is pretty easy to solve – replacement of the aforementioned transfer case vacuum switch. A GM tech
bulletin from several years ago noted that this part has been redesigned to prevent the hot, expanding fluid from pushing its way past the
transfer switch and corrupting the vacuum lines. Accessing the switch is a bit awkward (it’s a GM vehicle…what isn’t?), 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 compartment.
The root cause of the problem is pretty easy to solve – replacement of the aforementioned transfer case vacuum switch. A GM tech
bulletin from several years ago noted that this part has been redesigned to prevent the hot, expanding fluid from pushing its way past the
transfer switch and corrupting the vacuum lines. Accessing the switch is a bit awkward (it’s a GM vehicle…what isn’t?), 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 compartment.
| 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.
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. |
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.
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.
| Vacuum cannister: "reserve" vacuum supply |
Vacuum source line
<-- from engine --->
<-- 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 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 3-way hose plug disconnected from the transfer case vacuum switch (very important), you'll see a nice 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 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 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.
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 is 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 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 effective.
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 is 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 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 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 beginning...now we get to enter the inside of the cab and see what's behind the dashboard.
uncomplicated. Unfortunately, the fun is just beginning...now we get to enter the inside of the cab and see what's behind the dashboard.
Replacing the HVAC actuators.....continue reading
| Spring 2011 |
| The photo above is actually from my Blazer, which had similar issues (read about it here). 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. |
| Inset: Click on photo for larger view of the hose connector. |
Transfer case vacuum switch -->
(hoses disconnected)
(hoses disconnected)
| Vacuum hoses (3 lines) |
Thank you, Canadian
Steve and s10forum.com
for explaining to me the
mechanics of all this.
Steve and s10forum.com
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
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