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The Infamous S-series Transfer Case Switch





Using vacuum to power the transfer case...probably seemed like a good idea at the time.



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.


What in the world 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.


Using engine vacuum may not be as popular now as it was when the S-series was alive and well, but it was the real deal back then. And it could be a real pain to fix if it broke.





With the dash opened up on my 1996 GMC Sonoma, some of the HVAC vacuum lines can be seen. The roundish 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 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 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, the transmission fluid level will 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 must 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

A leak in the HVAC system can happen in several places behind the dash. Most often, the hiss sounds like it's coming from behind the HVAC control selector. For anyone not familiar with the transfer case switch situation, the diagnosis might be a bad HVAC control unit. Replacing the $150 unit might fix the issue for a short time, or maybe not at all.


Loss of HVAC functions

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 too much ease.


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 HVAC 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:





The photo above is from the transfer case of my Blazer, which had similar issues. 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 from the Sonoma. About half a gallon's worth. The transfer case was about as full as it could possibly be.





Above: This brings back memories. Check out the old-school cassette tape player from my 1996 Sonoma (with Auto- Reverse and Dolby noise reduction!). I was too cheap for the CD player option when I ordered my Sonoma. The HVAC controls were pretty simple, but when transmission fluid entered the vacuum lines, the mode selector eventually stopped working correctly.





Problem identified. Let's get to work on fixing it.



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.


A 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 compartment.


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.







Above is a very wet vacuum hose plug. This fits onto the 3-pronged vacuum switch.





New transfer case switch. Use a 7/8" wrench.





And now...the cleaning of the vacuum lines





Where the real fun begins!



After replacing the vacuum switch, the vacuum lines will need to be cleaned 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 the cap which connects the three vacuum lines 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). 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.





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 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.





Filled with fluid



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.





Above 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.


When transmission fluid enters the vacuum lines, eventually it settles somewhere. The 4WD actuator is 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). 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 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.​





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, excess ATF inside the transfer case can push past the seals and corrupt the vacuum system.


Now, we'll move on to the HVAC actuators. Please click on the link below:



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