|GM S-Series 4WD
|With either option, two things are happening when you push the 4WD button or throw the lever into 4WD. First, the
transfer case shifts and delivers power to the front drive shaft. With push-button 4WD, the shifting is done by a shift
motor that is mounted to the transfer case. With manual 4WD, the lever on the floor (above right) is what shifts the
This alone will not give you 4WD. The second action that has to take place is the front wheel axles must be joined
together inside the differential housing. This gets the front wheels turning by way of the front drive shaft. The best I
understand it, a collar must slide across the two axle shafts and join them up. When the collar slides and the axles are
joined, the front drive shaft will power the front wheels (a very lay person's explanation...).
So, transfer case engagement plus front hub lock equals 4WD. Let's take a look at troubleshooting the mechanical
and electronic components of the 4WD system.
The most common cause of non-functioning 4WD? Vacuum leaks. What, you say, does vacuum have to do with
4WD? Well, the auto designers of the world always knew that internal combustion engines generate a good deal of
vacuum, which can be used for a variety of purposes. It's free energy, more or less, and can be transferred easily
throughout the vehicle. Over the years, General Motors and most other car and truck makers have put vacuum to use
in powering certain vehicle systems. For example, in the HVAC system of S-series trucks, most of the well-hidden
doors and chambers which direct air to the proper vents are powered by vacuum.
Vacuum is also used to pull the cable that slides the collar inside the front differential housing and joins the axle
shafts. Without vacuum, the cable won't move. The transfer case can do its thing, but if the cable doesn't pull the
collar, 4WD is just a pipe dream. That cable is pulled by a pneumatic actuator under the battery tray. The actuator
gets its vacuum when the transfer case shifts to 4WD. Shift forks, or whatever they call this apparatus inside the
transfer case, push up against a spring-loaded ball that is part of the transfer case vacuum switch.
|At left is the push-button 4WD from my 1996 GMC Sonoma. This was an option; standard 4WD trucks came
with a manual shifter on the floor, as pictured on the right.
|First off, a disclaimer: I am not a mechanic. I have never been a mechanic. Anything you read here or anywhere else
on this website is based on my own personal experiences, or the massive amounts of information I've gleaned from
the Internet on whatever topic I choose to ramble about. If you've found this website, you were probably in need of
help, or you were really, really bored. What follows is yet another of my attempts to share what I've learned, or at least
point you in a better direction. Or, simply entertain you with my unsophisticated descriptions of what goes on inside
GM S-series vehicles. In this case, the topic is four wheel drive.
Every year around Thanksgiving, the S-10/Blazer/Sonoma/Jimmy forums are abuzz with questions about
non-functioning four wheel drive. Snow comes, and at the worst possible time, no 4x4...very annoying. It's happened
to me. As I've researched the 4WD system on these vehicles, the problems really come down to one of two things: 1)
Something mechanical; or 2) Something electronic. Now, if you don't have push-button 4WD on your dash, the
electronic part of this is a lot simpler. If you do have those convenient buttons, you'll have a couple other components
to troubleshoot if your 4WD gives out.
All three of the S-series vehicles I've owned had electronic shift transfer cases. These are the trucks with those handy
dash-mounted buttons that, with one push, give you 4WD on the fly. If your truck doesn't have those buttons, you've
got a lever on the floor.
|This switch, pictured left, sits on top of the transfer case. The ball on the bottom is what is pushed up by the internals
of the transfer case. There are 3 tubes sticking out the top, each connected to a vacuum line. One line comes from
the vacuum source under the hood; another line goes to the actuator under the battery tray (pictured above right);
and the third line is a vent line that tees off into 2 separate lines. When the ball is pushed, it opens a valve that lets
the vacuum through the switch and to the actuator.
Sounds fairly simple, right? Well, vacuum can be finicky as trucks age. The rubbery vacuum lines eventually break
down and begin to leak - especially the ones under the hood that are subject to engine heat. Usually there's a high
correlation between non-functioning 4WD and non-functioning HVAC controls inside the cab, because both systems
Besides leaks in the vacuum lines, another common reason for loss of vacuum is sometimes the line becomes
disconnected. One of the most common of these is the vacuum line that connects to the vacuum reservoir. On my
1996 Sonoma, whose reservoir was a plastic ball attached to the underside of the hood, a disconnected line is pretty
easy to spot. On my 2004 Blazer, the reservoir is tucked inside the driver's side fender. The connection is just about
impossible to see, so sometimes people will notice a disconnected vacuum line, don't know were it goes, and simply
cap it off. That might get the vacuum system functional again, but in certain driving conditions there won't be enough
reserve supply of vacuum to control the HVAC system or keep the front hub locked.
|Besides non-leaking vacuum lines, obviously the actuator itself has to be working properly if 4WD is going to function.
The rubber diaphragm can't have any holes or rips. It also shouldn't be full of transfer case fluid. Why would it be full
of transfer case fluid? Well, read all about it. If that's the case, then the transfer case vacuum switch is bad, either by
letting transfer case fluid into the vacuum lines or by sticking open. It the vacuum switch is stuck open, the actuator will
continuously pull on the hub-lock cable. That's not really good for the front end and can eventually result in a grinding
noise. Long-term, here's what can happen: Grinding Gears. Or, this can happen.
And speaking of actuators, there are several inside the dash, each controlling a door or flap that directs air to the
proper vents. If any of these are leaking, there may not be enough vacuum reserve to overcome the leak and keep
the 4WD actuator pulling on that cable. For this reason, troubleshooting your 4WD vacuum issues may not be as
simple as checking for leaks in the vacuum lines or in the actuator under the batter tray. You may need to look at the
vacuum lines and the actuators in the dash.
For trucks with an electronic shift transfer case, electronic gremlins can rear their invisible heads and cause
disfunction in 4WD shifting. The electronics are made up of three primary components: 1) Transfer case control
module ("TCCM"); 2) Dash buttons; and 3) Transfer case shift motor (sometimes called an "encoder").
The brains of the operation is the TCCM, located behind the passenger side kick panel under the dash. When you
push one of the 4WD buttons on the dash, the TCCM tells the shift motor what to do. It also keeps track of faults in the
system, and will eventually shut down the shift motor if enough faults are measured.
The transfer case shift motor does the grunt work. It shifts the transfer case into one of three positions: 4-Hi, 4-Lo,
and back to 2-Hi. This is pretty much what the floor lever does, if your truck doesn't have electronic transfer case
The shift buttons on the dash are there not only to request that the TCCM tell the shift motor what to do, but also to
help you troubleshoot the TCCM. This is where the GM service manual comes in handy. I've transcribed the GM
instructions for troubleshooting the 3-button 4x4 system:
|The vacuum reservoir "ball" is shown on the left. The vacuum cannister from the 2004 Blazer is on the
right. It fits into a well-hidden space in the fender well. The vacuum line connection is facing down.
|Basically, if you push one of the buttons on the dash and they all blink for a short time and you don't get the action
you were expecting, the TCCM is telling you that it's recorded enough faults that there's something wrong. It won't let
you shift the transfer case. In the troubleshooting guide, there is a procedure where you can make the TCCM tell you
what's wrong. It communicates the faults by making the dash buttons blink on and off. Depending on how many blinks
you get, and the sequence of the blinks, the TCCM will identify what part of the electronics isn't working correctly.
The TCCM identifies one (or more) of four Diagnostic Trouble Codes ("DTCs"), each with a troubleshooting
procedure. Some of the more common problems are a faulty transfer case shift motor, a corroded electrical
connection on the TCCM, and a bad TCCM. One out of those three is a cheap fix. The other two are a bit more
The best part of reading DTCs is that it gives you a shot at narrowing down the problem. I've read a lot of Internet
chatter from guys who just start replacing this or that, without knowing exactly what the problem is. I've never had any
problems from my 4x4 electronics, so I can't speak to how well the troubleshooting guide works, but it's there for your