Blazer 4wd question

Ok, I have read every thread that I can find on the 4X4 system.
Still not sure what Im up aginst. So hopefully someone can help.

I have a 1995 Blazer Lt 4x4. When I push either 4Hi or 4Lo all it does is flash the light on the dash and I can hear the relays behind the dash clicking. On the outside of the truck I cant hear anything making any noise. If I pull the line off the actuator under the battery I know there is no vacuum on the line at all. If there is any its not enough to feel with my finger on it.
I have also checked out the actuator and cant find any cracks or anything. The vacuum lines also appear to be connected and look to be in fair condition.

What Im wondering is the actual order of operation of the 4x4 system for this truck. And does the transfercase motor actually do anything when shifting in to 4Hi? Like I said there is no noise under the hood or back by the transfer case when I push the button on the dash.

So what would be the first thing missing? Is it the motor on the transfer case that moves then the sensor on the transfer case then the diff actuator then the diff actuator vac switch then the light goes solid on the dash?

I am not a 95 expert - all mine are 98up. But you should have a vac switch on top of the transfer case from the posts I read. Only 4Lo moves the encoder motor to pull in low gear and it does not acivate for 4Hi. There is a sensor on the xfer to signal that its engaged for 4Lo. For 4Hi the front actuator must engage or couple the front pass side axle and that too has the sensor on it.

For 4Hi the front actuator must engage or couple the front pass side axle and that too has the sensor on it.

So the first thing that is supposed to happen is the vacuum is passed to the actuator for the front diff?
Where is the point of origin of the vacuum. Is it the fitting on the transfer case?

When you push the 4x4 button, hi or lo 4x4, the actuator engages "actuates" the gearing within the transfer case to lock up the front axle to send rotational power to the front differential. The switch on top of the transfer case is turned on (for lack of a better word) as the transfer case engages, not before. This happens by the switch being turned on by the gearing moving inside the transfer case. This sends vacuum to the front actuator, the actuator then pulls the cable to lock the front end and engage the the front differential.

The point of origin for the vaccuum is at the engine, the intake for a '95 CPI its usually located on the passenger side, back of the intake, facing t he firewall.

When you push the 4x4 button, hi or lo 4x4, the actuator engages "actuates" the gearing within the transfer case to lock up the front axle to send rotational power to the front differential. The switch on top of the transfer case is turned on (for lack of a better word) as the transfer case engages, not before. This happens by the switch being turned on by the gearing moving inside the transfer case. This sends vacuum to the front actuator, the actuator then pulls the cable to lock the front end and engage the the front differential.

The point of origin for the vaccuum is at the engine, the intake for a '95 CPI its usually located on the passenger side, back of the intake, facing t he firewall.

Thanks for the reply.
So there is activity in the Tcase first then the diff?
It would seem that the T-case switch or the motor is the first missing link.

Any suggestions how these can be checked? Im hoping I dont have to drop the T-case completly before I find out what the prob is.

So there is activity in the Tcase first then the diff?
It would seem that the T-case switch or the motor is the first missing link.


Yes activity in the t-case first, then the diff. If the transfer case is not locking up/engaging the front axle, thats where you turn your attention to first. Even if you hear the clicking by the relays when you push the 4x4 button(s) and the 4x4 button stays lit, you have to physically grab and turn the front axle to see if its locked. If it doesn't turn when parked, your t-case is working.

You can actually engage your t-case manually by removing the actuator motor and turning the lever, but at this point I don't think its really necessary until you can verify the encoder motor and push button system is working.

Thanks again. I will pull up some information that I have at home to see if I can get a better understanding.

I did just read an old post from Blazee, where he said the t-case does nothing for 4hi selection so Im kinda confused. Would I get any kinda code if the switch was bad? Im thinking maybe its not reading the proper position for Hi vs low.

Thanks again. I will pull up some information that I have at home to see if I can get a better understanding.

I did just read an old post from Blazee, where he said the t-case does nothing for 4hi selection so Im kinda confused. Would I get any kinda code if the switch was bad? Im thinking maybe its not reading the proper position for Hi vs low.
That's the way the system is described to work in the manuals that I read. It stated that the actuator on the diff is used to change from 2wd to 4wd, and that the encoder motor was used solely to change from high range to low range. It stated that transfer case was direct driven and supplied power to the front diff at all times.

From my experience with the system, I don't believe that the manual description is correct. I think that the system operates similar (if not exactly) as blazes9395 states. The main reason I think that the info I read was wrong is because when in 2wd, the front drive shaft can be easily turned by hand.... if it were tied directly to the rear driveshaft then you would not be able to turn it, so something in the transfer case must engage it. I have rebuilt a ton transfer cases, but never the type used in these vehicles, so I can't say with 100% certainty the exact internal operations.

This is what I found
"Front axle assembly on "T" Series models uses a 7 1/2" ring
gear. Front axle assembly has an electric 4WD engage/disengage feature
which allows shifting in or out of 4WD while vehicle is moving (under
most conditions). The 4WD feature is shifted by a vacuum actuation
system. See Fig. 1.
The vacuum actuation system consists of a vacuum switch and
vacuum actuator. Shift mechanism in transfer case triggers vacuum
switch to apply engine vacuum to vacuum actuator after about a 3-
second delay. The vacuum actuator, in turn, pulls on shift cable which
pulls on shift fork in axle. This connects the right axle output shaft
to the front axle differential. Torque is now available to front
wheels."

So according to this, I should have vacuum at least up to the switch on the right inner fender? I didnt think to check that I only checked for vacuum at the actuator. So I guess I know where to look next. Im just not sure which line on the transfer case I should be looking at. They all seem to be in good condition at that end though.

Try this also:
The most common problem we see with these systems is an encoder failure. This means the computer module that controls and monitors the 4WD shifting motor doesn’t know where it is. The computer needs to know the location of the shifting motor to perform correctly. If the computer loses location information it stops responding. Diagnostic codes can be extracted but, because you have not used your 4WD much it may be all that you have to do is to clear the computer so it can start over. Turn the ignition switch off and remove the fuse marked T-case (it should be a 20 amp fuse). Wait a minimum of 3 minutes then replace the fuse. Once the fuse is replaced turn the ignition switch on & off 5 times. Then start the engine and push the 4 Wheel HI button. If the system starts working, great. Be sure to exercise the 4WD system regularly to prevent a reoccurrence. If the problem returns then you need a technician familiar with your 4WD system to get into some diagnosis.

So I have been working on other problems (Lower a-arm bushings and fuel pump) and I havent revisited this. Im going to try to get this figured out this weekend.

I know there are no codes from the TCCM. Already checked and the only code that was there was cleared upon ressetting diagnostic troule codes.

At this point I need to check a couple of things. First off, is my TCCM on the passengers side under the kick panel?
Also seems like the encoder really is the most likely suspect. What is a good way to diagnose that? Should I just pull it and put 12 volts on it? Which would be the hot pin?

When reading the factory explainations of the system operation it basically states that the front left wheel is a drive wheel in 2wd. I say that because it says the front drive shaft is always connected, and the front left cv is always coupled to the diff. In turn this says the front left is always connected. We all know that is incorrect. My guess is that the left front cv is in fact always connected to the diff, butt not in the transfer case. I think that happens when you hit the 4hi button.
Otherwise the only action that the TCCM would take upon actuating the 4hi button would be to switch vacuum to the front diff actuator.

That being said, has anyone ever herd of the Transfer case vacuum switching machanism going bad?