'91 won't engage in 4wd...

I fixed the 4WD consol finally and fixed a vacuum line leak under the truck near the transfer case. The front driveshaft IS engaging but the half shafts aren't. I am suspect of the front diff shift cable assembly.
Anyone know how these work? Can it become out of adjustment? Is it common for them to break? Is the shift assisted by vacuum or anything electrical? Its the small Blazer if that matters, 4.3 auto. Unfortunatly my crappy manual doesn;t go into any detail about this stuff.

thanks

I had a similar problem with my blazer, the hubs would lock and it would feel like it was in 4wd but the front wheels wouldn't spin because the transfer case wasn't, the problem was a non operational vacuum pump. to test this locate the diaphram, it looks like a cruise control diaphram, it has a silver ring around it, and a line coming in and going out of it, you will be able to push the middle in and it will make a sucking sound, if it doesn't make a sucking sound there is a vaccum leak, try to put it in 4wd and then press the diaphram in and put a pair of vise grips on the line that is pulled back, that will engage your half shafts. If it doesn't work then check to see the front hubs are locking.

Oh yeah, the diaphram is usually located on the passenger side over the wheel well under the hood, it might be somewhere else on your blazer, mine was a little older but you should be able to find it, just make sure if your vehicle has cruise control it isn't the one going to the throttle, if you put pliars on that it will lock open your throttle and seem like you have the truck floored.

Ok just to not leave a dead thread...

I found the problem. I had to remove the battery tray to find the vacuum actuator(nice location GM!) My cable is frozen solid. I have a parts truck and checked it, the cable/diaphram seems to operate freely so I can likely swap that one in.
For those searching and finding this thread for similar problems from what I understand the system works like this.
When you shift into 4WD the transfer case mechanically shifts and engages the front drivshaft. There is some sort of vacuum switch on the transfer case that sends vacuum thru a line up the pass fender well which dissapears under the battery tray. This line collapses a diaphram, which looks just like the type on cruise controls, and that pulls a cable that goes to the front diff to engage the half shafts. All of this seems to be just to prevent the added drag of having the front driveshaft freewheel when in 2WD. Why they didn't just put another cable to mechanically shift the front diff I have noooo clue. I had a vacuum leak under my truck which I found right away and now it appears I have a frozen front diff cable. I will fix it tonight if thats all it is

Cheers

I had a similar problem in my 93 4x4. I found that if I pulled the 4x4 selector just past 4x4 high and then back that it would engage.

Ok last nite I changed the shift cable for the front differential. Mine seemed stuck so I put in the one from my parts truck since it seemed in much better shape and moved freely. The truck still does not engage the front wheels. I checked for vacuum to the diaphram and it was not getting any so the problem must also be on the transfer case.
What is the deal with that part of the system? What do I look for and can whatever sends the vac signal to the diaphram be easily seen/removed from under the truck?
Thanks

My problem is very similar. I have a 94 4.3 with the electronic transfercase, 233. I changed the vaccum sensor on the transfer case but It did not work. So I checked the servo or diaphram (whatever it is called) under the battery. I come to find out that the rubber lining surrounding the diaphram is torn. Would this be the cause of my 4wheel drive problems? I have checked everything prior to this. I checked fuses, the TCCM and the Encoder all good. Changed the vaccum sensor on the Transfercase. Now I need to know now to fix the torn rubber on that vaccum thing that pulls the cable to engage my wheels to spin. Everything is spinning but the wheels. I had it up in the air and saw that the rear wheels and both driveshafts were spinning but the front wheels were not. I was in 4 Wheel HI with the indicator light on. Also having a problem getting into 4 LO. What is the part that I need? The thing looks like a cuise cotrol unit with the cable coming out of the middle and vaccum lines going into it. What is this part called? And where can I get it?