My '98 Blazer with 3 button dash has got some issues. I posted a question about it earlier but at that time was unsure whether it had actually locked together like I assumed. Now I'm sure and and am hoping somebody can tell me why. It initially locked up when I was in 4hi. I got it back in 2hi but found out then that the front axle was locked together. It unlocked itself and I forgot about it. A few months later, I was going 70 down an interstate in 2hi and the front axle tried to lock together again. I realized what had happened and then pulled the vacuum line and got it to quit trying to lock up. I finished my journey home and have just been driving it that way ever since, with no vacuum line hooked up. I finally decided to make time to fix it with the winter coming. I hooked up the line and it seems like everything is working again except it now won't lock the front axles together in 4lo. I'm not sure if it's not trying to lock them together or if there is nothing left to try with. My questions are, what would cause it to do this and what did I break when it unsuccessfully tried at 70 mph. Searching has yielded little help nor has any of the tech articles. This is not a matter of the front axles locking in with the rear, it's the front axles locking together with each other. Thanks

 

The vacuum being applied to the actuator would signal to me a bad Transfer Case Switch. It's a simple part to change and relatively inexpensive (I think like $15-20) As for problems inside the axle, that is another story. It could have messed up any number of things. There is an exploded view floating around here of the front axle, but I couldn't find it so here is a link of the diagram.

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The shift fork (#17) Could be damaged; the connector (#9) or the actual axle shaft(#23) could be stripped.

Hopefully that diagram will give you some insight if the vacuum switch does not fix your problem

 

So basically I'm going to need to tear it down to see what all I damaged. I'll just try to have some of the possibilities on hand when I go to do the job and return what I don't need, no biggie. I know I broke something because previously when it was still trying to engage under vacuum, it was making a very loud clicking/grinding noise like something was not long enough to fully engage. Now under vacuum it doesn't do anything, no noise or engagement. I'll post what I find when I get it apart.

 

1- The link provided by JBsnowboarder is a good one and I agree with his advice about the "vacuum switch on the Xfer case". They get bad and are cheap and easy to replace. Make sure you get the hoses back correctly.

2- The vacuum line to the front axle control should work like this; With the car idleing in park and the right front axle lifted so the tire is free to turn; in 2X the wheel should turn free, in 4X there should be vacuum on the line, to the muffin, the muffin should move about 1/4" the splines should engage and the right wheel should not turn freely. Check this with a separate vacuum pump, or a hose to the engine, what ever works. Make sure vacuum engages it and no vacuum disengages it. http://dl.dropbox.com/u/837001/4WD%20Vacuum.JPG is a drawing of the muffin etc.

Notice in this drawing, that "B" is a VENT to Atmosphere. Many don't think this thru, but there needs to be an air-filter on this hose. Lack of a filter can cause dirt to get sucked in, and pass thru the plastic "Switch" on the transfer case which WILL cause it to FAIL. Make sure the Vent had a air filter. My experience with failed vacuum switches it that fail in the 4X4 position. Don't replace the switch unless you correct the cause of the failure if possible.