We had a good winter storm here in Missouri last Sunday. I drove my 98' Blazer thinking (and kind of hoping) I could finally get a chance to use the 4x4. Of course the day I need it the most it didn't work. I've replaced all vacuum lines, actuator, transfer case switch, cable. The diaphragm is good as I just finished pulling the battery and using jumpers to start it up. It's pulling in and holding for all it's worth. I thought maybe I had a vacuum leak somewhere as my cruise control works only half the time and my cab air recirculate will go off on it's own after 10 or 15 minutes. I have the 3 push buttons no automatic that hold a steady light without blinking telling me the computer doesn't see any problems. (Yes I've pulled the passenger side trim to check the connections on the 4x4 computer-all clean.) I just drove it after replacing the gunked up PCV valve and felt the front drive shaft-free spinning in 2x4 but I noticed in 4x4 it's stiff-but I can still turn it by hand. Unless I stop and go into 4-lo (which halfway locks it in but I can hear it popping from slipping) and then 4-hi the driveshaft doesn't even offer any resistance.
The only thing I can think of is something is going on inside the axle and I'll have to pull it to check everything. When I did have it sort of working last year it would 'pop' whenever I pulled up a hill or accelerating which made me think it's not engaging all the way. Enough to make the computer think all good, but not quite enough to solidly lock in. I've spent countless hours and lots of $ trying to figure this very aggravating problem out. Any thoughts, advice, or simply a kind word would be most welcome. What could cause the axle to not engage all the way short of rounded off teeth?

 

Is it possible there is something not working correctly in the transfer case?

It sounds like you tested the front axle by trying to turn by hand the front drive shaft? Under what conditions did you do this, with one or both front tires off the ground?

My understanding is that if the front axle is engaged via vacuum applied to the actuator under the battery, then the front axle should then act like an conventional axle; i.e. with both wheels off the ground, if you rotate one wheel, the other will rotate the other direction (assuming the drive shaft to it is not rotating. I would think doing this test could help isolate the problem to the axle or to the transfer case.

I recently saw a video of a professional mechanic working on an S10 pickup who with the truck on the lift, set up a camera to record what the front tires and front drive shaft do when engaged with the engine running. (This of course would be something only to be done with your safety taken into account.)

In the same video he also mentioned that there is a sensor where the cable goes into the front axle that can go bad. I think it informs the computers if the front axle is engaged or not. That is another thing to check.

 

If that sensor was bad, wouldn't the axle still lock in and I would get a blinking light? Always had a solid light. Last week I raised the right tire off the ground and could free spin it no matter if in 4x4 or not. My understanding is if the axle is locking in I wouldn't be able to spin the right tire.

 

That is correct unless your 3-button transfer case is not going into 4WD. In any case, as long as the front driveshaft doesn't spin when you spin the right tire (left tire on the ground), the axle is not locking in.

 

Yep. I fooled myself for a moment once by not realizing that when the transfer case is not-engaged but if the axle was, if one of the front tires is on the ground, spinning the other would only spin the front axle drive shaft. (see picture below)

If the axle is not engaged, then spinning the right hand side would only spin the right hand axle. Spinning the left hand wheel (which is directly attached to the differential, unlike the right side) would spin either the other output shaft that is internally disconnected from the right side axle OR rotate the drive shaft. I would think the former would be more likely as there would be less friction on it. The result being you cannot tell what is going on.

The ultimate test is to raise the front end so that both tires are off the ground engage the 4wd* with the engine running at idle in Park and do the following:

• Spin one of the front wheels and see if only the other wheel spins. If yes then likely the front drive shaft is engaged and the transfer case is keeping the front drive shaft from rotating.
• If you can turn the front drive shaft and the front tires spin in opposite directions, then the axle is engaged but the transfer case is not engaged.
• If you cannot spin the driveshaft but spinning one of the front wheels does not spin the other, then the the transfer case is engaged but the axle is not.

This is what I would do but please, if someone know better, be sure to post it.

*I don't know if the front axle stays engaged when the motor is turned off. These tests might have to be done with the engine running.


The right axle is part #2. You can see the engagement mechanism at parts #9-#23. With the the right axle shaft disengaged, with the drive shaft fixed in place and the left side wheel spun, the output from the differential on the left side would spin deep inside the differential housing.

 

Christine, thanks for going through the trouble of posting that diagram. I just came in from taking the battery out for the umpteenth time and disconnecting the cable/diaphragm assembly. I found out that I can pull the shaft out no problem with my fingers while turning the drive shaft and it will solidly lock in. When I push in the diaphragm with my fingers I'll feel it hit the sensor detente and it wants to stop there. The diaphragm is smooth without any leaks or hang ups but when I connect the new cable it'll pull in about 3/4 of the way, and that last bit it stops unless I push a little harder. I stuck vice grips on the cable pushed in all the way and my front end is definitely locked in. When I put it all together I did a test drive around the parking lot and it isn't 'crow stepping' like it would in 4x4. I have no clue what that little hang up could be.

 

Hmmm. On another thread I have heard of the return spring (#21) for the axle engagement mechanism going bad. Could that be the problem?

BTW, I wasn't quite sure of what you meant by the second half of the sentence after "but": "The diaphragm is smooth without any leaks or hang ups but when I connect the new cable it'll pull in about 3/4 of the way, and that last bit it stops unless I push a little harder." What is being pulled in?

Regarding the sensor: I think the front axle engagement sensor is located at #17 or #16. If the axle is engaged but the transfer case control module does not know it, the transfer case might not also engage. (I'm speculating at this point but the sensor must be there to do something!)

FYI, although I've made a bit of a study of these axles and transfer cases, I'm now getting to the limit of my understanding of them. LOL I hope what I have been able to suggest has helped.

 

What I meant was when I was pushing in the diaphragm with my fingers with the axle cable connected I can feel it always stop about 3/4 of the distance. It's enough to hit the sensor that tells the 4x4 computer the axle is locked in, but it needs to travel about another 1/4" to lock the axle in. The cable free travels most of the way but I always feel this resistance right where the axle starts to lock in-I can push on the diaphragm a little harder with my fingers and it'll finally lock the axle in. I don't think the vacuum can overcome this resistance. Oh and to add insult to injury my front differential is now leaking gear oil around that engagement shaft the cable hooks to....it never ends with this truck. I'm tempted to get rid of the awful vacuum system and install one of those manual cable pulls.