How does the vacuum system work on 4wd? I have no 4wd

[jdmartin]

I tried to do a search but didn't find much applicable here.

How does the 4wd system work? When I shift into 4wd I don't get any 4wd, light or anything. This is from being parked but I also tried moving and didn't seem to get anything.

I reviewed the vacuum diagram that shows the routing of the vacuum hoses and started there. According to the diagram, engine vacuum attaches to a red hose that goes to the transfer case, and then green, yellow, blue and black (at some point) comes back to the front axle.

I started the Jeep and disconnected the red hose inside the engine. I had good vacuum there. I disconnected the harness at the front axle and had no vacuum whether it was in 2wd or 4wd, it made no difference.

When does the transfer case apply vacuum to the front axle - all the time, or just in certain situations? And what port, or is it all of them, does it apply vacuum to? I couldn't seem to reach the transfer case with the crossmember panel in the way, so I couldn't check for vacuum there.

I should mention that a couple of weeks ago the 4wd light was staying on all the time, but when I gave it gas it would go out. It flickered like this for about 100 miles, then went out completely and has not come back on.

[Joe Dillard]

-=- originally posted by jdmartin -=-
When does the transfer case apply vacuum to the front axle - all the time, or just in certain situations?

On my '91 (before swapping to the Warn Hub Conversion Kit), vacuum was present at one side of the front diff vacuum hose fitting while in 2wd all the time. As soon as I put the tcase in either 4hi or 4lo, the vacuum switched from one hose fitting to the other.

-=- originally posted by jdmartin -=-
And what port, or is it all of them, does it apply vacuum to?

It switches back & forth according to tcase position.

If you'd like; I could take some pics of which side does what according to tcase selection. But remember, mine isn't able to be hooked to the front diff any longer so you will need to look at what I mention and which side does what & when.

PM me & I'll do this if you'd like.

[bestagents]

manifold (engine) vac goes to TC switch. In 2WD the green line to the axle has vac on it, in 4WD the yellow has vac on it. the blue is a vent and is connected to nothing. one yellow line goes to the switch on the firewall that turns the light on.

vac need not stay there to keep the axle in 2 or 4 wd. once the vac motor moves the shift fork, it will stay there without any vacuum

[Janster]

More often than not - it has nothing to do with your vacuum.

Did you inspect the vacuum actuator on your Dana 30??

Pull the cover off (drain the oil) inspect the shaft, collar, and shift fork.

Pull the shift fork completely OUT.

Have someone sit in your jeep and switch it IN and OUT of 4WD while you watch the shift fork...... Does the shift fork move back-n-forth??

If YES - Then your vacuum is fine.

Stick your hand inside and move the collar back-n-forth to make sure nothing is seized up.

THat's all I remember...I got rid of my D30 2 years ago!

[YJSeminole]

try changing the vac switch on the transfer case, they get gummed up pretty easily and its only like 15 bucks.

[cucumber]

Guys, I'm still having this problem with my Jeep. It's a 93 YJ. I replaced the vaccuum switch on the transfer case, and that was the initial problem because the Jeep wouldn't shift out of 4wd. Well now it don't shift into 4wd. Someone told me it's the shift fork, because my vaccuum is fine. Where can I get a shift fork, or is it possibly something else?

[mark_k]

I have the exact same problem on my '91 YJ. Seems like I could go through one vacuum switch a week. Even did the same thing where it was stuck in 4wd and when accelerating hard, 4wd would go off for a short period of time. I dont believe its the fork, because if you take that vacuum swith off the tcase and kind of bang it on a hard surface you will see crud coming out of it, put it back in and it will all work fine for about 2 days until the same thing starts happening. I havent had a chance to figure out what is causing this yet so i cant be too much help, but i would start by check all the vacuum lines and making sure they are clear and have no cracks or anything. If you figure out the exact cause of this problem,. please let me know.

[tregreen]

Joe Dilliard would you mind posting those vaccuum photos here if you end up taking them. Would make a great point of reference.

[tregreen]

Joe Dilliard would you mind posting those vaccuum photos here if you end up taking them. Would make a great point of reference.

[wcjp]

Swapping to TJ axle shafts is always an option.

There was a right up by someone here on JU about converting to a homemade posilock for about $20. This also gives you 2low. I think the link is at home unless someone else can post it. The kit eliminates the problematic vacuum.

[mRogers]

This may not solve the problem if it is more than the vacuum, but it's still a good thing.

home made axle disconnect (http://home.earthlink.net/~stevenschreiber/cabledis.htm)

[cucumber]

Is that homemade Posilock good for a Street-driven jeep? Also, where might I be able to find some TJ axleshafts?

[muddinjeep]

What makes you think its the shift fork? Open the shift fork box and see if you can slide the fork back and forth, does it slide easily? I had some problems with mine. Some rust had formed on either edge of the shaft and prevented it from sliding. I took it out and used some wet sandpaper to remove the rust. Fixed my problem..You can also buy a new shift fork in Quadratec magazine. HTH

[mRogers]

-=- originally posted by cucumber -=-
Is that homemade Posilock good for a Street-driven jeep?

It won't give you any problems, works just the same as it always did except you have more control. When on the street you will have your t-case in 2hi just like always. When off road you can engage your t-case(4hi-4low) without your axle being engaged, thereby giving you 2low. This will give you much greater torque in 2wd. An example of a time to use this might be if you need to make a tight turn and don't want to do it in 4wd. Then when you need it in 4 you just flip the lever switch and when the gears mesh it will lock up for you, no more slowing down or stopping to engage it. These are just some examples of it's possible uses from a guy who is not extremely experienced, I just read a lot.