A common issue with the YJ is a failure in the automatic vacuum system that locks and unlocks the hubs when going in and out of 4 wheel drive. I recently suffered the issue myself, so I thought I would share the experience so that others can refer to this to help with their troubleshooting.

To make a long story short, when I disengaged 4 wheel recenty, the dash indicator on the dash did not go out. I was several miles from home and I was concerned about driving it any distance with the 4 wheel engaged.

I moved into a paved area and turned the jeep in sharp circles. Had it been in 4 wheel drive my tires on the front would have chirped on the pavement due to there not being any slippage. Luckily, they did not chirp, so I thought I had a failure in the dash indicator.

On the drive home, I noticed the jeep would not get up to power and I knew that I had an issue. Fortunately, I made it to the house without any issues, and I put the jeep to bed for the night.

OK. There is a problem with my 4 wheel drive, so where do I start? Well, I started right here at wranglerforum.com. I read through several thread where particular YJ 4 wheel problems were discussed. There was plenty of information, so it got me on track for some troubleshooting.

cont.

Here's a couple diagrams we've shared on the forum that I used to get an idea of how everything is run.

Here is some text that I have borrowed from myself in another thread.

The vacuum lines for the 4wd axle disconnect system always seem to get unhooked, melted, or ripped totally off. Here is a way to check to see if everything is hooked up right, and if it isn't, how to hook it up the right way.

On top of your transfer case will be four nozzles that control your front disconnect system for your 4wheel drive.

The red line coming from your transfer case is the hose for the vacuum source(constant). That will hook up to a hose running behind your air cleaner, if it is still there. Take a look and you will probably find the remains or the original red line. If you don’t have the remains of the original line that is hooked to the constant vaccuum source, find a port on your intake manifold that has a constant vaccuum(or ports on your carb).

Take apart the plastic housing around the vacuum harness(don't break it) to see the lines. Connect what seems to match up for now. Your best bet is to cough up the $30 that a new vacuum harness costs at the dealer and install that instead of driving yourself insane with broken lines.


(blue line) Leave hanging, it has a check valve on it which gets hooked to nothing.
(red line) Hooks up to a good constant vacuum source off of the manifold.
(green line) goes to the shift motor on the front axle. Horizontal on the shift motor.
(black soft vacuum hose) goes to the plug on your firewall by the battery for the 4wd indicator on the dash.
(yellow line) goes straight to the shift motor; one of the lines that goes into a two line plug verticle on the shift motor.
(black hard line) goes with one of the yellows to the shift motor. Plug located vertically on top.

Ok. I was pretty sure I had a failure in the vacuum system that actuates the shift motor on the front axle. I lifted the jeep up and examined the vacuum lines from the shift motor and tracxed them all the way to the transfer case. I found that my vacuum lines are in near perfect condition without any breaks in them. Also, everything was hooked up just as it should be according to the diagrams.

What now? Well, I had the jeep safely lifted, so I decided to start it up and watch eveything in action. I started with the jeep in 4 high, which engages the tranfer case and triggers the vacuum switch that is mounted on the top of it. The switch changes the vacuum on the shift motor on the front axle and locks the hubs in. One at a time, I pulled the vacuum lines off the shift motor to see which line was pulling.

The line on the left had vacuum on it, and there was no pressure on the line on the right side of the diapragm on the shift motor. Stands to reason that if I shift the transfer case back into 2 wheel drive, the vacuum would change over to the other vacuum hose because the vacuum switch on top of the transfer case had changed positions, right? Well, what I discovere was that the vacuum did not change and I still had pressure on the left line and not the right.

Aha!

I retraced my vacuum lines back tot he switch and re-verified that all was connected as it should be. Everything looked fine, so I decided it was time to pull that switch out and take a better look at it.

The switch has a vacuum harness with 4 ports that plug right into the top of it. Also, it is keyed, so when you pull it off, it will only go back on one way. Th switch just unscrews out the top of the transfer case and I just used my channellocks to get it loosened and out.

Lo and behold, the switch has a ball vearing looking plunger in it that is pushed and the way in and will not come out even with me beating it on the frame of the jeep. Great! I now know what my problem is. However, I'm thinking to myself "This dang little switch is going to cost me an arm and a leg". Not the case.

O'Reillys had the switch for 15 dollars. AutoZone and all the others also carry the switch and apparently this thing is a common switch on vehicles and has several applications.

I brought the new switch back home and screwed it in and reconnected the vacuum harness to the top of it. What's next? Well, I get back in the captain's chair of the jeep and start it up. I still have it in four high, so I shift in back into 2 high. The dash indicator instantly went out. Problem solved!

I took it out for a rough ride today and all is back to normal.

cont.

Here's a couple pics of the switches so that you can see the difference. Take note of how the new switch has the plunger all the way out and the old one is sunken in.

Also, I take crappy pics, so they look really bad...

Anyway, the job is really very easy and it took me a bit longer to type it out that to actually do it.

The 4 wheel engages and disengages immediately now, so if yours is a bit sluggish, do yourself a favor and change that switch! It might save you from being stranded. If it does happen to you though and the vacuum keeos your hubs locked, reverse the vacuum hoses on the diapragm on the shift motor and it will unlock the hubs. This will get you where you can drive your jeep home without the worry of ruining your locks.

and that is all.

Here's a shot of my shift motor.

Don't make fun of my rust. It's only a little surface rust. Yours will look the same when it is old and has as many miles as mine does.

I'm glad you got it fixed, Amerijeep.
Can this thread be stickied so it'll be easy for others to find? You've done a great write-up and it'll be a big help to others having the same trouble.

ok. It's stuck. I hope it reads ok. I didn't proof it at all and just went with it. I used to right tech articles for a magazine. It was easier, because the editor makes you look good...

Thanks. It reads ok ,to me.

Good info. I'll see if this is my prob. tomorrow.

Thanks for the post. Your diagrams where a big help. Instead of taking the jeep to a shop I found a crack in my vacuum line and replaced it for $3 and was back in 4 wheel drive.

great post amerijeep all good info... just finished a rebuild and its something thats on my list which is getting shorter all the time. love the nod to those serving too.

Thanks this is my first day on the forum and ur post already helped me figure out my problem with my 4wd thanks

I second Raymond. The pictures are the most helpful to me. I am going out in a bit to look at mine b/c I have problems with mine and figured it was something to do wit the transfer case

My Wrangler would not unlock the hubs and all the lines are colored black on my '92. Difficult to figure out what was going on until I read your post.

Thanks...

This helped me out. Mine was all disconnected. I rehooked everything up but would not engage. I found the yellow line was cracked. I spliced in a rubber vac line that seemed to fix it. When I engage the trasnfer case in and out of 4wd the indicator would go on and off if it was only engaged for a few seconds. I figured it was fixed, not so. If I engaged the 4wd for more than a few moments it would NOT unlock the hub when I returned to 2wd. The 4wd indicator stays on even though the transfer case is disconnected. I tested this by going to 2wd and the drive shaft to the front axle would spin freely and when in 4wd the drive shaft would engage with the transmission correct? I disonnected the yellow line which removed the vac from the diaphragm and removed the 4wd indicator. I figured I was set. But not so, when I took it for a drive it seemed a bit less in power and much noiser. I am assuming that the transfer switch sticks when left in 4wd for any length of time. That will be replaced here shortly. I did swap the the 2 vac lines so that the yellow is now in the disengage diaphragm. I am assuming this will disengage the lock, but the yellow line has 2 connections and will actuate the 4wd indicator on the dash. so My dash will have the 4wd lit but not actually be in 4wd. All this seems to be a pain the ass. I have read about the posi-lok but before I go and spend another few hundred bucks Iwant to know if installing the posi-lok is a better way to go or just living with this convoluted (but convient) vacuum actuated system.
Thanks,
Bern

I had the same problem but mine went out on the trail. I didnt know I was in 2x4 most of the day till I got really stuck on a rough hill. I replaced the system with the posi-lock from Quadratec and I LOVE IT !!!!! I can engage the front at about any speed and then when I want to I just shift to 4hi/low. I can also disengage the front and leave it in 2lo for really sharp turns. Also i dont have to worry about cutting vacum lines on the trail.

I had the same problem. I decided to take out the vac. line altogether and put in one piece TJ shafts. I have loved it. Never have to worry again. I put in the block off plate and it is great took me about 30 minutes and 40 bucks.

AmeriJeep.
Great detail troubleshooting...
BTW - For some reason the pictures do not show.
Can you re-send?
Thanks,
f

cant see pics.. would be nice...

my 4 wheel drive indicator is always lit.. being able to see those pics would definitely help me troubleshoot this..

Hi!

I'm encountering the same problem on my 89' 2.5l.
The first axle engages and disengages very good, but the light is still on.
Where I can start to find solution for this?
Is this switch which is near the battery dead or is the switch on transfercase dead?
Like I understand the switch which is on top of the transfer case engages and disengages front axle and that's it. It doesn't affect 4wheel indicator on dashboard?

my light won't come on but the 4wd works due to a manual cable converstion. The bulb is good, do you think that switch controls my light with the opposite problem?

I'm not seeing any pics? What gives?

Could you email some pics to me? I'm having an issue like this as well, However I noticed we seem to have both lines pulling a vacuum all the time? Do you think this is caused by the same valve?

hey im new to this forum and i just bought a 93 sahara and my indicator light just stays on. i think its always engaged due to all the noise im getting inside the cabin. just drove the rig home tonight and i want to resolve this problem this weekend. could you send me the diagrams? none of them are showing up

i'm havin this very same problem, i'll have to look at when i get time to this weekend... good info tho!!

+1

I just wanted to say great write up. After buying a yj this past weekend I had the same problem. So I went down to my local autozone and picked up the vacuum switch. After 5 mins in the parking lot it was fixed. This really must be a common problem because I found the part in the help section for $15. Thanks again.

It looks like the pics are gone and I no longer have them to re-post. I'm glad this is helpful still.