Trying to decide a course of action, I recently acquired a 99 4x4 XJ in less then ideal shape. A project I called it... So, after I finished the interior revamping, I am moving to the undercarriage. There I find I have a front axle that I don't think is a 99. The XJ has the "part time" Command-Trac 4WD with the auto transmission (AW4). While using the XJ to pull broken fence posts with concrete out of the ground, I found the front wheels were not pulling. The rear was churning the ground with both wheels...(trac-lok differential)! I had the 4WD system in 4W-Low and if felt like it was engaged. So I took a quick look at the front to make sure the drive shaft was there (it was) and I see these vacuum lines coming from the axle but not connected to anything. I have had a few XJ's in the past, but dealt more with overheating, leaks and etc. Replaced a engine in a 90 XJ, waaay back but have never messed with the drivetrain. I know I have the 8.25 rear axle with 3.55 ratio gears, and should have had a Dana M30 front axle according to the build sheet.

So what steps should I take? Should I raise the XJ off the ground and see if it will turn the front wheels in 4WD, should I plan on replacing this "vacuum" axle with a donor from Upull it? I am really stumped as to a course of action and could use some ideas!

In other news, the interior is comfortable and looks good still need to decide on a radio for it, and I got the AC working! Still diagnosing a "airbag" light, though.

 

Wait. Are you saying your axle is one of the earlier-versions that has the vacuum disconnect on the passenger side??

Because the late model D30s shouldn't have any vacuum connections. Just a vent line that isn't connected to anything else. Are you certain you aren't looking at your vent line?

 

It may have a vacuum activated locker in the front axle. You'd have to pull the cover off to know for sure, especially since it appears to have a TruTrac or other lunchbox locker in the rear. Usually front lockers are air pressure, cable, or electrically actuated, but anything goes when doing modifications. Air pressure types need an on-board compressor or air bottle.

Get someone to scan it with a DRB Scan tool and it will bring it what's wrong with the airbag. It maybe just the clockspring in the steering wheel. Mine showed "Open circuit to firing squib" which is the connection thru the clockspring.

 

so many times a shop that needs to replace a front diff in these cherokees don't know there were changes throughout the years, and get one from a wrong year.
i bought a 93 a few years ago with a vac diff in front. i went through all the receipts in the glove box (was owned by a lawyer) and found one from canadian tire that replaced the front axle. didn't know i guess and installed it anyway. you'd think they'd see that it was physically different. but they installed it anyway.
guess they didn't test it once it was in.

i just did what i did in the past for a quick fix since it was winter (that's how i found out), and pulled the vac housing and clamped the collar in place with a hose clamp. worked like a charm till i sold it to a friend. he didn't care as he only wanted the clean low mileage motor and interior for another build. body was rotten at this point.

 

Sorry, I should have posted a pic with my post..
This is looking forward at the front axle from the passenger side. The vacuum lines are facing the camera and there are 3 that originate at the axle.


The second pic is also looking forward at the front axle from the passenger side but shows the driver's side of the axle.


Hopefully, the pics will help identify what a previous owner installed on my "project". Is it a keeper? Should I check the gear ratio on it?
Any suggestions on checking the function of the front drive train's function?

As far as the airbag goes, the clockspring is the only thing I have not changed (well that and the passenger airbag) I will get it scanned to help pinpoint the problem.

For general information, the vehicle appears to have been originally sold in Texas according to the build sheet so it stands that it has spent its duration here. At least, there are no rust issues!

 

It's strange because that looks like a high-pinon dana30 pumpkin with a low-pinon tube on it, which must be wrong because I don't think that would work. Maybe its just the camera angle. Anyway, that's definitely the vacuum disconnect you'd find in early renix-era XJ's (92+ iirc). It's my understanding that (from 4wd) the vacuum disengaged the front drivetrain with a sliding fork so that the front driveshaft didn't spin when not in use, whereas on your '99 the disengagement happens in the transfer case and the front driveshaft just spins with the wheels. You could probably make it functional but I would pop the cover and lock the front axle. Then it will behave more like stock.

https://wranglertjforum.com/threads/...20/post-558004

 

Here is what you need to know:

Your front axle is from an older XJ. It is a Dana 30 high pinion, an explanation of that is the pinion gear is above the center-line of the ring gear, not below it as most drive axles are. This feature is a good thing. Your axle also has the CAD or center axle disconnect, this is a not so good thing, but not the end of the world either. The CAD is a vacuum operated fork system that locks the two piece passengers side axle together by sliding a spline collar across both axle shafts (effectively making it a one piece axle). The CAD system is know to fail and be problematic.

So picture this, you put it in 4WD, the rear end pulls but the front end nothing... if the CAD is not sliding the collar to engage the axle, the open differential will apply power to the wheel with least resistance, in your case the passenger side non connected axle, so even if the transfer case is in 4wd, your inner passenger axle shaft is the only thing spinning up front because it isnt "collared" to the outer shaft.

There are many ways to deal with this. The quick cheap fix is to remove the CAD cover housing and manually slide the collar over both axles, then use a "radiator hose clamp" to shoulder against the collar keeping the passengers side front axle locked together. This will work for as long as you want it to. There are other ways to fix it, there is a manual cable retrofit called a "posi-lok" that slides the collar manually from inside the cab. You could fix the CAD system. And some people choose to replace the passenger side axle with a later model one piece axle shaft. Search for doing this, it is a good idea to replace both drivers and passenger side if you go this way (the u joint size is different on late model axles) so you keep the parts similar. this is a direct drop in with no modifications required, but there is a bit of wrenching involved.

I tried to explain it as simply as I could, assuming you are not familiar with whats going on inside the axle housing. If you are past this point, good, if not, I hope this helps.

 

So........I should keep the front axle and either lock the collar with a hose clamp or install a "posi-lock" that will manually slide the collar into and out of the lock position?

ridesdirt........I think I followed your post to a understanding. I guess I am still wondering why only the rear wheels engaged? For instance, I got off road (4WD-HI) in some dry loose material and applied ample throttle. Both back wheels dug in and left ruts, starting to "fishtail" while the front did nothing. No sign either side engaged.

Can I check function by raising the front wheels and spin the tires by hand? If so, would a sign of front axle engagement be the front drive shaft turning?

 

I have an 89 with the vacuum disconnect, and I am currently in the process of getting rid of that POS.
couple of things -

1) do those vacuum lines go anywhere? (from your picture it looks like they are capped off, and don't go anywhere)
2) having a 99, if they are connected somewhere does it know to engage the vacuum system when you put it in 4wd if they are? (Answer: I seriously doubt it).

SO what happens, is you engage 4wd, and the front drive shaft spins, but the diff in the front sends power to the side with the least resistance, which means it sends it to the passenger side, which is not actually connected to the wheel. on a system that is meant to have this vacuum setup, when there is a vacuum leak, we run into the same problem. Hence why i am ditching it on my 89.

You have several solutions -
1) you can get a manual engagement tool that allows you to engage the fork from inside the cab - cheapish option, being under $200: http://4x4posi-lok.com/app_jeep.html
2) you can modify the vacuum/fork bit so that the fork is always engaged. cost: really cheap, just need a plug and a taps. .
3) swap over to solid axle shaft for the passenger side. This is the more expensive option, depending on what you do, but you can go to a junk yard and get an axle shaft from a later jeep, you'll need seals, etc, but it's the more "permanent" change. I am dropping A LOT of money to do this option, since I am upgrading to chrome-molly shafts, installing a locker, etc. you can get a kit for under $500 for new axle shafts, block out plate, and a few other bits. . but you can get used shafts, or just the passenger side non chrome-molly, etc to make it cheaper.

 

excellent video info above... and comments.

so the way this works, the driveshaft spins the pinion gear (inside the axle housing), the pinion gear spins the ring gear. This is where the spinning force changes direction from the drive shaft spinning along the vehicle, to the axle shafts spinning perpendicular to the vehicle. pretty straight forward deal. To complicate the design we have what is called a differential (google it - would take me 8 paragraphs to explain how it operates). A differential allows the rear wheels to spin at different speeds, this is good when you go around a turn, the outside wheel has to travel further, so it does spin faster than the inside wheel.

in you rear axle assembly, you obviously have some sort of "positive traction" unit. what this does; when power is applied, it applies equal power to both rear wheels. there are a lot of ways the rear end can accomplish this, for this conversation, let just say you have a LSD in your rear axle assembly (LSD stands for limited slip differential). So when you spin the rear tires, both are spinning and you can fish tail the rear end.

in your front axle assembly it is obvious you have a "open" differential, this is the way 99% of all cars are. When you apply power, the differential applies power to the wheel with the least resistance. Since you have a two piece axle shaft on the passenger side of the "pumpkin" (center section of the rear end where the ring and pinion gears are), your front power will always go to the inner most half of that two piece axle shaft. So inside your axle assembly, there is this short axle stub spinning (it is not connected to the passenger front tire until the slide collar locks the two piece axle together), if you slide the collar to lock the two piece axle shaft together, then the differential will send power to whichever front tire has the least resistance.

If this makes sense to you, here is what 4 wheel drive actually is...
with open differentials in the front and rear, you essentially only have a 2 wheel drive vehicle, 1 in the front and 1 in the rear. but it can be either rear and either front, the one that will spin the easiest (not a good thing - you can get stuck on wet grass)
your set up, with LSD in the rear and open in the front, you have a 3 wheel drive vehicle, 2 in the rear and 1 in the front (when it is working correctly)
if you have LSD or "lockers" in the front and rear you can finally achieve 4 wheel drive! yeah!
And, most rear wheel drive vehicles only power 1 rear wheel, so they are essentially a 1 wheel drive...

clear as mud???

 

Sorry guys! I never thought about the fact that he might have an early front axle installed for repair. I now think he had better check to make sure the gear ratios match. It wouldn't make a difference with the axle disconnected, but definitely will once that axle is engaged. Chances are they are both 3.55, but maybe not.

 

Got a chance and checked function as the video did. In 2 WD the front driveshaft, turns by hand freely. In 4 WD-High, the front drive shaft does not turn.

aroundincircles, thanks for the video it helped me understand more! To answer your question, those vacuum lines (3) are "open" to the air on the free end and connect to the actuator on the axle on the other end. I don't think the axle is original to the vehicle and as for as I can tell the collar does not join the 2 axle halves on its on. I figure I will have to "fix" the collar in position. My local U pull it has a Dana 30, I can pick up for $200 if I can't do the collar fix.

 

Dave, rather then take the "pumpkin" down and count teeth can I mark the drive shaft and see how many revolutions it makes with one turn of the tires? Or do I have it reversed?

 

you can count drive shaft turns. 1 rotation of the tire and count driveshaft revolutions,

there could be a tag on the diff cover, if there is the gear ratio should be stamped on it. both front and rear.
also, if you pull the cover, the ring gear will have the ratio stamped on it, no need to count teeth.

 

To check the gear ratio on an open differential if you are only rotating one tire you have to count the driveshaft rotations and multiply it by 2 because of how the spider gears work. Also FYI an open differential always sends the same amount of torque to both sides, but since energy takes the path of least resistance if one wheel has less traction than the other it will be the only one that spins, the other tire while not moving it is still receiving the same amount of torque.