Question about front axle/4wd.

[sandalscout]

So, I'm extremely new to 4wd systems, and doing some planning for future upgrades of my XJ. I've got a vacuum actuated Dana30 front axle, and I think I know the answer, but why does a front axle need to be "locked in" if the transfer case is not in 4wd mode (and therefore no power to the front axle)? Is it due to the fact that the front wheels turn at different speed while cornering?

What exactly does the vacuum actuator do to lock in the axles? Does it push over a sleeve onto the axles to turn them?

Also, what other methods are used to lock in front axles besides manual hubs?

Thanks!

 

[sandalscout]

Okay, I found a reference to a "sleeve" sliding over with the vacuum motor, but I'm a little more confused, now that I think about it. Would'nt a sleeve to lock the 2 sides together essentially be a locker?

 

[86cj]

It has to do with the transfer case spinning. If you have manual hubs and both sides are unlocked that means the front driveshaft is not spinning along with the gears in the front of the transfer case. You would not be able to shift the transfer case while moving as both transfer case outputs (driveshafts) are not spinning at the same speed and the gears would not mesh together. It only takes one axle shaft to be engaged to make that happen and most 4wd's built today spin the front driveshaft at all times, allowing shift on the fly, your vacuum sleave just brings both axle shafts into play for proper 4WD. Only your differental can provide a locker, both axleshafts are engaged even in a open differental.

I am sure there are other ways to explain it, I hope this makes some sense for you......

 

[IH8RDS]

There are a couple option that you have to upgrade.

You can get a a vacuum disconnect adapter that eliminates the 2 piece shaft and replaces it with a one piece.

0r you can replace the axle. I personally would replace the axle with a newer XJ Dana 30 that will have stronger 296 axle shafts. Look for a HP out of a newer 97-99 (in 00-01 they went with a LP design and are less desirable). These axles are plentiful in the junkyard and expect to pay about $150 - $200 in a u-pull-it. Just figure out you're gear ratio and get one that is the same. Most of the gears will be 3.55.

 

[sandalscout]

That does make some sense, and I do understand the reason that the transfer case works the way it does, but I guess what confuses me most is that if the front differential is not a locking type (like mine from the factory) why would the vacuum actuator even be needed? Why not just make it "engaged" all the time? If the front diff is going to allow slipping anyway, the front tires can spin at different speeds on dry pavement, why doesn't it just stay "engaged" all the time?

Your reference to modern 4wd spinning the front driveshaft at all times would explain this just fine, but with older ones (that don't spin the front DS constantly), it seems like the transfer case and the vacuum motor do essentially the same thing, just at different places in the drivetrain, and providing more things to break (I now that the transfer case does more, but I'm looking only at 2wd versus 4wd modes).

Thanks

 

[IH8RDS]

Okay, I found a reference to a "sleeve" sliding over with the vacuum motor, but I'm a little more confused, now that I think about it. Would'nt a sleeve to lock the 2 sides together essentially be a locker?

Here is a good explanation

http://www.cherokeetalk.com/forum/showthread.php?t=1102

 

[sandalscout]

Scott, thanks for the info, it'll come in handy once I get to that point, if ever. I've found some cool articles on common upgrades, just trying to figure out the logic behind the system now!

That links is very handy, but I still don't quite get my it matters if the axles disconnect in the first place.

 

[IH8RDS]

There is one more option I forgot to add. This takes away the manual actuation and converts it to manual.

Product ID: PSL900

http://www.midwestjeepsupercenter.com/products.asp?cat=181

 

[86cj]

Scott, thanks for the info, it'll come in handy once I get to that point, if ever. I've found some cool articles on common upgrades, just trying to figure out the logic behind the system now!

That links is very handy, but I still don't quite get my it matters if the axles disconnect in the first place.

It does not matter, most MFG's just let both front axle shafts spin all the time, manual hubs are almost non existant today. Your front open differental allows turning slip. If both your axle shafts stay connected to the center axle diff it works fine, it just adds some drag, you don't need the vacuum sleeve.....

I used to leave my CJ's front hubs locked all winter, without a locker in the front it just adds some drag spinning the guts, no other issues....

 

[I Leak Oil]

You know, when I got my first 4 wheel drive, a 77' blazer, the best thing I did was get a shop manual. You will need it anyway but it's got lots of info and pictures/diagrams. I like picture books!
Jason T.

 

[sandalscout]

I've got a manual, and I see HOW it works, what I want to know is WHY did they put it in in the first place? Seems like something else to fail.

 

[I Leak Oil]

You're right it is something else to fail. It was most likely designed like that to:
A) Make the ride better when not in 4wd.
B) Make it more convenient for the average Joe.

It certainly isn't more reliable, stronger or cheaper than a single piece axle.
I guess for a 100% answer you'd have to get into the heads of the Jeep engineers who decided to use the disign.
Jason T.

 

[sandalscout]

Thank you for the reply Jason, I thought it was something that just made sense to everyone else, and I was missing out on something.... glad to see it's not really all that explainable.

Thanks!

 

[BigAl]

Here is one mod I did to a dana 30. I took apart the vacuum motor assembly and put a small piece of fuel line on the shaft, then reassembled. This keeps it locked at all times, basically the same as the new dana 30s. When Vaccum is applied the fork slides on the rail. The vaccum lines get brittle and leak with age, so I just by passed them

 

[98dango]

The reason the front is. You have a NP231 transfer case. This case dose not have a Viscus (sp) coupling or a sun gear system. This means with the front diff turning all the time you may experance vibration. This allows you to disconnect one side of the drive system. All in the name of conveance my good friend. Yes its just one more thing to go wrong. This system was abandon after 1991. No nothing on the truck changed. The auto manufactures just learned as every wheeler know you can run with the hubs locked in if every thing is in good shape. I have done away with this pain in the rear. All i did was install a long side axel shaft from a 97+ jeep TJ. This also upgrades the u-joint size.