I copied this from the 4 Banger Forum and it explains the whole vacuum system
"58 years ago, there were no such things as manual locking hubs until Arthur Warn invented them. Since then, they have been standard on many trucks up until the 80’s when there was a shift towards a simpler design. This new design would help streamline the manufacturing and save companies millions.
At first, they did not want to get rid of the ability to unlock the front wheels. So instead of disconnecting each wheel independently, why not just disconnect the axle shaft to one side? Why one side you say? This all goes back to how the differential works. The driveshaft turns the pinion which then turns the ring gear and differential. Since both tires have traction, the vehicle moves. As most of you already know, this isn’t true when one wheel looses traction. All the power is then transferred to the side that has no grip. This mean, while your tire is spinning in the air you are not moving. The same principle applies to the axle disconnect. Disconnect one axle and the differential will send power to the side that is not connected.
To disconnect the axle shaft, it is first cut in two. Both ends are splined and a collar is used to connect the two shafts. The collar is held in place by a shift fork that is connected to a vacuum actuator. The actuator has a diaphragm inside, when vacuum is applied to one side and pressure to the other, the diaphragm moves. The movement pushes a rod that moves the shift fork and ultimately the collar which gives you 4wd.
The system is pretty straight forward but had one Achilles heal, it needs to be sealed. To create vacuum, the system relies on the engine. To switch the system on and off it relies on a switch at the transfer case. To connect all this together, plastic hoses are used. Since the newest Jeep to use this was built in 1995, they have been around a while.
The hoses are exposed to the elements. Like all plastics, they can also dry-rot which makes them brittle and they do crack. In 1992, Chrysler redesigned the layout for the vacuum lines to help limit the possibilities of leaks. Instead of three vacuum lines to the vacuum actuator, newer models only use two. The extra line was used to send a signal to the dash to let you know it was in 4wd. Now, the switch was connected directly to the housing at the axle.
With proper maintenance, the vacuum system will work for a long time. But what if it doesn’t? There are a few options for you. The simplest one would be to fix what’s wrong and maintain it afterwards. This could mean replacing the lines, diaphragm or switch.
Another options is to replace the vacuum disconnect with something cable actuated. Posi-Lok has a series of cable kits that replace the vacuum setup. Simply remove the old lines and the disconnect housing at the axle. The kit comes with a new housing and a long cable attached. Drill a hole through the firewall and connect the switch to the dash and you’re done.
Yet another consideration is to swap the two-piece axle shaft for a one-piece shaft. This setup first started appearing on the Cherokee’s in the mid-‘90’s. The Wranglers didn’t see this happen until the redesign in 1997. This removes all extraneous moving parts from the equation but does have its drawbacks. Now the all of the components in the front differential will be moving. It means more wear and tear and a slight loss in power, performance and fuel economy. The biggest problem is the front driveshaft. Jeeps that uses the vacuum disconnect did not have their front driveshaft balanced. This was done to save cost since the driveshaft didn’t turn when in 2wd but because all of the components in the axle are turning, the driveshaft will also turn. This was one of the first big problems when the 1997 Wrangler was first released. To solve this, the front driveshaft will need to be balanced.
If you happen to own a early Wrangler, both axle shafts will need to be replaced since the U-joints used are 260x series while very late model YJ Wranglers and all new Jeeps use 5-297x. Now if you happen to own one with the 5-297x series U-joints, all you will need to do is swap the passenger side long shaft.
There you have it, the vacuum disconnect system in a nutshell. "
There is a very good description of the system. Now as Dava mentioned the posi-lock system is the common cure, but when mine went, I just swapped the motor out for a different one. I have no interested in pulling a cable thru the firewall.
Topic: 4x4 not engaging (Read 1475 times)