One of the mighty downfalls of the YJ's axles (aside from the d35c) is the Center Axle Disconnect. Every YJ has them stock, and if you haven't been acquainted with yours yet, it may only be a matter of time!
So what is the Center Axle Disconnect? Its the little box on the passenger side housing of your front d30 axle, it houses a shift collar that is vacuum operated. The purpose of the CAD is to use vacuum to slide a shift collar and lock the 2 piece axle shafts together, thus allowing you to be in 4wd. What may end up happening is the vacuum motor will fail, or vacuum lines end up cracking/splitting/etcing, causing your jeep to no longer engage 4wd. This can end up being a headache trying to replace the vacuum motor, or trace down broken vacuum lines.
This thread is meant to give you several different options to work around the potential unreliability of this vacuum system. It's also worth noting by doing modification number 1 or 3, that your axle shafts, and front driveshaft will be in constant rotation while driving. According to the engineers at Jeep this will roughly translate into a 0.1, to 0.3 loss in miles per gallon. The TJ, and JK wranglers do not have a CAD, as it was ditched after the 1995 model year YJ.
1) Replace passenger side 2 piece axle shaft, with solid 1 piece axle shaft.
(This particular workaround will give you the strongest results!)
Note: to retain 4wd light with this conversion, please see THIS THREAD
a) Sourcing a solid passenger side axle shaft. These can be found in 92-99 XJs (Cherokee) and 97+ TJs.
The 96-99 XJs, and any TJ axle shaft will have the larger 297x u-joints. (Make sure you don't get one with CV joints!)
If you do find 96+ shafts, you might as well grab the drivers side and upgrade too!
b) Purchasing a drivers side axle shaft seal
c) Either fabricating a block off plate, or re-using the old shift fork housing
How to do it!:
1. Apply emergency brakes, and chock the rear tires, jack front end up and support with jack stands.
2. Remove tire, and the cotter pin on the axle nut. Remove brake caliper, there are two 13mm
bolts on the back. Pull rotor off wheel studs. Torque setting is 11ft/lbs
3. Remove axle nut, it requires a 36mm six point socket
this may require a breaker bar, and an air impact wrench if you have one. Be sure to use an impact socket! Torque setting is 175ft/lbs
4. Remove three 13mm twelve point
bolts on the back of the unit bearing. Torque setting is 75ft/lbs
5. If the unit bearing is stuck, a BFH with a gentle hit in the right places, paired with a pry bar can make easy work. It may benefit you to use anti-seize on the contact areas to make this step easier in the future.
6. Place a small container oil catch container under the CAD housing, unplug vacuum lines from the housing, use either bolts or vacuum line caps to plug them with. Remove the four 11mm
bolts holding the housing on and remove housing. Let any oil drain from the housing.
7. Slide the outer axle shaft out and remove the collar on the shaft. Pull the inner shaft out. Remove the seal on the right side of the CAD housing.
8. Seat the new seal, you can be creative here, or rent a seal puller from an auto parts store.
9a. If you decide to make a back off plate to replace the vacuum housing, use the existing housing as a template for size and bolt positioning. When reinstalling use a gasket sealer, such as RTV to seal the back off plate to the housing.
9b. You can also reinstall the existing vacuum housing back in its original location, you do not need to hook anything back up to it if you don't want to, as it will be useless.
10. Trace back vacuum lines, and wiring harness and remove as necessary. In the case of the wiring harness, just move up into the engine bay.
11. Repeat steps 2-5
for drivers side if you are replacing this shaft as well.
12. Grease splined shaft ends of replacement axle shafts, and where the axle seals ride along the shafts.
13. While sliding in new axle shafts, support them next to the axle tubing as to not catch on the axle seals.
14. Reinstall the remainder of the components in reverse order, make sure to torque to spec.
15. Refill differential with recommended gear lube. Typically 90 weight for open differential. If an aftermarket Limited Slip Differential has been installed, you may also be required to mix in a friction modifier.
-Special thanks to smithy of jeepforum, bolt sizes and torque specs were provided from his writeup.
2) Replace system with Posi-Lok cable system
Not to be confused with a posi-traction or limited slip, or any other type of locker, the posi-lok cable installation will allow you to mechanically engage and disengage the shift fork. The benefit of this will allow you to run in 2Lo in addition to 2Hi, 4Hi, and 4Lo.
4x4 Posi-Lok - 4WD Engagement Systems
3) Inexpensive vacuum shift motor fix
(This is by far the cheapest solution if you're working on a budget!
Originally posted by Coyote_97TJ
1. Remove the four 11mm
bolts that secure the shift motor to the axle housing and remove the shift housing assembly, have a drip pan ready.
2. Remove the 2 "E" type retaining clips that hold the shift fork on vacuum motor and the 1 "E" type retaining clip that holds the vacuum motor to the shift housing.
3. Pull the vacuum motor out of the shift housing which will slide the shift fork off the shaft.
4. Take the shift motor to the hardware store.
5. Buy a long spring that is just large enough to slide over the shaft of the vacuum motor shaft and take it home.
6. Slide the spring onto the shaft of the vacuum motor, put the shift fork back inside the shift housing and slide the vacuum motor back inside the housing so as to fully compress the spring. The spring will be too long so make a rough guess as to how much to trim off so that the vacuum motor will go all the way back into the shift housing.
7. Once the spring is the correct length slide the vacuum motor back in and install just the 1 large "E" type retaining clip that holds the vacuum motor into the shift housing. The other 2 are no longer needed.
8. Slide the shift collar inside the axle housing over to engage the front axle.
9. Apply RTV to the housing and install back on the axle housing making sure to align the shift fork up with the center of the shift collar inside the axle housing.
10. Refill differential with recommended gear lube. Typically 90 weight for open differential. If an aftermarket Limited Slip Differential has been installed, you may also be required to mix in a friction modifier.