Center Axle Disconect Function and Trail Repair Explained
How the Center Axle Disconect System on a Solid Axle works. Short Cuts
by: Stevan Gajic


As you probably noticed, Dodge Rams and Jeeps no longer come with Manual Hubs. All Rams and some Jeeps come with a vacuum-actuated center axle disconnect (CAD) system to disengage the front axle while in 2wd. This helps to improve fuel economy over non-disconnecting axles, and also allows true shift-on-the-fly engagement of 4wd. The downside is that there are vacuum lines that can be damaged off-highway, causing the front axle to disengage as well as adding more moving parts to fail. At the very least anyone that does any trailwork with their Ram should know how the CAD system works and how to engage it should the vacuum line be damaged.

CAD Function

The left wheel is connected to the axle directly in a similar fashion to a full-time 4wd axle. The right axleshaft consists of two shafts: one engaged to the differential, the other to the right wheel. A sliding sleeve connects the two separate shafts when 4wd is engaged. While in 2wd, the left wheel turns the spider gears and through it, the right inner axleshaft. Since the two right shafts aren't connected, the right wheel is free to turn independently of the left. While the spider gears spin, the ring gear, and thereby the front driveshaft, stay stationary -- allowing for a decreased amount of drag on the driveline.

When you engage 4wd, a synchronizer in the transfercase spins up the front driveshaft; at the same time, a vacuum switch opens, causing the vacuum motor in the CAD to try and lock together the right axle shafts. Engagement does not occur until the splines on the shaft lines up with the ones on the sleeve. As the sleeve slides over, it trips an electronic switch, turning on the 4wd light in your dash.

A cutaway of the CAD system.
The vacuum motor and housing.
The vacuum circuit used to actuate the front CAD

Trail Repair

If you do damage the vacuum line, an easy way to re-engage the front axle is to remove the housing. After removing the vacuum motor, reinstall it but, instead of engaging the fork between the e-clips, install the farthest clip so that it pushes on the fork. This way the spring that otherwise holds the axle disengaged now holds it engaged. When reinstalling the motor, make sure to slide the sleeve over manually or you'll never reattach everything. You may need to raise the right tire and turn it to align the splines. Depending on how you're oriented, you may loose a significant portion of your axle fluid when you remove the housing so be prepared to top off the axle fluid.