The Mystery of the Vacuum Disconnect Dana 30

Old 11-30-2008, 03:24 PM
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Join Date: Apr 2008
Location: Northern Illinois
Posts: 2,237
Year: 90
Model: Cherokee
Engine: 4.0
Default The Mystery of the Vacuum Disconnect Dana 30

One of the most often asked questions is why does my vacuum disconnect not work. Here is an explanation of how it works, why it breaks and things you can do to fix it.

First, what is the vacuum disconnect?
Well, its really nothing more than than a small vacuum motor, a shift fork and a collar to connect the axle halves.

Here is a picture of the vacuum disconnect, still in place, but with the skid plate removed.

Once we remove it we can see the shift fork and its little tips.

The axle collar that connects the inner and outer shafts giving you 4wd.

What happens when I put my Jeep in 4-wheel drive?

When you shift your shifter handle, it supplies vacuum to one side of the motor and the shift fork slides over and moves the collar to connect the outer shaft to the inner shaft.
The first photo is the vacuum motor in the 4wd position, the second is the collar engaged in 4wd.

When you shift out of 4wd the vacuum is applied to the other side of the shift motor and the fork goes back and it moves the collar back.

The first picture is the shift motor in the 2wd position. The second is the shift collar in the 2wd position.

Why doesn't my shift motor work correctly?
There are really only a few reasons that the shift motor doesn't work.

One of the reasons is that the shift fork itself becomes worn and sloppy on the shift arm. The shift fork can be replaced but many times if you are going to go to this effort its simpler to replace the entire assembly.

The little black tips in this photo are one of the common reasons that the vacuum disconnect fails to work properly.

These are also replaceable and quite cheap, but if you go to the trouble its just as easy to replace the entire unit.

Can I test my vacuum disconnect?
Yes. With a vacuum pump you can apply vacuum to one port or the other and see if the motor slides the fork back and forth. Apply vacuum to the port closest to the differential to shift the fork into 4wd. Apply vacuum to the port closest to knuckle and shift the fork into 2wd.

Any other information on this misunderstood part of a Jeep?

You need to add about 5 oz. of the same lubricant that is in your differential in the shift motor housing.

Hopefully the answers your questions about this misunderstood part of your Jeeps 4wd system.
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