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Discussion Starter #1 (Edited)
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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Makes me wonder if that setup is weak? There's only a collar connecting both shafts.
 

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I'm somewhat ignorant on the subject, as while it seems all my piles had CADs at some point (hell, my dodge even has one), I typically delete them or swap axles.

Anyways, the typical failure is either the vacuum motor, the lines to it, or the vacuum switch in the tcase. NOT the shift collar. The splines on the shaft there are huge, I've never measured them but I'm going to say in the area of 1.5". And the collar is likewas quite large. You will reliably break the outer, or the u-joint, before anything internal in the CAD.
 

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Discussion Starter #6
I'm somewhat ignorant on the subject, as while it seems all my piles had CADs at some point (hell, my dodge even has one), I typically delete them or swap axles.

Anyways, the typical failure is either the vacuum motor, the lines to it, or the vacuum switch in the tcase. NOT the shift collar. The splines on the shaft there are huge, I've never measured them but I'm going to say in the area of 1.5". And the collar is likewas quite large. You will reliably break the outer, or the u-joint, before anything internal in the CAD.

Ive got some pics of the splines I will post later when I am at my computer.

But you are correct.. normally the failures are from the vacuum motor or the vacuum lines. Or I have seen some instances of the tips coming dislodged and falling off so the shift fork doesn't move correctly.
 

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great write up.
I shimmed my fork over last month had no problems at all. it was about a 20 minute job.
now I have a grinding noise that started monday comming from the front but I have not had time to see what it is. I hope I can take a look at it today.
 

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great write up.
I shimmed my fork over last month had no problems at all. it was about a 20 minute job.
now I have a grinding noise that started monday comming from the front but I have not had time to see what it is. I hope I can take a look at it today.
did u find out what the grinding was?
i have some weird noises too. my vac lines to this thing are very sloppy if it indicates im in 4wd is that accurate or will it still say? i have a weird cv axle sound
 

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Discussion Starter #11
did u find out what the grinding was?
i have some weird noises too. my vac lines to this thing are very sloppy if it indicates im in 4wd is that accurate or will it still say? i have a weird cv axle sound
The switch that operates the 4wd light is on the transfer case and completely seperate from the vacuum disconnect motor. So the light will still light even if the motor itself fails to operate correctly.

SOME early XJ's had CV shafts in the front rather than U-joints. But normally that was only ones with the full-time 4wd, and therefore no vacuum disconnect.
 

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I have a disco dana 30 on my 1990 xj. There are no wires going to it, or any place for wires. It does have the two vacuum lines on the actuator thing like yours, but another one on the right side of it. Does anyone know how many different types Dana made and and if any are more or less reliable? Also would anyone by any chance know where the sensor for Part-Time 4wd is?
Thanks
 

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Discussion Starter #13
I have a disco dana 30 on my 1990 xj. There are no wires going to it, or any place for wires. It does have the two vacuum lines on the actuator thing like yours, but another one on the right side of it. Does anyone know how many different types Dana made and and if any are more or less reliable? Also would anyone by any chance know where the sensor for Part-Time 4wd is?
Thanks


The particular axle I used for the pictures was out of a 95 Wrangler with ABS. My 90 doesn't have the wires either. The system operates exactly the same way though. The problems are usually due to either the vacuum lines failing or the diaphragm inside the motor failing. So they are all about the same reliabilty. If your are asking where is the sensor that lights the dash lights for part-time, its located on the transfer case.
 

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I have a disco dana 30 on my 1990 xj. There are no wires going to it, or any place for wires. It does have the two vacuum lines on the actuator thing like yours, but another one on the right side of it. Does anyone know how many different types Dana made and and if any are more or less reliable? Also would anyone by any chance know where the sensor for Part-Time 4wd is?
Thanks
The third line goes to a vacuum operated electrical switch on the firewall just under the heater fan motor. When the vacuum motor engages the axle, it sends vacuum to the switch which turns on the 4WD light on the dash.
 

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The third line goes to a vacuum operated electrical switch on the firewall just under the heater fan motor. When the vacuum motor engages the axle, it sends vacuum to the switch which turns on the 4WD light on the dash.
this is what i thought! i have a new prob my 4wd light goes on and off. sometimes it wont come on at all and i dont think im getting 4wd either.for a while it would come on but go off as soon as i hit the gas and come back on as soon as i let off
 

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Discussion Starter #18
this is what i thought! i have a new prob my 4wd light goes on and off. sometimes it wont come on at all and i dont think im getting 4wd either.for a while it would come on but go off as soon as i hit the gas and come back on as soon as i let off
Vacuum leaks will cause that problem. Start chasing and checking EVERY vacuum line. You may have a cracked one that you don't even realize is no good.
 

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Vacuum leaks will cause that problem. Start chasing and checking EVERY vacuum line. You may have a cracked one that you don't even realize is no good.
right, so you think its more likely a vac leak than the unit being bad having a bad diaphram?
 

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i just bought to vac units at the boneyard and hooked em both up under the jeep to the actual hoses. and mine has the 3 lines too and no wires like the one in the pic.so the both engage and disengage fine but not once did my 4wd light come on.so i havent removed mine.i think i might have a cracked line going to the switch.ive hear of guys locking the coupler. what are your guys thoughts on that? also how do you get manual hubs?is there a way?
 
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