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Discussion Starter · #1 ·
Hello all!

Just purchased a 91 YJ 4.0 that needed a front axle assembly. I put one in out of a 89 YJ. My issue is that I replaced the entire vacuum harness with a new 89 harness to work with the CAD. As most used jeeps someone else has been under the hood! Here are the issues I need help with...

1. Where do I plug the red line for constant vacuum?
2. Where does the black soft rubber hose go? It is T'd into the harness.
3. The black rubber plug on back firewall had this hose shoved into it that was connected to the manifold. I don't think it goes there what plugs into it?
4. Where do these electrical plugs go? They both are near the transfer case. One has been cut or torn but the plug is connected.
5. The 89 CAD doesn't have an electrical plug in where my 91 did, will this cause my 4X4 light not to work?

Thanks in advance for any help!





 

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I don't have it all but here is what I got and some pics.

The red line comes the intake at the rear there is a plug with two outbound. The red line goes behind the engine T's into the heater control to supply vacuum but continue to the passenger side under the battery tray some goes to the T-case some come back that way to the axle.

The 89 had a 4X4 light switch mounted on the firewall passenger side a little blue vacuum switch. The 91 I guess is on the axle itself I have an 89 so don't know.

The Tcase has two switches one is the reverse light and the other is the 4X4 vacuum switch

Here is the pics and diagrams hope this helps.

I have never had a workiing vacuum operated CAD so best I got.
 

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red-vacuum from engine to transfer case switch
green-vacuum from t-case switch to front axle to disengage 4x4
yellow-vacuum from t-case switch to front axle to engage 4x4
blue-connects to t-case switch and then just has a small filter on the other end ( connects to nothing)..this allows for releasing the vacuum on the line not being used
black-from axle to vacuum switch..this controls the idiot light

The 4WD Indicator Light in the 91-95 YJ is the gauge closest to the drivers side in the center gauge cluster, to the left of the fuel gauge as pictured below. "4WD" With a graphic of the drivetrain will appear in the upper half of the gauge, the bottom half is not used.

In the 87-90 YJ the 4wd light is in the center of the dash along with the seat belt, parking brake, and high beams. Pictured below.

How it works:

To understand how this light becomes lit up when you shift your transfer case into 4wd, can be a bit tricky! It's not necessary to understand exactly how it works, but it will help you make sense of the workaround.

It all starts in the transfer case, the 4WD Vacuum Control Switch is located on top of the transfer case, if viewed from the back it is to the right of the shift linkage. You should see a rubber bushing with 4 tubes coming out.

Red is constant vacuum source to the transfer case
Yellow uses vacuum to engage the shift motor on the front axle CAD
Green uses vacuum to disengage the shift motor on the front axle CAD
Blue is release for the vacuum change, which will release vacuum in the unused hose

When the transfer case is placed in 4Hi, the 4WD Vacuum Control Switch will send vacuum being produced by the intake manifold though the Red line, down the yellow line and engage the shift motor. Once the collar successfully engages, the indicator switch on the vacuum motor will relay to your 4WD light in the cab, to turn on.


Success! Now we somewhat sorta know how this works! So if you've replaced your 2 piece axle shaft with a solid shaft, regardless of how you dealt with closing off the CAD, even if you left everything hooked up as it was before, its advised you follow these directions to make sure you don't lose your 4wd light, or if you already ditched the vacuum lines, you've lost the 4wd light already!

Send me a PM I have a great word doc that will not post with pics and part numbers to make it all work. I can attach it to an e-mail
 

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Discussion Starter · #4 ·
Can someone post pics or email me pics of how the vacuum lines look at the manifold. I have to re-create them and need a visual aide!
 
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