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Old 11-21-2013, 09:35 AM   #1
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4WD troubleshooting: no vacuum signal

1988 YJ - I do not get a vacuum signal to either the engine bay indicator lamp switch nor to the FWD axle motor .
I am looking for the origin of the vacuum signal .
I rebuilt the 4WD axle motor several years ago and all was AOK .
Now, 2 years later , no vacuum signal .

I read that there is a switch on the transmission that may be involved as an upstream source .
Can someone provide a vacuum schematic showing the origin of the vacuum signal and its path prior to the front axle 4WD actuator motor?

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Old 11-21-2013, 11:31 AM   #2
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Old 11-21-2013, 11:44 AM   #3
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There's a vacuum switch on the transfer case. They're available.
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Old 11-21-2013, 02:45 PM   #4
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I was able to locate the transfer case 4WD vacuum switch .
There appears to be 4 colored lines coming out of it .

My concern remains the same : can anyone identify how the engine manifold vacuum reaches the transfer case 4WD switch .
I believe that I am not getting vacuum to the 4WD vacuum switch on the transfer case .
I spent hours trying to trace the mysterious red line that connects to the 4WD vacuum switch . No luck on tracing ; it vanishes into the unknown .
I believe that the yellow line turns on 4WD engage .
I believe that the green line turns off 4 WD dis-engage .
There are 2 more colored lines : red (possibly vacuum source from engine )
plus there is a blue ( vent ??) .
Can anyone shed more light on this ?

I am hoping to get a schematic of the vacuum source which heads to a vacuum reservoir , then through a check valve , then to the red line , then to the transfer case 4WD switch .
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Old 11-21-2013, 03:54 PM   #5
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Here is the diagram Red is to the vacuum source.


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Old 11-21-2013, 06:35 PM   #6
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Yes , thank you for the red line schematic to engine manifold vacuum source.

I am still plagued by the issue of not being able to trace the red vacuum source into the engine bay to find its source.
I ripped apart the floorboard 4WD stick shift , I sneaked a look at the 4WD transfer case switch to see the red/ blue/ yellow/ green lines .
I cannot put my fingers on the red line to be able to trace it into the engine bay.
How do folks do this? The transfer case blocks my access.

I am crawling on the cold dirt reaching / straining/ using digital camera to get a better look to find the red line somehow / somewhere connected to a vacuum reservoir or to a manifold vacuum source. No luck .
I am tempted to pull hard on the red line until it snaps and then lay in some new vacuum manifold rubber tube.
I realize that might be a one way street that I prefer not to use.
Has anyone done this tracing?
If so, how?
I can see that the red line should supply a constant vacuum source;just can't verify that it is correctly connected w/o seeing it connected
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Old 11-21-2013, 07:24 PM   #7
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Look at the back of the intake manifold for a nipple and there should be a line that goes to the firewall 4X4 light switch. Once you find that one you have the iinput source. There are also restrictors on the line and it may have a broken line from old plastic or the restrictor is closed up.

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

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Old 11-22-2013, 06:07 AM   #8
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Jokerchief- thank you for your input.

I will again study the firewall 4WD switch to identify its associated vacuum source. Hope it should be a direct connection to manifold vacuum or a tee'd manifold vacuum. Do not see any vacuum reservoir in the loop.

My preliminary investigation 2 days ago revealed that the firewall switch did not receive any vacuum , nor did the yellow line for engage the CAD , nor did the green line for disengage .

I was under the impression ( incorrectly? ) that the firewall switch was a split-off tee from the yellow line.
I thought that the firewall indicator lamp switch was downstream of the transfer case 4WD switch. True/ not true ?

It was my understanding that the firewall lamp indicator (switch) would not receive vacuum until the transfer case 4WD switch selected the vacuum source to be channeled into the yellow line. True/ not true ?

Can you confirm that the firewall switch is upstream of the transfer case switch and that the firewall indicator lamp always gets constant vacuum even tho 4WD is not selected?

That runs counter to my understanding of the pathway.
I have conflicting data.

I understand that I need a constant manifold vacuum source from the carburetor base ; just cannot see the setup / disconnected / broken lines in my YJ.
Hoping to isolate why I am not getting the vacuum source.
Any information is appreciated .
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Old 11-22-2013, 06:26 AM   #9
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Here is my transfer case 4WD vacuum switch .
I understand that the red line should supply constant manifold vacuum to the 4WD vacuum switch.
I cannot trace the red line to its vacuum origin .
Any guidance on tracing upstream back to the manifold carburetor vacuum source? I believe that there is supposed to be a vacuum reservoir in the upstream source. I cannot reach this red line without taking it to a service garage / lift /remove transmission support brace. ( $$$). Where does the red line plug into the reservoir ? Where does the red line plug into a tee'd manifold vacuum source?
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Old 11-22-2013, 12:04 PM   #10
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Jokerchief 462 : I just reread your post and realized that you wrote :" back of manifold" . I did not pick up on that earlier . I will now check the "back of manifold" area , not look at the manifold vacuum ports at the base of the carburetor. This " back of manifold" area might be the connection area that feeds vacuum supply to the red line .
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Old 11-22-2013, 01:19 PM   #11
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Thanks to your info , I traced the red line from the transfer case 4WD switch to the tee'd manifold vacuum supply located on the battery side of the engine; that is where the red line connected to a check valve --> manifold supply--> vacuum reservoir located at the driver windshield corner ( round black plastic sphere).

I verified that I got a vacuum supply / feed to the transfer case 4WD switch.

The vacuum signal may not get correctly routed through the 4WD transfer case switch.
I need to verify the switch functionality.
Time for a new topic .

Thanks to all who voiced their ideas. Every bit helps me understand.

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