1990 Jeep Wrangler - 4WD Vacuum Line Question

Hi all,

New to the forum so hello! Wanted to see if I could get some assistance with what I am looking at and if there is any need for concern.

1990 Jeep Wrangler - 2.5 L engine w/ ~126K miles

There is a set of vacuum lines that are wrapped in a black plastic protector (similar to the electric wires) - they are running to what I believe is a solenoid on the front differential, I think it is a something that engages the front axle when you engage 4wd, kind of don't know the specifics. Anyhow in the picture below, there are color coded vacuum lines (for reference, this is passenger side engine bay by the oil dipstick, right by the large vacuum canister on passenger side firewall). I noticed the blue and red vacuum lines that I have circled in yellow go nowhere, the blue looks like it was cut and just sits there, while the red looks like it should have connected to something in the past. Does anyone know or have a picture of where that connects or know anything about the blue line? The 4wd has worked fine for me, just found the vacuum lines not going anywhere odd...

Thanks for the help!

lma1989 said:

Hi all,

New to the forum so hello! Wanted to see if I could get some assistance with what I am looking at and if there is any need for concern.

1990 Jeep Wrangler - 2.5 L engine w/ ~126K miles

There is a set of vacuum lines that are wrapped in a black plastic protector (similar to the electric wires) - they are running to what I believe is a solenoid on the front differential, I think it is a something that engages the front axle when you engage 4wd, kind of don't know the specifics. Anyhow in the picture below, there are color coded vacuum lines (for reference, this is passenger side engine bay by the oil dipstick, right by the large vacuum canister on passenger side firewall). I noticed the blue and red vacuum lines that I have circled in yellow go nowhere, the blue looks like it was cut and just sits there, while the red looks like it should have connected to something in the past. Does anyone know or have a picture of where that connects or know anything about the blue line? The 4wd has worked fine for me, just found the vacuum lines not going anywhere odd...

Thanks for the help!

View attachment 121650

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Flyer58 said:

Yes those vacuum lines did go to the front axle disconnect vacuum motor in it's original form. Many owners have switched over to a solid axle, cable controller, or just locked that vacuum motor in the locked position. Or possibly it's all still there but the vacuum lines have become disconnected.

At the back end they will go to a vacuum switch on the transfer case and the vacuum source is from the intake manifold.

To see what you actually have you'll probably need to remove the vacuum motor on the front axel and see what's there.

The electrical connector goes to the 4WD light on the dash

View attachment 121654

View attachment 121655

View attachment 121656

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I cannot tell you how much I just learned about this, the pictures are so helpful, never had it explained to me like this. It makes so much sense with a visual. According to the diagram you have above, that actually matches with what I am seeing. The red vacuum line not going anywhere with the connection appears to be a check valve from the diagram. And the blue one I am seeing not attached to anything appears to be a vent, that maybe is just missing the filter on the end of it. I'll have to check but to my knowledge it hasn't been switched to a cable controller. I had an '88 Cherokee we were on an offroad trail and the 4wd wasn't working, no power to the front axle and it ended up being either a busted vacuum line or that vacuum motor, cannot recall at the time. It brought to light for me the complexity and potential failure of the vacuum related nature of engaging 4wd on these lol. Thank you so much for the help!