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Crack in A/C Control Unit causing vacuum leak?


Jess
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I made a post about my vacuum leak awhile back and you guys helped me figure out what's actually going on. Now I'm actually trying to fix it lol. So I have an 89 with the 4L/4WD and without going into tons of backstory, I pulled my dash to fix a water leak like 2 years ago. Sometime after that, I noticed that when I have my AC/Heat/Vent/Anything on and I'm climbing up a hill, the AC switches to defrost until I release the gas. Then, it goes back to whichever mode the controls are set to. It will do this occasionally on level ground but it's much more pronounced in 3rd/4th up a hill.

 

I inspected the Vacuum reservoir in the front bumper and it appears fine. I looked over the lines in the engine compartment, can't see anything there either. I thought maybe it was the stupid vacuum disconnect for the axel since it's so loosy-goosy but disconnecting that and plugging the lines made no difference.

 

I removed the stereo and lower dash to inspect everything I can see hoping I won't have to take the rest of the dash off but I'm not seeing anything clearly broken or loose except some cracks on the AC Control unit itself:

crack.jpg.44b0adcad01796839d5936592ae8d783.jpg

 

There is another screw on the right side of the unit that has a similar crack as well and just flexing the unit will spread the crack apart. But I don't know how vacuum is routed through there and if it's possible that this could be the source of a leak. 

 

I have a Chrysler Service Manual for the 1990 Jeeps. I don't think there's much change between 89 and 90's as far as this is concerned? But I went through all the modes on my truck and they matched up with this (except my control unit doesn't have "Max AC"):

manual.jpg.c933823c2d34e71afd45ce2153cf83f6.jpg

 

So that at least gives me some sense that I reconnected the lines correctly when I reinstalled the dash. It's entirely possible that there are cracks on the lines I missed - They are all original with 267k miles on them so I'm sure they could all be replaced. I'm trying to follow lines back and inspect logically but as you guys know, they're not all super easy to get to. 

 

Jess

 

 

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No “Max AC” means you should have a none AC Jeep. Or none ac control unit.

The heater/ac controls are 1986-96 interchange. Only the 1984-85 have a different vacuum switch.

I’ve had older cars rot out the vacuum harness for the HVAC, and was lucky enough to find a factory replacement, it’s color coded so I guess if you had to it’d be easy to make your own.

Our 91 does the same kind of thing with vents switching to defrost, the default safety measure, on a big hill going into the city. The rest of the time it’s fine, and I haven’t tried to recreate the problem since I replaced the intake/exhaust gasket.
I’ve checked all the vacuum lines under the hood and replaced the vacuum reservoir and it still had the problem. Far as I could see the under dash vacuum harness was ok.

But like I said ours only does this occasionally so I’m not sure where the problem is either. I’ve never removed the AC control unit to check the vacuum switch.
BTW I can’t tell from the picture where the cracks are.




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BTW I can’t tell from the picture where the cracks are.

Yeah - Sorry. I thought about highlighting it. It's running diagonal right above that hex-cap screw.

 

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How do you know the vacuum ball is holding vacuum?

I don't really. I should just order a new one since it's cheap. Side note, I've done several of the tips on your website. Thanks for putting that together, it's awesome.  

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