Defrost won't turn off. ONLY Defrost works. - JeepForum.com
 
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post #1 of 12 Old 09-24-2017, 08:39 PM Thread Starter
BrentM94
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1993 ZJ 
 
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Location: Lake Isabella
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Defrost won't turn off. ONLY Defrost works.

So this is unusual.

My defrost is stuck on. Heater won't work. Just defrost.

Which is REALLY NOT GOOD!!! My windshield is broken due to CalTrans doing road work. It happens and I didn't seal the crack soon enough. My bad. This happened about 2 years ago.

Well I never want to run defrost due to the crack running across the driver side.

So this might help, I know my headlight switch is broken. I know when I put my new alternator on, upon first start up it sounded like something short circuiting. Which only happened twice, tightened my output cable down and it's been good since.

This must of happened shortly after, I replaced the alternator about four days ago.

So I have no idea and I'm hoping someone can enlighten me on what this could be. I'd say maybe the blower motor went out but that can't be the case because it's still blowing air on the windshield.

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post #2 of 12 Old 09-24-2017, 08:46 PM
kg6mov
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Do you have the manual hvac controls and a vacuum leak?


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1984 F250, 460 BB, T19, work truck/future tow pig
1987 Mercedes 300D Turbo, the diesel daily, 30mpg in style

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Originally Posted by Texas ZJ1 View Post
I heard someone yell "mall crawler" from the back porch. I'll go see who said that. Be right back.
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post #3 of 12 Old 09-24-2017, 08:56 PM Thread Starter
BrentM94
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Quote:
Originally Posted by kg6mov View Post
Do you have the manual hvac controls and a vacuum leak?
No I don't have a manual for hvac controls. The heater hoses look good. Vacuum leak, not sure where to look that would pertain to the heater.

Oh and I just checked it again it does turn off. Only off and defrost. All other settings are not working.

Fan speed works, temperature adjustment works, A/C button lights up and works.
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post #4 of 12 Old 09-24-2017, 09:37 PM
speedbrake
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I believe what kg6mov was getting at was 'do you have manual heating/cooling controls as opposed to Automatic Temperature Control (ATC)?'. The reason is, with the manual temp controls, your vacuum system is plumbed into the back of your flow selector (floor vents, bi-level, dash vents,defrost, etc). When your turn that flow selector, it routes vacuum to different actuators in your HVAC box to open/close the correct doors to give you the flow where it's desired. The default mode when there's NO vacuum, or not enough, is to send all your air out thru the defroster vents. Which pretty much is how you described the original problem. Check for a vacuum leak. It's likely your problem..
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post #5 of 12 Old 09-24-2017, 10:30 PM
nicyousuckk
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x2 vacuum leak. If you do indeed have manual controls for the HVAC system, check the vacuum lines in the engine compartment, passenger side. Theres a Tee fitting right near the firewall that sometimes cracks, leaving you with all HVAC air being routed at the windscreen.
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post #6 of 12 Old 09-25-2017, 01:06 AM
Uniblurb
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^ Agree, and below is a photo of my 96 4.0 where the small black vacuum line tees-off and goes through the firewall. Without this vacuum going to the manual system controls it will default to defrost every time. Don't lose the small vacuum line down inside the firewall if it is broke which makes it a major problem to run a new line.

Here's the photo and I installed a small piece of split rubber line on the smaller line so the metal won't cut the plastic line.
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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post #7 of 12 Old 09-25-2017, 01:17 PM
Damien_
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Sorry wrong thread..
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post #8 of 12 Old 09-26-2017, 08:23 PM Thread Starter
BrentM94
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Quote:
Originally Posted by Uniblurb View Post
^ Agree, and below is a photo of my 96 4.0 where the small black vacuum line tees-off and goes through the firewall. Without this vacuum going to the manual system controls it will default to defrost every time. Don't lose the small vacuum line down inside the firewall if it is broke which makes it a major problem to run a new line.

Here's the photo and I installed a small piece of split rubber line on the smaller line so the metal won't cut the plastic line.
Okay it's likely that it broke. Sounds like that's the problem. I'm going to take a look tomorrow. I'll let you know what I find. Thank you all for the help.
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post #9 of 12 Old 09-26-2017, 08:37 PM Thread Starter
BrentM94
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I checked the T. it's good then I followed the line all the way to the battery. Of course underneath the battery. I'm not sure what it's called but I call it the battery holster, haha. I had to remove that when I replaced the alternator. The vacuum line hooks up underneath there.

I forgot to hook it back up when I bolted it down.

So now my question is. Does it go to anything important under there. It works if I plug it up. So what exactly does it do underneath there? Right under the battery.
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post #10 of 12 Old 09-26-2017, 08:45 PM
kg6mov
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Under the battery is a vacuum reservoir. It's just a sealed can, but it helps with cruise and hvac when the manifold isn't producing enough vacuum.


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1984 F250, 460 BB, T19, work truck/future tow pig
1987 Mercedes 300D Turbo, the diesel daily, 30mpg in style

Quote:
Originally Posted by Texas ZJ1 View Post
I heard someone yell "mall crawler" from the back porch. I'll go see who said that. Be right back.
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post #11 of 12 Old 09-26-2017, 08:49 PM Thread Starter
BrentM94
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Quote:
Originally Posted by kg6mov View Post
Under the battery is a vacuum reservoir. It's just a sealed can, but it helps with cruise and hvac when the manifold isn't producing enough vacuum.
Right on, thank you.
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post #12 of 12 Old 09-27-2017, 04:04 AM
Uniblurb
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At idle there's high vacuum in the intake so that line plugged would work fine. But you'd find out during high acceleration or climbing hills that stored vacuum in the reservoir is needed or it's going to default to defrost again. Glad you found it!

96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 96 4.0 XJ (son's)


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