Heater stuck on defrost vents - JeepForum.com
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post #1 of 21 Old 01-30-2018, 12:24 PM Thread Starter
BorderWalker
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Heater stuck on defrost vents

As title says. My Hvac seems to be stuck on the defrost vents, even when changing the blower controls. Not sure if this is a one time thing or if it might prove persistent.

Manual controls, no ATC.

Anyone got an idea what could cause such as this to happen? Vacuum line issues? Something stuck somewhere?

I know I don't hear the "Hiss" sound I used to hear when I'd turn my blower off.


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post #2 of 21 Old 01-30-2018, 12:28 PM
1996maroonzjf
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Vacuum leak for sure.
The manual HVAC is controlled by vacuum and defrost is the default position if it loses vacuum.
My ZJ has a slight leak, it will blow out of the defrost in any position under high RPMs with cruise on. Cruise control is other main user of vacuum in ZJs. It operates in all positions under normal driving conditions around town, or with cruise off.

-Tim
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post #3 of 21 Old 01-30-2018, 12:30 PM Thread Starter
BorderWalker
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Originally Posted by 1996maroonzjf View Post
Vacuum leak for sure.
The manual HVAC is controlled by vacuum and defrost is the default position if it loses vacuum.
Well, to be fair the last position I had it set to blow on was defrost + Floor vents. I'll check after work if it's also blowing out the Floor vents or not. Could only check defrost vents.

If it IS a Vac leak, is this self contained to just the HVAC system, or might I expect other systems to start having issues?

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post #4 of 21 Old 01-30-2018, 12:36 PM
wingless
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There are two damper doors that must be in specific positions for defrost-only operation.

One is the floor off / floor shared door shown on the left here.





The other is the dash vent / defroster vent door, with the motor / actuatuator shown on the left here.



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post #5 of 21 Old 01-30-2018, 04:34 PM
JonMopar
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I had this problem last week. Vacuum leak at the "T" section in the vac line at the back of the firewall was the fix it my case. A line had popped out of the T

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post #6 of 21 Old 01-31-2018, 06:52 AM Thread Starter
BorderWalker
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...So I had a lightbulb moment this morning. Problem was still persisting last night and this morning, as well as Cruise refusing to work.

Then I remembered. When I was installing the Relay harness for my headlights on Sunday, There was a hose I unplugged which was attached to the battery tray.

That wouldn't happen to be a Vacuum hose related to the heater and cruise controls, would it...?

Cause if so, I may have had an idiot moment.

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post #7 of 21 Old 01-31-2018, 07:01 AM
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Quote:
Originally Posted by BorderWalker View Post
...So I had a lightbulb moment this morning. Problem was still persisting last night and this morning, as well as Cruise refusing to work.

Then I remembered. When I was installing the Relay harness for my headlights on Sunday, There was a hose I unplugged which was attached to the battery tray.

That wouldn't happen to be a Vacuum hose related to the heater and cruise controls, would it...?

Cause if so, I may have had an idiot moment.
I don't see how a hose to the battery tray would be vacuum, are you sure that wasn't just a drain line in case of battery leak?
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post #8 of 21 Old 01-31-2018, 07:06 AM Thread Starter
BorderWalker
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It had no visible outlet to collect fluid in case of a leak, and it went upwards from the battery tray, not down. Seemed to go back towards the firewall.

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post #9 of 21 Old 01-31-2018, 07:38 AM
GOLDWING
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There is a vacuum reservoir under the battery tray.

GW
1996maroonzjf and dwsschatte like this.

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post #10 of 21 Old 01-31-2018, 08:55 AM
1996maroonzjf
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Quote:
Originally Posted by 1shotleft View Post
I don't see how a hose to the battery tray would be vacuum, are you sure that wasn't just a drain line in case of battery leak?
There is a vacuum reservoir integrated into the battery tray. It holds vacuum while the engine is off.

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post #11 of 21 Old 01-31-2018, 09:07 AM
1shotleft
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Quote:
Originally Posted by 1996maroonzjf View Post
There is a vacuum reservoir integrated into the battery tray. It holds vacuum while the engine is off.
See how much I'm learning.
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post #12 of 21 Old 01-31-2018, 10:17 AM
GOLDWING
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Quote:
Originally Posted by 1996maroonzjf View Post
There is a vacuum reservoir integrated into the battery tray. It holds vacuum while the engine is off.
While this is true, there is a check valve that holds the vacuum, the purpose is to keep vacuum sufficient to operate the cruise control while climbing long grades. At greater loads are put on the engine the vacuum of the engine tends to decrease and the reservoir helps supply it.

GW

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post #13 of 21 Old 02-01-2018, 07:29 AM Thread Starter
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Lesson learned folks. When your putting everything back together, TRIPLE CHECK EVERYTHING.

I felt around blindly for that hose I mentioned. Turns out I had forgotten to plug it back in. Once I wiggled it onto the nipple on the underside of the battery tray, suddenly my vents were working perfectly and Cruise was back in action.

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post #14 of 21 Old 02-01-2018, 08:29 AM
GOLDWING
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Quote:
Originally Posted by BorderWalker View Post
Lesson learned folks. When your putting everything back together, TRIPLE CHECK EVERYTHING.

I felt around blindly for that hose I mentioned. Turns out I had forgotten to plug it back in. Once I wiggled it onto the nipple on the underside of the battery tray, suddenly my vents were working perfectly and Cruise was back in action.
I would agree and add one thing.....

When you've finished a repair on your ZJ and then coincidentally you have a different problem, take a look at the things you just worked on. Chances are you created the second problem while working on the first problem.

GW

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post #15 of 21 Old 02-23-2018, 09:08 PM
digdug18
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does anyone have a layout for where all the vacuum lines go to and from? I'm guessing that I've the same problem, as my vents change on their own.
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