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Old 06-17-2013, 09:46 AM
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Single Zone HVAC Problems

I have been fighting with this annoying behavior with my 2004 Columbia Edition (same controls at a Laredo) with the 4.7 HO for quite some time. The air flow will shift from the setting selected on the control knob back to the default "defrost" setting. This happens any time the engine goes under load. I know this is vacuum-related and have taken the following steps to fix the problem:
- Checked and replaced the check valve between the throttle body and the HVAC controls and cruise control/vacuum canister (located behind the bumper)
- Replaced bad line from inner fender/frame rail to cruise control
- Dealer replaced hard line that runs from throttle body to the HVAC control and inner fender/frame.

I have also noticed that when the vehicle goes under load I can hear a vacuum noise in the the passenger compartment. This leads me to believe it could be a vacuum fitting under the dash. I am looking for a vacuum diagram but have not been able to find one for under the hood and in the dash.

Can anyone help with this? Any suggestions?

Thanks!
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Old 06-17-2013, 12:39 PM
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Re: Single Zone HVAC Problems

There seems to be a check valve also inside the cabin on the lower side of the HVAC housing, did you check that one as well?
But because you can hear some vacuum leak, you should remove the AC panel and check from there is there is any hose damaged. Maybe one of the actuators is leaking? But only by opening it up you will discover what the problem is.
The only vacuum diagram i could find is hereby attached.
Attached Files
File Type: pdf Vacuum control.pdf (66.3 KB, 3 views)
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Old 06-18-2013, 02:48 PM
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Re: Single Zone HVAC Problems

Thanks! I will look for the check valve under the dash and the control unit in the dash. I hope to get to that later in the week.
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Old 06-26-2013, 08:47 PM
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I have the same set of circumstances, and will be curious how things turn out.

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Old 06-26-2013, 09:55 PM
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Re: Single Zone HVAC Problems

Still go this on the list. Life has been getting in the way and will be that way for the next week. I will post up my finding.
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