AC blowing out defogger/defroster vents
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Cadillac DeVille 1985 to 2005 including:
1985-1992 Fleetwood, 1993 Sixty Special, All FWD Forum Discussion, AC blowing out defogger/defroster vents in Past Cadillac Vehicle Discussion; This is my first post here. My mechanic installed a used AC compressor in my 95 DeVille (4.9L V8) yesterday, ...
  1. #1
    EdwardP is offline Cadillac Owners Member
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    AC blowing out defogger/defroster vents

    This is my first post here.

    My mechanic installed a used AC compressor in my 95 DeVille (4.9L V8) yesterday, a new compressor was too cost-prohibitive. Once it was installed and charged, the cold air was blowing out the defogger/defroster vents atop the dash and not the front vents.

    A quick Google search on this problem showed that in 90% of the search results, the problem was a vacuum leak.

    Could someone offer guidance as to where to begin looking for this? I am bringing the car back Friday morning for this, but last night, I discovered something that looked like a vacuum hose had come apart from somewhere way down in the rear of the engine but could not see exactly where.

    Due to the location of the compressor, they had to lift the engine up in order to remove/replace and perhaps this hose became disconnected during that time, as the air (cool or warm) had blown out correctly prior to this.

    Thanks for any advice. I appreciate it.

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    noahsdad's Avatar
    noahsdad is offline Cadillac Owners Connoisseur
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    Re: AC blowing out defogger/defroster vents

    Not too familiar with that body (so I may be talking out of my butt here), but most of the time the vacuum leak is inside the cabin. The airflow doors are all controlled by the Climate Control System sending signals to another module called a programmer, which directs vacuum to the proper flapper doors inside the HVAC ducts. Start the engine and listen for any hissing under the dash, starting with the parking brake pedal area.

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    Re: AC blowing out defogger/defroster vents

    You are on the right track. I would agree with Noahsdad, but since they worked on the car and the engine was "lifted" I'd suspect a line was pulled loose somewhere under the hood. Fist place I'd look is the cabin supply line. It should be plugged in to the firewall on the passengers side.

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    EdwardP is offline Cadillac Owners Member
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    Re: AC blowing out defogger/defroster vents

    I stared the car and heard what sounded like a mild hissing under the drivers' side dash, it was closer to the emergency/parking brake pedal. No hissing heard on the passenger side though.

    Evidently, this had been like that all this time. Earlier in the season though, the heat would correctly come out of the bottom vents, then when the temperature was gradually reduced, the same air would then come out of the front vents.

    But now, the air conditioned air only comes out of the top (defogger/defroster vents) and nothing out of the front or bottom - even when the temperature is increased, the warmer air still comes out of the top.

    In any event, I will let the mechanic know of this apparent leak heard under the dash near the emergency/parking brake, as well as what I discovered under the hood last night.

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    Re: AC blowing out defogger/defroster vents

    Quote Originally Posted by EdwardP View Post
    I stared the car and heard what sounded like a mild hissing under the drivers' side dash, it was closer to the emergency/parking brake pedal.
    You found your problem. Parking brake release diaphragm is leaking. Until you get it fixed, pull the vacuum hose and plug it. You won't have the auto brake release, but you probably don't now anyways, but you WILL have A/C air flow out the dash vents.

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    EdwardP is offline Cadillac Owners Member
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    Re: AC blowing out defogger/defroster vents

    Is this diaphragm something that can be repaired?

    I searched for the part (Google) and nothing came up specifically for such an item.

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    Re: AC blowing out defogger/defroster vents

    It can be replaced, not repaired. Take a peek under the dash at the parking brake release mechanism. You can identify it by the vacuum hose leading to it. Be advised, you need to be a bit of a contortionist.

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    Re: AC blowing out defogger/defroster vents

    First post as well. I have the same problem with mine. It is very aggravating. Cold air blows out the defrost and to the rear seat. I can get cool air to come out the front vents if I set the temperature to around 77 degrees, but this is usually uncomfortable down here in Florida. I have no vacuum leak though. And I have been through the trouble codes, for which there are none stored. When I go into manually manipulating the inputs in the diagnostics, I can get air to blow out the front vents as well. But only while in diagnostics.

    I also have a problem in the winter where hot air blows out the passenger defrost and passenger floor, but the drivers side defrost and floor is barely warm. The only fail I get is for the electric shocks in the front. And that is an entirely different can of worms. Thanks.

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    Re: AC blowing out defogger/defroster vents

    When I go into manually manipulating the inputs in the diagnostics, I can get air to blow out the front vents as well. But only while in diagnostics.
    What inputs are you manipulating? That will probably tell you where your problem is.

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    Re: AC blowing out defogger/defroster vents

    By the way this is on a 94 Deville, I forget exactly what they are called, without looking in the service manual again. But, there was a range between 0-100 of where the air would blow out and you could adjust the number value to test the various methods of delivery. I think the vent delivery was in the correct aspect ratio for the number I had selected.

    My parking brake works as advertised as well. I actually dont know what I should expect when I turn the A/C on to Auto. I know the compressor clutch should engage, but is it only supposed to blow out the front vents, unless you have another mode selected?

    This is one of those things that I hate working on. I hate repairs that require removing trim pieces, as I usually end of breaking the plastic, i.e. insides of doors. But I refuse to trust another mechanic to tell me he found the problem.

    If there was a problem with the vacuum wouldn't it affect the front vents all the time. Like I said before if I am within 5 degrees of outisde air temps, and I can get it to work, but its just not a pleasant cool.

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    Re: AC blowing out defogger/defroster vents

    The pre 2000 Devilles did not have a mode control (bi-level). A/C comes from the dash vents and heat comes from the floor.

  13. #12
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    Re: AC blowing out defogger/defroster vents

    Quote Originally Posted by MRneatfreak View Post
    I actually dont know what I should expect when I turn the A/C on to Auto. I know the compressor clutch should engage, but is it only supposed to blow out the front vents, unless you have another mode selected?
    Here's the short version of the thought process behind Cadillac's HVAC design in the 80s and 90s. You adjust the desired temperature for the cabin like the thermostat in your house. You turn on AUTO. That's it. AUTO engages the A/C compressor whether you desire cooling or heating. The A/C system removes humidity from the air, then a mix door either directs the chilled air directly to the cabin, or sends it over the heater core to produce warm, dry air.

    The Climate Control Panel sends signals to a device called a programmer, which then directs vacuum to flaps inside the cabin ducts, and directs air based on your choices. A/C always emerges from the dash vents, heat from the floor, defrost toward the glass. This is why misdirected airflow issues almost always originate with a vacuum leak in the cabin.

    ECON is a less sophisticated mode that does not engage the compressor. Theoretically you will get better fuel mileage using ECON, but less comfort. It merely draws outside air into the cabin through the cowling. It does not cool it at all, but will produce heat if your CCP temp setting is higher than the outdoor temperature. On damp winter days, it's better to use AUTO because the dry air will clear foggy windows much faster.

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    Re: AC blowing out defogger/defroster vents

    ECON is a less sophisticated mode that does not engage the compressor. Theoretically you will get better fuel mileage using ECON, but less comfort. It merely draws outside air into the cabin through the cowling. It does not cool it at all, but will produce heat if your CCP temp setting is higher than the outdoor temperature.
    It DOES cool IF the outside air is cooler than the cabin air. It is basically AUTO with an A/C bypass (no A/C).

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    Re: AC blowing out defogger/defroster vents

    True, in the same way the old VENT controls would let cooler air in, but only us old guys remember those. I just meant it doesn't actively chill the air.

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    Re: AC blowing out defogger/defroster vents

    but only us old guys remember those.

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