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Thread: 96 DeVille AC vent to defrost oscillation

  1. #1
    Paul R is offline Cadillac Owners Member
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    Smile 96 DeVille AC vent to defrost oscillation

    I have a 96 DeVille, and when operating the AC (auto mode), cold air will flow from dash vents and then it changes to the defrost vents. This back and forth oscillation is continuous, back and forth about a minute from dash vents, a minute from defrost vents. Why does this occur,is it normal? If not, likely culprit? Thank you, first time post.


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  3. #2
    rodnok01's Avatar
    rodnok01 is online now Cadillac Owners Master
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    Wrong forum' a mod will move hopefully. No that is not normal. Have you check codes? Those actuators are vacuum controlled, could be a bad vac line causing issues.

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    jayoldschool's Avatar
    jayoldschool is offline GM RWD V8 addict
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    Re: 96 DeVille AC vent to defrost oscillation

    Off to FWD. Does anyone ever make this mistake the other way? Post in FWD, when they have a RWD?

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    Submariner409's Avatar
    Submariner409 is offline If it won't run, chrome it
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    Re: 96 DeVille AC vent to defrost oscillation

    We move them around all the time - as well as threads on Sevilles and Devilles being posted in "STS" and "DTS" and vice versa.

  6. #5
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: 96 DeVille AC vent to defrost oscillation

    No, it's not normal. Your system is vacuum operated and defrost is the default mode. You have a vacuum leak. Does it happen when you get in the throttle and then return to dash when you let off?

  7. #6
    Paul R is offline Cadillac Owners Member
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    Re: 96 DeVille AC vent to defrost oscillation

    I apologize for posting incorrectly. 96 DeVille FWD.
    Quote Originally Posted by jayoldschool View Post
    Off to FWD. Does anyone ever make this mistake the other way? Post in FWD, when they have a RWD?

  8. #7
    Paul R is offline Cadillac Owners Member
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    Re: 96 DeVille AC vent to defrost oscillation

    Quote Originally Posted by Ranger View Post
    No, it's not normal. Your system is vacuum operated and defrost is the default mode. You have a vacuum leak. Does it happen when you get in the throttle and then return to dash when you let off?
    I'm going to answer "yes" to this. At idle, it stays in the dash vent position, and then starts the back and forth between dash vents and defrost vents when driving. Does anyone have a diagram of the vacuum system related to the AC? Thank you again.

  9. #8
    rodnok01's Avatar
    rodnok01 is online now Cadillac Owners Master
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    It enters behind pass strut area, my vac hoses were toast in that area, then under dash there is a gaggle of them coming into junction blocks and sometimes the connections wiggle loose.

  10. #9
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: 96 DeVille AC vent to defrost oscillation

    My guess would be that you have a vacuum leak at or near the vacuum reservoir. That reservoir holds vacuum so that it can be used when you open the throttle and the vacuum drops, preventing this exact situation. Locate the cabin supply line at the firewall (#7) like rodnok said and follow it back to the reservoir (#10). Also check the check valve (#14) I'll bet you find a leak somewhere.

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