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1996 Deville 4.6
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51 Posts
Discussion Starter #1
I was having the same problem many of you had, the ac would blow out the floor and defrost but not the dash vents. I checked vacuum lines. There is one vacuum line coming out of intake manifold, to some type of vacuum switch? Then it goes to another vacuum switch and then to the line going through the firewall to the inside. I disconnected the line from the 2nd switch to the firewall, hooked up a direct line from the manifold, and the dash vents worked great.

Question: The 1st vacuum switch has an electrical line going to it and the 2nd one does too. What are the purpose of these electrical lines? I suppose somehow it switches from floor to dash or something. Hoiw does the car know to switch from lower vents to dash vents?

I put a T between the vacuum line from the manifold to the 1st switch, directly to the line going into the firewall to get the dsh vents to work but would like to figure out what exactly is wrong. Any help appreciated.

Rich
 

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White Diamond '03 DHS (with DTS floor shift)
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86,975 Posts
The line going to the "switch" is most likely the EVAP line. It is going to the control valve and solenoid. The line going to the firewall is what provides vacuum to the HAVC & parking brake release, but I don't recall it going through any switch before entering the firewall. Has anyone messed with the vacuum lines? The switching from floor to dash is done by the programmer aka ACM (Air Conditioning Control Module) which is down line (under the dash) from the firewall. On the pre 2000 Devilles heat goes to the floor and A/C goes to the vents. Since putting direct vacuum to the firewall line solves the problem, it obviously lies in that "switch", though I can't figure out what it is or why it is there.
 

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1998 Cadillac Deville Sedan
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897 Posts
Well the wierd thing is I accidentally killed my battery the yesterday(left a hardwired stereo equipment on), the battery was at 3.2v so it was REALLY dead(probably needs a new battery). After a jump the car came back on and of course I lost all the memory settings but the air was blowing out of the dash vents again :confused:

Maybe try unhooking your battery for a few minutes?
 

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1996 Deville LaSalle edition
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17 Posts
You know, there is another post in the forum about the same problem, and he says that if he shuts his car down and then restarts it it clears the problem...for a while, and then it comes back. It seems like at its core this isn't a vacuum problem, but an electrical one, since the HVAC system in these cars isn't simply electric or vacuum, but a combination of both.

My AC doesn't work at all right now, so I don't have any usefull advice for you...but I would maybe try and replace the vacuum switches by the manifold before the firewall and see what happens. That is assuming the problem comes back...
 
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