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Discussion Starter #1
99 deville..with ac on auto the airflow switches back n forth from ac vents to defrost..would that be a..programmer issue
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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(I think the partial vacuum setup was still used in the 1999s......)

Check the vacuum line from the passenger end of the intake manifold to the firewall and into the cabin/HVAC stack.

Does your parking brake auto release work? If not, it's a vacuum leak somewhere.
 

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Discussion Starter #3
(I think the partial vacuum setup was still used in the 1999s......)

Check the vacuum line from the passenger end of the intake manifold to the firewall and into the cabin/HVAC stack.

Does your parking brake auto release work? If not, it's a vacuum leak somewhere.
park release works...also vac source is good..can find no leaks...while driving it will just switch to defrost..and later go back to ac vents..so i am thinkin what tells it to switch..back n forth..with out commands from ac control head?
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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you - the MODE is vacuum controlled -
and when you loose vacuum DEFROST is the default position -

long hard acceleration dumps vacuum -
coasting/deceleration develops the highest vacuum -
 

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Discussion Starter #7
you - the MODE is vacuum controlled -
and when you loose vacuum DEFROST is the default position -

long hard acceleration dumps vacuum -
coasting/deceleration develops the highest vacuum -
yes i get that..just sitting in park it will do it...gonna check the vac res..under the fender..so far i found no vac prob...till tomoro...thanx much
 
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