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1990 Vette only defrost

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Old Aug 15, 2010 | 11:53 AM
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Default 1990 Vette only defrost
Hey guys this 90 Vette only blows out of defrost. The C68 control does not have any codes in it. I'm thinking a/c programmer, anyone think it could be anything else?
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Old Aug 15, 2010 | 01:38 PM
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It could potentially be the vacuum lines themselves, or no vacuum source to the programmer.
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Old Aug 15, 2010 | 03:07 PM
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As Aurora 40 said check the supply line from the intake manifold, there is a check valve that is notorious for failure, when vacuum is lost defrost is the default mode for directing airflow.
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Old Aug 15, 2010 | 03:44 PM
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Originally Posted by Y-ME
Hey guys this 90 Vette only blows out of defrost. The C68 control does not have any codes in it. I'm thinking a/c programmer, anyone think it could be anything else?
Blend door
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Old Aug 15, 2010 | 04:27 PM
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How do you check the c68 for codes??
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Old Aug 15, 2010 | 05:20 PM
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Originally Posted by red 90
How do you check the c68 for codes??
http://www.corvetteactioncenter.com/...agnostic+Codes
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Old Aug 15, 2010 | 08:55 PM
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Originally Posted by red 90
How do you check the c68 for codes??
Press both fan speed buttons at the same time til 00 pops up on the control display then hit the auto fan button to cycle through any codes.
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Old Aug 15, 2010 | 09:43 PM
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Originally Posted by Aurora40
It could potentially be the vacuum lines themselves, or no vacuum source to the programmer.
This is right where I'd start. Especially with no codes. I think defrost is the default (i.e. with no vacuum, defrost will flow), and the opening/closing of the vents is controlled by diverting vacuum on/off to different vents.
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Old Aug 16, 2010 | 09:00 AM
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I would check for vacuum, with the engine running pull the vac line off of the vac ball located in the front left corner of front bumper, put your finger over the end of the line and feel for suction. if there is none trace the line back for cracks or breaks in the line. good luck..
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Old Aug 16, 2010 | 12:31 PM
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Originally Posted by toptechx6
As Aurora 40 said check the supply line from the intake manifold, there is a check valve that is notorious for failure, when vacuum is lost defrost is the default mode for directing airflow.

You know I feel like a dumbass cause I know about the vacuum lines behind the engine are 95% chance whats wrong. Slipped my mind, found a broken line, I'm sure that was it, can't wait to reconnect battery after replacing the impact sensor to confirm.
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Old Aug 16, 2010 | 01:44 PM
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Originally Posted by Y-ME
You know I feel like a dumbass cause I know about the vacuum lines behind the engine are 95% chance whats wrong. Slipped my mind, found a broken line, I'm sure that was it, can't wait to reconnect battery after replacing the impact sensor to confirm.
Short Term Memory Issue ? Good Luck.
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Old Aug 16, 2010 | 06:08 PM
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Yup she switches vents now.
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Old Aug 17, 2010 | 08:31 AM
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Originally Posted by Y-ME
Yup she switches vents now.
Glad to hear all is resolved
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Old Aug 17, 2010 | 09:24 AM
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I was having the same troubles, except mine was defrost and feet. Went out and found a crack in the line. Fixed.. Thanks for the thread.
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