A/C randomly goes from vents to defrost - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 07-21-2010, 12:46 PM   #1
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Default A/C randomly goes from vents to defrost

I have a 91 Bonneville SSE and while I was driving with the A/C on it automatically switch from blowing out the vents to blowing out the defrost. I hit a bump in the road and it switched back to vents. It switches at random times and with it blowing out the defrost it doesn't cool as well. If I hit the dash hard with my hand it will switch back to vents. Does anybody know what the fix is for this problem? (My hand is getting sore.)
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Old 08-04-2010, 01:22 AM   #2
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mine does the same, if anyone has a answer to this i would greatly appreciate it aswell.
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Old 08-08-2010, 02:59 PM   #3
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First check the vac lines under the hood, they should activate a damper-door that switches exhaust flow location. On my '88 the hose had just popped off (near the firewall between middle and driver'* side if I remember correctly). Might be a one-way valve there too which could be stuck, which couples a couple different lines into one.
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Old 08-11-2010, 03:37 PM   #4
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I checked the vac lines under the hood and everything is connected. I sprayed some WD-40 in the one-way valve. I hope that takes care of the problem.
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Old 03-02-2011, 01:40 PM   #5
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replace vacuum lines. check vaccum lines behind the ac cluster if u want.
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