defrost control problem - AC Auto Repair Help Forum


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bonicellijhb
New User

Dec 4, 2009, 1:56 PM

Post #1 of 3 (1703 views)

defrost control problem	Sign In

I have a '94 gmc sonoma. The defroster/air conditioning will only run in the defrost position. When moving the selector from defrost around to all of the other settings defroster stays blowing. I do not hear any vacumm while moving selector, however a/c compressor does turn off and on when in a/c and max a/c. I have checked all of the components of the control panel and they seem ok. I do not have any plugged vacumm lines at the selector for them. I looked to see where vacumm lines run to and they disappear into the dash. Dash would have to be removed to look further. I can not even see the door in the vent system to check it. I am a mechanic by trade. I have worked on jets, helicopters, cars, and currently industrial manufacturing equipment, I have been a mechanic full time for 26 years and am at a loss how to proceed. I would be eternally grateful for any advice with what to do next.

Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Dec 4, 2009, 4:30 PM

Post #2 of 3 (1688 views)

Re: defrost control problem	Sign In

The mode selection is done through a series of air doors that are operated by vacuum. When there is either a partial or total loss of vacuum, the system will default to either defrost or on some vehicles, heat. Acceleration can also have an effect on the doors for the same reason.

Look under the hood for a small black plastic hose coming through the firewall. Follow it to the vacuum source looking for any breaks, cracks or leaks. Also follow any other lines that branch off because they may go to a vacuum reservoir (storage tank) and a leak there will cause the same problem. Using a vacuum gauge to determine if you have full vacuum at any given point is the best way to find it. You can cut into the line anywhere and splice it back together with a piece of vacuum hose. Find the broken line, you found your problem.

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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.

bonicellijhb
New User

Dec 4, 2009, 5:54 PM

Post #3 of 3 (1681 views)

Re: defrost control problem	Sign In

Thank you very much for the help.

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