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There is a vacuum switch that controls an air gate that diverts the air flow to the proper outlet. Blowing air out the defroster is the default setting of the switch. Most common causes of this problem are vacuum leaks. Check the vacuum hose on the passenger side engine compartment first. It ususally leaks going through the fire wall. Regardless this problem is most often always the result of a leak somewhere in the vacuum system not the failure of the switch itself. If it is a diesel check for proper operation of the vacuum pump

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โˆ™ 2007-12-20 07:01:39
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Q: Why does AC and heater blow air through defrost vents and will not blow air into cabin vents on 2000 Ford E350 van?
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That's probably steam from a leaky heater core.


Why does heater defrost vent not open on a 1989 Sunbird and cold air blow from dash vents but no air blows from heater defrost vents and where are the enter and exit hoses for the heater coil?

I don't know why the vent would not open for the defrost, but it may be related to the fact that your heater core is blown. If your getting nothing but cold air out of the vents when the heat is on, your core is finished and if the core is finished then it is likely related to your vent not opening/switching. Something''s wrong with your heating either way.


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Most likely a bad or disconnected vacuum hose to the defrost/floor selector in the heater ducting.


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Over time debris and other things can collect near the heater core, its possible that something small and plastic has fallen down through the defrost vents and gets hot when the vehicle is running. You may need to disassemble the heater and remove the items and perhaps vacuum it out.


Why is there no air coming out of vents can hear air blowing under dash board?

The year, make and model would help but many vehicles use vacuum to operate the direction of air flow through the floor vents, dash vents and defrost vents. If the vacuum supply is cut off for whatever reason the system defaults to defrost only. So if you only have the defrost mode working chances are the vacuum supply is cut off. Check the HVAC vacuum supply line under the hood for damage. If that system seems to be working but you have insufficient air flow through all of the vents, check/replace the cabin filter if applicable.


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Does the heater vents on a 2004 2500 dodge diesel operate on vacuum or electric My truck only blows out of the vents on the dash occationaly it will switch to defrost for me?

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I had a Chevy van of this vintage and I found a vacum hose leak in the controls that would make the heater/defrost door default too defrost open. lisining carefully under dash or under hood may pinpoint leak. I had a Chevy van of this vintage and I found a vacum hose leak in the controls that would make the heater/defrost door default too defrost open. lisining carefully under dash or under hood may pinpoint leak.


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I have a 1986 GMC half ton and my defrost doesn't work very good it pushes most of the air through the dash front vents and not defrost but it still blows good hot air?

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Why won't air come through the vents?

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