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What would make your AC and heater only blow out of the defrost vents if you have it on normal AC or vent or mix?

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2011-10-18 21:07:11
2011-10-18 21:07:11

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Jeff

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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.

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.

Check the following.......... 1. Vacuum leak, broken vacuum line or disconnected vacuum line. 2. A/C - heater inside car door operation.. 3. A/C - heater controller..

Any of the following can cause this......... 1. Bad a/c - heater controller.. 2. Bad a/c - heater controller inside door operation.. 3. Bad, broken or cracked vacuum line/lines..

Leak in heater core. Replace core.

depending on what year and make it is, the most likely issue is that you have a valve that moves to divert air to the upper vents, floor vents, and defrost vents and it is either stuck in the defrost position, is broken, or a cable/connector has come loose. The valve is usually in the center of your dashboard up against the firewall. The air flow direction in many vehicles is operated by vacuum controlled servos. If the vacuum supply is cut off, the system reverts to it's default position which is the defrost mode. My guess would be that the HVAC vacuum supply line under the hood is disconnected, pinched or collapsed.

There are heater doors on the heat blower box that direct the path of the heat. Apparently the heater door to the floor vents is stuck closed.

If we are talking rain water not engine coolant I would suspect the drains under the cowl are plugged up with debris (leaves and such). If it is engine coolant then I would suspect the heater core is leaking.

I would think that is a vacuum or a linkage problem with the heater control assembly (where all the buttons/slides are for changing between floor/defrost/vent) Had a diaphragm go bad on one once, and in a jeep cherokee, a slight collision with a guard rail knocked a vac line off for the vent controls, and ac only worked from defroster.

ihave a 1986 chevette that when i turn defrost on it blows out heat and when i put heater on it blows out only cold air? ihave a 1986 chevette that when i turn defrost on it blows out heat and when i put heater on it blows out only cold air?

Blown Heater Core - The heater fan blows past the heater core and if it has a hole in it your antifreeze ios being blown onto your windshield.

Warm(er) air rises - if vents were high up there would be little circulation without fans.

check your vacum line to door by the gas pedal for vacum

You probably have a jammed or otherwise defective vent door.

The compressor is coming on because you have the heater set to defrost. Take it off defrost and the compressor will not come on. I would suspect the compressor is locked up.

If that smoke is more like steam, you have a leaky heater core.

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.

This is a very common problem and you need to check for a vacuum leak, broken vacuum line or a loose vacuum line......

The motor which blows air through the vents.

Check the heater core, it is probably bad. It could be heater hose connections to the heater core, but I would bet on the heater core going bad.

Because you have a vacuum leak in the system or your vacuum canister is leaking. When you accelerate the vacuum from the engine will drop. That will cause your system to loose vacuum and the floor/defrost door will go to its neutral position. To keep the vacuum up you have a canister that will hold vacuum. It must be air tight.


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