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Jeff
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..
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.
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.
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.
Leak in heater core. Replace core.
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.
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?
You probably have a jammed or otherwise defective vent door.
Leaking heater core