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Defrost thru Panel/Floor Selector Switch
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coopsterj
New User
May 1, 2009, 1:04 AM
Post #1 of 5
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Defrost thru Panel/Floor Selector Switch
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1997 Chevy Astro Van (Cargo), V6, 98,000 miles. Heat and/or AC only comes out of the defrost and floor vents, no matter where the switch is set. (ie., panel, floor only, floor/panel, etc.) Both Heat and A/C work, just where it's not needed; all the time! If it is pertinent, the backlight behind the fan switch is out, however all the other lights work. I welcome your input.
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Hammer Time
Ultimate Carjunky
/ Moderator
May 1, 2009, 5:05 AM
Post #2 of 5
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Re: Defrost thru Panel/Floor Selector Switch
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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.
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Tom Greenleaf
Ultimate Carjunky
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May 1, 2009, 11:46 AM
Post #3 of 5
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Re: Defrost thru Panel/Floor Selector Switch
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Correct me if I'm wrong Hammer Time but doesn't this use and electric actuator motor for the doors?
This may be the same issue we discussed off site with broken nylon parts or the dang thing does't work. Pretty sure the default will be defrost (some will still go to floor outlets) whether vacuum controlled or not if not jammed up??
Any recent underdash work on this vehicle?
T
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Hammer Time
Ultimate Carjunky
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May 1, 2009, 12:19 PM
Post #4 of 5
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Re: Defrost thru Panel/Floor Selector Switch
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No, it uses electric for blend doors and vacuum for mode doors.
From Alldata
Air Distribution System Notes
Control of airflow through the HVAC module is regulated by vacuum-actuated doors. At various positions of the knobs on the control assembly, mode doors mix and direct cooled, heated, and outside air through the air ducts.
In case of vacuum line/harness leakage or hose collapse, it is not necessary to replace the entire vacuum harness assembly. Replacement can be made by cutting the hose and inserting a plastic connector. If an entire hose must be replaced, cut the hose off at the connector and then attach new hose directly to the vacuum solenoid control assembly.
Vacuum System
All valves except the temperature valve are controlled by engine vacuum stored in the vacuum tank in the engine compartment. Vacuum is routed by a vacuum switch in the control assembly to either of the spring-loaded three-position vacuum actuators. One vacuum actuator is linked to each valve.
The mode knob controls the vacuum switch. For each mode knob position, the vacuum switch will direct engine vacuum to specific hoses and vent the remaining hoses. When a hose is vented to normal air pressure, the vacuum actuators internal spring will expand and hold the associated valve in one position. When vacuum is routed to the hose, the force of the vacuum overcomes the internal spring pressure and moves the valve to another position. In this way, the mode knob positions the valves for the selected operating mode.
Ducts and Outlets
A system of ducts and outlets directs air to the passenger compartment. In cases of poor air output, defroster, heater, air conditioning, and vent ducts should be checked for obstructions such as leaves, dirt, or objects that may have fallen into the ducts from the passenger compartment.
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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.
(This post was edited by Hammer Time on May 1, 2009, 12:20 PM)
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Tom Greenleaf
Ultimate Carjunky
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May 1, 2009, 2:17 PM
Post #5 of 5
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Re: Defrost thru Panel/Floor Selector Switch
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Thanks - your info sources for the assorted models is better than anything I have. So true about the vacuum line underhood usually (cheap plastic sometimes) gets broken easily and that has to be ruled out,
T
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