7 Position HVAC Control Function

Discussion in 'Interior Restoration' started by K1ng0011, Dec 29, 2017.

  1. K1ng0011

    K1ng0011 Veteran Member

    111
    1
    Mar 22, 2014
    United States
    I have a 1972 camaro factory heat/ac. In have been looking into how the factory AC HVAC system works to restore it. I know the 1970-72 camaros use the 7 position heater/ac control. I feel like this is going to be a stupid question. Does anyone actually know what each position does? Difference between MAX AC and Normal AC? Difference between Defog, Deice? You get the idea. I cant seem to locate what all of these do on the forum anywhere. I would like to understand how each position works to understand it as a whole better. One a side note it seems like its a secret that people may not know about, but if you move the cold/hot lever left to cold it will seem to stop. At that point you can push it a little bit more and it will go a little further and it open your kick panel vent on the passenger side.


    A9600111.JPG

    Off: Activates the purge door valve. to push air out of the AC/heater case into the engine compartment.
    AC MAX:
    AC Normal:
    Bi Level:
    Vent:
    Heater:
    Defog:
    Defrost:

    AC/Heater Vacuum Diagram: http://nastyz28.com/tech/heat-ac/72acvacdia.jpg
     
  2. RS1979

    RS1979 Veteran Member Lifetime Gold Member

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    Jun 18, 2013
    Memphis, TN
    AC MAX: The compressor clutch stays engaged to give you max cooling
    AC Normal: The same clutch engages and releases when the pressure reaches a certain point
    Bi Level: Blows air into the lower vents and upper vents
    Vent: Blows outside air into the car. No heat or A/C
    Heater: Engages the heater
    Defog: Blows hot air on the inside front window to remove the fog
    Defrost: Removes ice or frost by blowing hot air on inside of front window
     
  3. K1ng0011

    K1ng0011 Veteran Member

    111
    1
    Mar 22, 2014
    United States
    I think that these different settings vary which of the doors that open. Such has the kickpanel door and door under the cowl. Either letting fresh air in or keeping it out kind of like a recirculate feature on a newer car I guess?

    Max AC: My understanding is that the AC compressor is turned off and on due to pressure or too cold of temp with the ambient temperature switch next to the evaporator. So this does not make sense to me.
    AC Normal:
    Bi Level: I have never used this but I thought air came out of the floor vents and dash vents at the same time in all positions but defrost.
    Heater: I can set mine to vent and turn the bottom slider to hot and I get hot air.
    Defog: Does this use outside air or something from the flapper under the cowl grill?
    Defrost: My understanding is that on most modern cars the AC compressor is used to de-humidify the air to defrost your window quicker. Is that the case here?

    Doors

    * - Plenum door: Under the grill where your windshield wipers are.
    * - Kick panel Door: Under the passenger kick panel.
    * - AC/Heater Diverter Door: Somewhere in the heat/ac suit case under your dash.
    * - Defroster Door: At the end of the heater suitcase under your dash.
    * - Blend Door: Connected to the hot/cold lever on the bottom of your controls. somewhere in the heater ac suitcase under your dash.
     
    Last edited: Dec 30, 2017
  4. twozs

    twozs Veteran Member

    8,670
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    Aug 5, 1999
    hopewell jct ny
    ac max recirculates the cabin air with no outside air . the cowl flap on the passenger side cowl opening will be closed and compressor is continuous . ac norm is cowl open introducing outside air . bi level is what it says . vent is cowl open no compressor . the temp can be controlled by the bottom slider . defog should engage the ac compressor and defrost is no compressor for cold weather . POA / THX valve systems had no compressor cycle or even a shut off when they ran dry back then . the cycling system you mention was like 78 and later . there is no pressure switch on the POA / THX system . with no real safety but the thermal limit which is on a clip on the suction line or wiring harness these compressors locked up on a pretty regular basis . It was more a mechanical system with valves and bypasses than electric switches ( the AC system not including the controls )
     
    Last edited: Dec 30, 2017
  5. K1ng0011

    K1ng0011 Veteran Member

    111
    1
    Mar 22, 2014
    United States
    Thanks twozs that is what I was looking for. That is good information thanks. So I pulled up a heater/ac panel for a later year Camaro I see that it just has the defrost feature. Based on what you said the defrost option on early 1970-73 Camaros did not run the AC compressor. So was this done to prevent damage to the AC system or prevent it from locking up in really cold weather? It makes sense to use the AC compressor to dehumidify the air and defog the windows.


    A9600115.JPG
     
  6. twozs

    twozs Veteran Member

    8,670
    130
    Aug 5, 1999
    hopewell jct ny
    the top control panel has a defog and deice . defog would be compressor for dehumidifying , deice is no compressor for ice melting on the outside of the windshield . this is what a 72 camaro would have . the lower one with the DEF selection was compressor but being that selector is for later years it would be with a clutch cycling system normally for coil freezing . freon or refrigerant is pressure / temperature sensitive . a can of Freon ( say R12 ) will have 80 pounds of pressure at 100* . that same refrigerant at 0* will have 0 pressure . this is how the pressure switch helps the system . empty , it senses no pressure and will not engage the compressor . it also works as a safety for compressor operation in cold weather as the colder the refrigerant the less pressure . what happens is , if the ambient temp is to low the Freon wouldn't evaporate and liquid Freon is sent back to the compressor ... we all know you cant compress a liquid ..the POA system senses the lower pressure and diverts the liquid back into the coil .
     

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