max a/c
- Thread starter milano
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WillSquared
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I'm not sure about that. Max ac may just open the foot vents. Good question.
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One should be cooling outside air and the other cools inside air.
Itali83
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Yeah it just closes the fresh air door to recirculate the air inside. So the a/c isnt trying to cool the hot humid outside air.
nc87k5
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Hey guys. I've got a question about my distribution box. When in max ac, it blows thru the vents and floor vents. I want to know is the floor vent door is closed by a spring or does the vacuum actuator close it? Also, where does fresh air come in for the system? When I have everything off, I feel air coming out the floor vents as well.
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chengny
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is the floor vent door is closed by a spring or does the vacuum actuator close it?
The temp door and floor/dash door are connected by linkage. They move in unison and are actuated by a single actuator (the one in front of the box - closest to the firewall).
Here are a couple of easy things to check. To see the floor vent door only requires removing the duct piece that points down towards the transmission tunnel (one screw up top):
Also, where does fresh air come in for the system? When I have everything off, I feel air coming out the floor vents as well.
The fresh air inlet is under the wiper cowling way over to the RH side. And, by design it does not fully close off - a certain amount of air is always allowed to flow into the cab. See the chart:
The temp door and floor/dash door are connected by linkage. They move in unison and are actuated by a single actuator (the one in front of the box - closest to the firewall).
Here are a couple of easy things to check. To see the floor vent door only requires removing the duct piece that points down towards the transmission tunnel (one screw up top):
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Also, where does fresh air come in for the system? When I have everything off, I feel air coming out the floor vents as well.
The fresh air inlet is under the wiper cowling way over to the RH side. And, by design it does not fully close off - a certain amount of air is always allowed to flow into the cab. See the chart:
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nc87k5
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The small white clip has been replaced and as far as I can tell, the linkage you are referring to is attached properly but the floor vent door is not closing.
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chengny
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The floor vent door is positioned directly by the actuator. It is only linked by a two section extension arm (and the white attaching clip). See these images:
Although the arm is made up of two parts - connected by a screw - I've never seen the screw come loose. So, if the floor vent door is not moving, a safe assumption would be a failed actuator or an issue with either the hoses or the control head.
BTW- that actuator is the only dual-acting type in the system. It requires a vacuum signal to move in both directions. It will stay wherever it was left without a vacuum signal. All the other actuators are spring loaded against the vacuum signal and default to that position when vacuum is lost.
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Although the arm is made up of two parts - connected by a screw - I've never seen the screw come loose. So, if the floor vent door is not moving, a safe assumption would be a failed actuator or an issue with either the hoses or the control head.
BTW- that actuator is the only dual-acting type in the system. It requires a vacuum signal to move in both directions. It will stay wherever it was left without a vacuum signal. All the other actuators are spring loaded against the vacuum signal and default to that position when vacuum is lost.
flyboy1100
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The vent on the passenger side floor/wall that draws air from outside only opens when I select Max, is this correct?
chengny
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The vent on the passenger side floor/wall that draws air from outside only opens when I select Max, is this correct?
Yes it only is supposed to be open when MAX A/C is selected - and only when that mode is selected. This is to allow the blower to pull air in from the cabin - air that has already been cooled and dehumidified - rather than hot/humid air from outside.
The recirc door and outside air inlet damper share a common vacuum line and are operated simultaneously. But, while the recirc door opens when the system mode lever is moved to MAX A/C, the OAS damper closes. It follows that the opposite happens when the system is in any mode other than MAX A/C. This is accomplished by using actuators that have opposite spring loaded default positions.
This ensures that the blower always has a supply of air.
See the vacuum schematic and logic chart below. When MAX A/C mode is selected, the valve behind the control head face plate opens port #4 to the main vacuum source and closes off it's vent to atmosphere. With vacuum applied to the actuators - via the orange tubing - they move from their default positions to their operated positions. The default position of the recirc actuator is closed - and the OAS default is open. Note that the only mode position that applies vacuum to the common OAS/Recirc line (on port 4) is MAX A/C:
So, if MAX A/C is selected (and port #4 is opened), the recirc door in the kick panel goes to open and the fresh air inlet under the wiper cowling is pulled shut - or as shut as it can be. The OAS damper is designed so that it cannot completely seal off the plenum under the cowling. I think this is to ensure at least some fresh air is always able to flow into the cab (it's probably a CO poisoning thing)
Yes it only is supposed to be open when MAX A/C is selected - and only when that mode is selected. This is to allow the blower to pull air in from the cabin - air that has already been cooled and dehumidified - rather than hot/humid air from outside.
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The recirc door and outside air inlet damper share a common vacuum line and are operated simultaneously. But, while the recirc door opens when the system mode lever is moved to MAX A/C, the OAS damper closes. It follows that the opposite happens when the system is in any mode other than MAX A/C. This is accomplished by using actuators that have opposite spring loaded default positions.
This ensures that the blower always has a supply of air.
See the vacuum schematic and logic chart below. When MAX A/C mode is selected, the valve behind the control head face plate opens port #4 to the main vacuum source and closes off it's vent to atmosphere. With vacuum applied to the actuators - via the orange tubing - they move from their default positions to their operated positions. The default position of the recirc actuator is closed - and the OAS default is open. Note that the only mode position that applies vacuum to the common OAS/Recirc line (on port 4) is MAX A/C:
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So, if MAX A/C is selected (and port #4 is opened), the recirc door in the kick panel goes to open and the fresh air inlet under the wiper cowling is pulled shut - or as shut as it can be. The OAS damper is designed so that it cannot completely seal off the plenum under the cowling. I think this is to ensure at least some fresh air is always able to flow into the cab (it's probably a CO poisoning thing)
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flyboy1100
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i guess the idea is the air coming from the fender/side is cooler than the wiper inlet? or maybe not as much airflow to allow the max/recirc to really function best?
i should really fix my a/c then i could test it out, lol. I think is is just low on r12 so the pressure switch won't close to activate the compressor. need to convert it to r134a someday, maybe next year.
i should really fix my a/c then i could test it out, lol. I think is is just low on r12 so the pressure switch won't close to activate the compressor. need to convert it to r134a someday, maybe next year.
chengny
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i guess the idea is the air coming from the fender/side is cooler than the wiper inlet?
You got it Jackson.
It probably isn't a huge issue in ND - but down south it is way more efficient to recirculate the cabin atmosphere than to displace it with outside air.
Other than to remove contaminants from the air in the cab (e.g. cigarette smoke), it makes no sense to cool/dehumidify incoming air when air that has already been conditioned is available and only needs a slight temperature drop.
The recirculated air in the cab is probably around 70 degrees and the outside air can be as high as the low 100's. The gas savings due to reduced compressor operation are considerable.
You got it Jackson.
It probably isn't a huge issue in ND - but down south it is way more efficient to recirculate the cabin atmosphere than to displace it with outside air.
Other than to remove contaminants from the air in the cab (e.g. cigarette smoke), it makes no sense to cool/dehumidify incoming air when air that has already been conditioned is available and only needs a slight temperature drop.
The recirculated air in the cab is probably around 70 degrees and the outside air can be as high as the low 100's. The gas savings due to reduced compressor operation are considerable.
87ChevyR10
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Just dont use max ac for too long. You'll eventually run yourself short of O2.
flyboy1100
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I have read those warnings on other vehicles just not sure how it is possible since they do still allow some fresh air in and the cabin isn't 100% air tight so some air will be flowing through at all times.
We have used it for hours on end in my Subaru and our old VW (sold now).
