There are two questions. First, MAX AC makes the air colder, usually by recycling the cooled air already in the vehicle. Normal AC pulls fresh air from outside the vehicle into the interior. Second, if your vehicle has an electronic control for the environmentals, you may have a bad control module (inside the section where the knobs are mounted) or the vacuum line may be damaged. Check to see if you can control where the air blows from when using other temperature controls. If you have full control except when you use the MAX AC setting it is more likely the electronic control module is bad.
you need to replace the vacuum.
Check for vacuum leak--either under dash or in engine compartment Check for bad vacuum motors under dash Defrost is the default position for this system There is a vacuum resovoir located behind your battery , it is for this condition. When you accel your engine doesn't produce vacuum due to the throttle plate being open so a resovoir is there to hold a vacuum reserve so that when there is low vacuum produced by the engine there will be reserve to hold the vacuum motor applied preventing the air from changing direction in your dash. When there is no vacuum the direction is at the defrost vents if you have a broken hose or no vacuum your ac would blow out the defrost all the time But since it happens when you accel it sounds like your vacuum resovoir is cracked thus not allowing it to hold a vacuum
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.
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You lost the vacuume supply coming from the engine, defrost is the default mode if a mafunction in the vacuume supply is lost.
defrost is the default mode. in most cases a vacuun leak is the culprit.
Sounds like a vacuum problem - leak either under hood or under dash -- could ba a bad vacuum motor under dash Defrost is the default position for this system
The mode door actuator has failed. We have instructions for repair at www.trailvoy.com
Check the vaccuum hoses going to your manifold- The one that controls air flow direction to your AC is probably split Astroman
the ducts can not be hooked up for the vents to work
When there is a problem with the vent system it dfaults to the defrost setting. Sounds like you have a problem in the doors that direct the air flow.
It is Likely a vacuum leak under dash leading to vents
you need to replace the vacuum.
henry14@yahoo.comh
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 floor heat vent and defrost to see if air comes from them. The vents will be vacuum controlled and a vacuum leak will make the vent doors divert air to the wrong place. Check the entire engine and climate control systems for a vacuum leak.