I found this on another site & hope it helps.
Arthur in Texas
"If the vacuum system functions normally at idle but goes to defrost during acceleration, a small leak exists in the system. The leak can be located by turning off the engine and using a hand vacuum pump/gauge to check for vacuum delay while selectively blocking off vacuum hoses. I would start with the vacuum tank and check valve under the hood. This is the most likely location of a leak".
It have battery
Loss of vacuum due to leak or bad vacuum caninster. Defrost is the default on this system and when vacuum drops it goes into default mode Check for leak either under hood or under dash area
It is conveniently located on the passenger side forward surface of the defrost air distributor. To access it, remove the dashboard cover, remove the air conditioner controls and radio, remove all right side lower dashboard fasteners, noise deadening panels, etc, pull dashboard assembly away from firewall, rotate defrost air distributor slightly clockwise, and you can reach in and remove the module (grey) with a 7mm open end wrench this guy is exactly right.
The 2005 Chrysler 300 rear window defrost relay switch can be found beneath the drivers side dashboard. The rear window defrost relay switch will be near the center console.
If central air conditioner turns on but is not blowing through the vents, check the filter. If it is dirty, change it. Change the thermostat into cool, this will defrost the air conditioner.
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You have a vacuum leak causing your vents to switch from dash to defrost.
The vacuum supply line from the engine is disconnected or broken. When the vacuum supply is cut off the system defaults to the defrost mode.
You probably have a jammed or otherwise defective vent door.
When there is a problem in the vent system it defaults to defrost setting. Check the doors & vac supply
When you select the defrost setting , the air conditioning compressor will kick in and out to help dehumidify the air , that's normal to hear the compressor kicking in and out
Check the vacuum supply from the engine. If it is disconnected or cracked the system will default to defrost only.
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no, only with defrost or ac on or if coolant temp is hot enough
There is most likely a vacuum leak on the line from the engine to the air conditioner control.
Check the following items......... 1. Vacuum leak/leaks. 2. Switch. 3. Controller.
Manual defrost is the instructions for you to manually defrost your freezer and automatic defrost is saying the freezer defrost on it's own.
Your unit is freezing up out side like a block of ice? if so then obviously you have a heat pump.. what is happenong is your defrost board or thermostat on the out side unit is no good and needs to be replaced.. if you cant afford service on it, to defrost the unit some what quickly, turn your air conditioner on for abour 5 minutes.. that should defrost the unit..
If the outside temperature is about 50 degrees Fahrenheit ( 10 * C ) or higher and the defrost or floor and defrost modes are selected the air conditioning compressor will cut in and out as it dehumidifies the air ( that's normal )
It is defrost.
Cooling fan is designed to come on only when A/C, defrost, or engine temperature exceeds 235 degrees
It only stays on when the air flow is turned to defrost. It does this because the air condition has a dryer on it that dries the air which defrosts the windshield better.
The moveable "baffle" inside the dashboard is not redirecting the air flow as it should. It's operated either by vacuum or an electric motor, in either case, repair is going to require the removal of the dashboard.
The first thing that you need to start checking for is a broken vacuum line/lines or one that has come off.............