It have battery
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.
Vacuum leak
It is just a simple fan blowing over a coil.
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The evaporator coil condenses the humidity in the air dehumidifying it.
You have a vacuum leak causing your vents to switch from dash to defrost.
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
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.
Turn it off and leave it off for an hour or two
You probably have mold on the evaporator coil.
The evaporator coil is probably freezing up which will restrict airflow.