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 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.
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..
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