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Performance Upgrades - Maintenance - Modifications - Problem Solving - Off-Road - Street Trucks. Covering the Explorer, ST, Sport, Lincoln Aviator, Sport Trac, Mercury Mountaineer, Mazda Navajo, Ford Ranger, Mazda Pickups, and the Aerostar. Featuring H.I. - Human Intelligence.
..There are a couple things to look for when chasing this problem...A blend door not operating properly, a vacuum line issue, or lastly, a bad switch...
..The first two would be what I would look into first..
The defrost (windshield) is the default for the heater/AC system. It is controlled by vacuum and if you are not getting vacuum to the switch, you get air to the windshield. As Tbars indicated, check the vacuum lines going to the switch first.
I had a situation where the heat/AC system worked fine until I accelerated. It would then default to the windshield. After considerable diagnosis, I found the check valve in the vacuum reservoir was broken. I replaced the vacuum reservoir and all is well now.
