Mike is right on. most of the time it stays cut off for 5-20 seconds or so after the WOT run is done (cant recall which spec is for which car; I think stangs are on the quicker side) - and since cold air continues to flow with the A/C compressor not running (try switching from A/C to vent sometime - cold air continues for 30-60 seconds), most folks dont notice.
What you say is true but it will not cause the air to blow out the defrost.Mustang5L5 said:
Sounds more like a vacume problem to me. Seen it plenty of times. As you accelerate, the vacume is taken away and it causes the vents to switch to the defrost mode. Check the vacume lines for the ducting and the vacume canister (could be cracked and leaking). Also seen the vacume switching valve in the dash go bad as well. Se if you can hear a vacume leak in the dash area.
The issue is the reservoir. It sits in the passenger side fender. If you follow the source vacuum from the tree, it comes to a three-way check valve on the passenger side of the firewall. One line goes into the cabin (the source for your controls), while the other goes to the reservoir. There is a leak somewhere.