I have a 93 F-150 with the 302. When I am going down the road at about 70mph and speed up to pass someone. My ac switches from the vents to defrost. Like someone turned the switch. Then it will not blow back out of the vents till you let off of the gas. Then it starts blowing out the vents like normal. Is this normal or is there a vacume that is not working right? Thank.

 

It's your vacuum. I'd be betting you vacuum reservoir is not working, or you have a leak somewhere else.

 

i had a car that had a broken vacum line it done the same thing

 

I have seen a few reasons for that. especialy on ford trucks. there is a check valve it the vacume line that leads to the ac/heat control it is located under the hood I am looking at a 91 and it has \ a 3 way check valve it comes directly off of the intake and goes to the a/c right at the firewall (passenger side at the a/c plenum good luck! the check valve is a one way valve if you unplug it you should be able to suck in but not blow back from the 2 outlets and vice versa from the feed. this is first to check if the check valve works you probably have a leak again good luck this is when a vacume tester comes in.

hope this helps
Mark

 

I recently posted a question in the Heating and A/C Forum that pertained to the same problem. The check valve was the culprit on my truck. I had even added a second vac res to the heater system to pick up the slack. That solution worked most of the time.

Good luck I hope your fix is as simple as mine was. Got one at the salvage yard for a couple of bucks.

 

Thanks guys for the good info. Is the control valve about 1 1/2" long and have 3 ports on it. Located just right of the blower motor. If so I pulled it off and checked it, and it was fine. If that is it I must have a nother vacuum leak some where else. Thanks again!