Hey guys its been awhile since I posted but here is a brief rundown of this weird problem on my 1995 4 banger.
a few weeks ago my heater control valve was leaking and replaced it with a motorcraft one. easy to replace but i noticed later that under 50 mph my ac blows cold but over fifty it starts blowing hot air so i turn it off to cool the compartment. well the heater on the floor vents keeps blowing intermittenly while i am traveling on the highway. In Texas the heat is above 110 degress, so u can imagine the heat inside the cab. I have checked the blend doors and vacumn lines and they seem operable and working. What can be the problem. Does she not love me any more and is making me suffer in this Texas heat.

 

Are you giving it full throttle when you do this? At WOT the A/C compressor kicks off. Do you hear the compressor at all after 50?

How did you check the blend door?

 

Your 4 banger does more than 50 MPH with the A/C running? I must have a lemon.

 

update. have not fixed the problem but did hear a suction sound when it switched from ac to heater mode automatically the other day. I will follow the vacumn lines from the heater control valve and others behind the dash. what is the black round ball that vacumns lines attach to on the right side of the engine compartment? thanks for the help. By the way, i have not had a chance to listen to the compressor after 50 mph but it does cycle when turned on.

 

i call it a vac canster it stores vac pressure.. you may have a small vac leak the faster you go the more vac you loose

 

Check this video out that I made:

 

that should be based on engine rpm not speed from the way i understand it. correct me if im wrong

 

The vacuum canister stores vacuum for use when there is no vacuum present in the intake manifold (pretty much any time the engine is higher than idle). It will really leak all the time, but will leak at a slower rate at idle, because the vacuum from the intake manifold is replenishing the vacuum being leaked out. It will completely leak all vacuum when the intake manifold has no vacuum for a certain time, depending on the size of the leak of the canister.

 

Thanks Shelton for the explanation on vacumn lines. I will go out and buy a vacumn guage to check the canister which i think is the culprit or may be the max ac actuator since i here a loud suction sound when the hot air starts blowing. Yet, i am still confused why my floor vents blow hot air when on the highway. The ac and heater controls are turned off at the control panel. Will report back when i do these tests probably until the weekend. Thanks again to everyone.

 

I found what was causing the hot air to enter the cabin when i got above fifty. it was the electronic white box above the blend door. somehow the white box has a nipple thingy that controls the blend door. it was out of place. I inserted the nipple thing into the blend door mechanism and poof no more hot air out the floor vents. thanks to every one that posted the help. now i have to replace the charcoal canister.