I have a 98 f150 4.6 2 wd my ac blows cold out of the defrost only!!!! And when I hit a certin rpm it goes to hot heater did the same but all the vents were workin at that time also

 

sounds like you have a problem with the blend door that operates where the air flows to could be as simple as a back vaccum hose or the blend door being broke

 

I have a '99 with the same problem. I was told it was the blend door being broke. I is a 7 hr job at ford and a 50 dollar part. I will take off from a stop light and it will go hot immedatly. Then I turn it off and a minute later I turn it back on and its cold again.

 

I ws also told it would be smart to replace the heater core while they are in there. and the hose "connectors" for the heater core.

 

Where's the blend door at any pics?

 

im not sure, but i think you have to take a lot of the dash components out to get to it. I read on her somewhere that you can take out the glove box and cut a hole behind it to access the blend door, but i can no find it now.

 

What is it going, don't take out the dash before you some easy tests to make sure it's the blend door.