i have a 2001 f150 and when i have the a/c on while when i aclerate the a/c goes to defrost does any one know what the problem is and how to fix it.

 

You have a broken vaccuum line somewhere. Look for the line where it goes into the firewall. That is the most common place to be cracked. Other than that you might have to take it to a shop and have it smoke tested to find the vacuum leak.

 

I wonder if the vacuum accumulator can is leaking or has a bad check valve. The accumulator looks like a black painted coffee can.