I have an 02 f150 Supercrew with the 5.4 and my motor surges from 1100 to 600 rpm any time my a/c is in any position except panel or floor. Have check the charge of a/c and it is fine. Any suggestions?

 

It seems that you already realize that the AC compressor will kick on any time the defrost is on. This is to dry the air coming out of the dash so you don't make the condensation worse on your windshield every time you turn it on.

The compressor is pulling power every time it kicks on to dry the air. My usual suspect for this would be the idle control valve. When was the last time you did a full tune up?

 

My truck did the same thing, I charged the ac and switched the ac compressor switch then unplugged the battery. Worked good for over a year. But I don't know exactly what part solved the issue. Also could be idle air control sensor. My truck is on occasions having a jumpy idle and my mechanic friend thinks it's the IAC sensor. Good luck man.

 

Also check your vac lines for the AC.....box behind the battery has a few vac lines that are notorious for going bad...vac leak=surge

 

Thanks guys I will look into all of these. And the truck only has 83k on it so havent done a full tune up yet. Probably gonna happen when it starts warming up a little tho. Thanks again