What could be wrong with my 89 F-150 with a 4.9l engine? Ever time I turn on the a.c. or the defroster it starts to idle up and down and chokes down. If I turn off the a.c. or defroster it will idle normal. I replaced the i.a.c. valve but it didn't help any. No one I talk to seems to have any idea what it could be. I know some of you will know what could be wrong with it.

 

It could be your compresor is going bad. Unplug the compressor and then try the defroster agian. see if this allows the truck to idle normally.

 

Ill 2nd that. As the only thing different from normal engine opp, when the AC and defrost is on, is the AC Compressor.