Hello..my 99 f-150 4.2L V6 when I turn on AC its idles up & down..what causes this?

 

The idle is designed to raise when the compressor cycles on, and lowers when it cvcles off.

 

With electronic control, the computer has an input from the A/C system to tell it to 'account' for the extra load.
It does this by opening the IAC more to let more air in a continuously varying control operation.
This should be transparent to the driver.
Not an increase in RPM or a drop but a steady loading of the motor with no changes observed.
The reason is the idle RPM is set in program to +/-50 hot rpm and is sensed by the crank sensor to keep it within those limits and not vary enough to be noticed. Same for the electrical load and power steering load changes.
If this were not the case you will have all kind of engine rolling and even stalling at times, just from idle load changes.
Suspect the IAC to be sluggish.
Remove it and do a cleaning to see if that clears the idle speed change.
You may also consider cleaning the throttle body and plate area.
Good luck.

 

Sounds like you might be low on refrigerant.

When mine was low the compressor would cycle on and off every 3-5 seconds.
Topped mine up with one of these.
http://www.autozone.com/autozone/acc...uestid=1821241

 

+1 Mine does the same thing when I am running low....problem solved!!

 

Thanks for everyone suggestions,will check out each one