So, recently I started having a tick and drop in RPMs from ~750 idle to ~500 then back to 750. I timed it one day at a red light, and found that its basically on an 8-9 second interval. I got home, popped the hood, and found my AC clutch engaging the pulley. It doesn't affect my driving, and I stay steady when I'm on cruise control. It happens when my AC is off, and I haven't noticed this before. The clutch will engage for about one second then disengage. Anyone know why it does that? It causes a pretty nervous shutter in the truck when I'm stopped.

 

AC compressor is used in Defrost mode as well as AC on

The AC evaporator is used to dry the air before it is blown onto the windshield, this makes for a faster Defrost as the drier air can suck more moisture off the glass

Turn off Defrost and see if AC compressor stops cycling on/off
If so then all is normal

In 2004 the computer controls the AC compressor so should respond quickly with higher idle as compressor comes on, it does that using the IAC Valve, may be time to clean it if its slow to react

 

On my 98 Merc Mountaineer with the 4.0 I also have that issue. Every setting except the front vents only. Any other setting will cause the clutch to click like that. Not just AC, heat as well. Pretty sure it's normal, if irritating.

 

Thanks for the information on the IAC, i cleaned it a few weeks ago when I pulled my throttle body off to give that a quick clean. I must have removed some carbon and that is what kicked it off. I saw another post of yours on IAC testing and mine wasn't opening like it should have. New one in now and runs like a champ.

 

Good work

Thanks for follow up post and the FIX