My 2000 7.3 ac clutch is turning on and off even when the defrost or ac is off...where should I start to diagnose this?

 

Your air gap might be too large for the A/C clutch.

Start here (post #3): https://www.ford-trucks.com/forums/5...ml#post4464164

Here are some more for you to peruse:

https://www.ford-trucks.com/forums/1...h-air-gap.html
https://www.ford-trucks.com/forums/1382776-air-gap.html
https://www.ford-trucks.com/forums/1...knowledge.html
https://www.ford-trucks.com/forums/1...gap-fixed.html
https://www.ford-trucks.com/forums/6...-question.html

This by no means the entire list. Plenty of us have suffered intermittent A/C issues for one reason or another. If you have a continuous cycling of the compressor, though, it is most likely the air gap for the clutch. It's a cheap fix compared to other options and worth a try IMO.

I recommend that you post your engine-related issues in the 99-03 7.3L section. Lots of people that know way too much about our trucks over there. Not all of us cruise over here in the general SD sections as our trucks are several generations old and not as relevant over here any more.

 

the 7.3L does not use a relay so maybe the function switch has continuity in the ac clutch circuit even when you have it in the off position ?
check the violet wire coming out of the function switch to see if there is 12v even when off or panel or floor. all other functions allow 12v through to the ac pressure switches

 

Go double-check the exact switch positions that it is observed to be operating in.

 

it happens when the floor vent is open and heat is on

 

Ok so I went to the truck again to check and it only happens when defrost is turned on or ac is on

 

I think it is only off in vent and off. My dearly departed 2002 F350 would run compressor in all but those two selections.
If it is lower 50's down to low 40's short cycling is normal. In the 30's it will run briefly and then go off for 45sec to a min or more. Once in the low 30's or colder it will not come on. The cycling is controlled by a pressure switch and the gas pressure gets lower with temp and will reach a point where pressure is not high enough to ever close the switch and operate clutch. Clutch gap being wide causes compressor to work OK and then after a few minutes the winding gets warm from current flow and conducted heat and current flow drops and that makes magnetic field too weak to pull clutch together given the wider gap and higher magnet requirement. Cutting off compressor for a bit lets winding cool and you get another few minutes of use before it happens again.

 

Then it's normal.

 

well not normal because the clutch only works for a split second and then disengages

 

Maybe, maybe not. You haven't provided enough pertinent information to determine that. THAT behavior is not what you asked about in the first place.

If its cool enough outside then it's normal to have very short cycling, it any engagement at all. If it's above 50°F, the cycling rate should get longer as the temperature rises and the load on the system increases. If the temp is high enough where you NEED A/C performance, then short-cycling is a concern.

Start by reading the HVAC FAQs, the link is in my signature.

 

With a CCOT system the pressure cycling switch for compressor goes open at 24 psi and closes at 42 psi (a couple of psi range to those numbers but open number can't be less than 21 psi without risk of evaporator freezing in higher humdity with low fan speed). When it is in the lower 50s and below, the pressure will drop very rapidly from the 42 close pressure back to the 24 open pressure just like when the system is low on refrigerant at temps 30 degrees or so higher.

 

I had been told that the AC compressor briefly cycles every so often when off. It keeps everything circulating to prevent issues.

 

The laws of thermodynamics controls the low end of engagement (as well as the upper). When the temperature of the system drops, so does the internal pressure. When the internal pressure drops below the engagement point of the pressure switch, the switch remains open and the compressor remains disabled. Since the pressure switch is designed to close on a descending pressure between 40 and 45 psi, that equates to a system temperature of 45-49°F. Check the owner's manual, it will tell you that compressor engagement below 45-50 degrees ore thereabouts isn't expected.

There is no getting around that behavior.

 

I remember having a problem defogging the windshield but what I originally asked was if this very short clicking is the system broken.

 

You STILL haven't provided adequate details and context in order to answer that question.