I have '04 F-250 with 6.0. I heard someone talking a mod. I think it has something to do with it not running it so much but still provide ventilation. Thanks.

 

I don't know the particular mod you are talking about.
However,
Disabling the AC so "it doesn't run so much" sounds like disabling the AC compressor so that it only runs in AC modes. Folks often ask about this, mainly for fuel economy reasons. The usual thought is "Why does my AC run when I have my heat on? Isn't that wasting gas? Is this a problem?, etc...".

It's not a mod that makes much sense, really.
You would lose the dehumidification (comfort factor) that comes from running the AC compressor in 'other" positions, for little to no gain in fuel economy.
The AC refrigerant loop is only a factor in MPG when it's under a heat load on warm days. The warmer the ambient temp, the harder the refrigerant loop works, the more power it takes to run. The system doesn't run, or it just idles along, below about 40 degrees, but takes LOTS of power at 95-100 degrees.

Being winter, you can test the theory easily before you modify anything. Disconnect the compressor clutch connector and tuck the wiring out of harms way. Now, drive the truck for a few weeks.

You will probably find that it's harder to keep the windows clear on cool/cold, damp days and that you feel uncomfortable even once the cab gets hot.
You might, ...maybe..., notice a .5mpg uptick at about 70 degrees in your fuel mileage but only if you have tracked it well over a period of time before and after.

 

Once I was on a trip.on the interstate in the summertime with the windows down. To my understanding I had the AC off but I thought I heard the compressor cycle.

 

Turn the control to OFF or VENT. There's your "mod".

 

There has been something called "cold air mod" for A/C systems which entailed stopping flow completely to the heater core when A/C is engaged. If this is what you're talking about I've done it and honestly using Max A/C works just fine for the same thing. In fact with a new compressor and orifice tube I'm getting somewhere near 40* leaving air temps even at 35 mph on an Ohio hot day of 90*.

 

Thanks to both. I'll make a note on the OFF or VENT & try it. I use the MAX a lot with window cracked since I believe the cab air only recirculates.